what is an assertion?

An assertion or assert statement is a way for me to tell Python, “DO NOT CONTINUE, if this statement is False”, or said a different way “GO TO THE NEXT LINE, ONLY if this statement is True”.

I use assertions in tests when making a program to make sure something is True about the program before I continue building. I use them to test ideas, without worrying about if I will remember the ideas later. I use them to test how programs behave, for example when given inputs.

Assertions can help catch things that make tests that were passing, start failing when I add new lines of code. They help me answer 2 questions

  • what is the same?

  • what is different?

The difference between my expectations (tests and ideas) and reality (what happens when the program runs), tells me what to change to make them match. The closer my program is to reality, the better.

preview

I have these tests by the end of the chapter

  1def test_assert_keyword():
  2    assert 1 + 1 == 2
  3
  4    assert '1' + '1' == '11'
  5
  6    assert 'I am' + ' alive' == 'I am alive'
  7
  8
  9def test_assertion_error_w_none():
 10    assert None is None
 11
 12    assert False is not None
 13
 14    assert True is not None
 15
 16    assert 0 is not None
 17
 18    assert 0.0 is not None
 19
 20    assert '' is not None
 21
 22    assert () is not None
 23
 24    assert [] is not None
 25
 26    assert set() is not None
 27
 28    assert {} is not None
 29
 30
 31def test_assertion_error_w_false():
 32    assert None is not False
 33
 34    assert False is False
 35
 36    assert True is not False
 37
 38    assert 0 is not False
 39
 40    assert 0.0 is not False
 41
 42    assert '' is not False
 43
 44    assert () is not False
 45
 46    assert [] is not False
 47
 48    assert set() is not False
 49
 50    assert {} is not False
 51
 52
 53def test_assertion_error_w_true():
 54    assert None is not True
 55
 56    assert False is not True
 57
 58    assert True is True
 59
 60    assert 0 is not True
 61
 62    assert 0.0 is not True
 63
 64    assert '' is not True
 65
 66    assert () is not True
 67
 68    assert [] is not True
 69
 70    assert set() is not True
 71
 72    assert {} is not True
 73
 74
 75def test_assertion_error_w_equality():
 76    assert None == None
 77
 78    assert False != None
 79
 80    assert False != True
 81
 82    assert False == False
 83
 84    assert True != None
 85
 86    assert True == True
 87
 88
 89def test_assertion_error_w_is_vs_equal():
 90    assert 0 is not 0.0
 91
 92    assert 0 == 0.0
 93
 94
 95def will_not_run():
 96    assert False == True
 97
 98
 99def test_failure():
100    # assert False == True
101    assert False == False
102
103
104# NOTES
105# a dictionary is not True
106# a dictionary is not False
107# a dictionary is not None
108# a set is not True
109# a set is not False
110# a set is not None
111# a list is not True
112# a list is not False
113# a list is not None
114# a tuple is not True
115# a tuple is not False
116# a tuple is not None
117# a string is not True
118# a string is not False
119# a string is not None
120# a float is not True
121# a float is not False
122# a float is not None
123# an integer is not True
124# an integer is not False
125# an integer is not None
126# True is True and equal to True
127# True is not False and NOT equal to False
128# True is not None and NOT equal to None
129# False is not True and NOT equal to True
130# False is False and equal to False
131# False is not None and NOT equal to None
132# None is not True and NOT equal to True
133# None is not False and NOT equal to False
134# None is None and equal to None
135
136
137# Exceptions seen
138# AssertionError

questions about AssertionError

Questions to think about as I go through the chapter

start the project

  • I name this project assertion_error

  • I open a terminal

  • I change directory to the assertion_error folder in the pumping_python folder

    cd assertion_error

    the terminal shows

    cd: no such file or directory: assertion_error

    there is no folder with the name assertion_error in this folder.

  • I use uv to make a directory for the project and initialize it

    uv init assertion_error

    the terminal shows

    Initialized project `assertion-error`
at `.../pumping_python/assertion_error`

    then goes back to the command line.

  • I change directory to the project

    cd assertion_error

    the terminal shows I am in the assertion_error folder

    .../pumping_python/assertion_error

  • I make a directory for the tests

    mkdir tests

    the terminal goes back to the command line.

  • I make the tests directory a Python package

    Danger

    use 2 underscores (__) before and after init for __init__.py not _init_.py

    touch tests/__init__.py

    New-Item tests/__init__.py

    the terminal goes back to the command line.

  • I use the mv program to change the name of main.py to test_assertion_error.py and move it to the tests folder

    mv main.py tests/test_assertion_error.py

    Move-Item main.py tests/test_assertion_error.py

    the terminal goes back to the command line.

  • I open test_assertion_error.py

  • I delete the text in the file then add the first failing test to test_assertion_error.py

    1assert False is True

  • I go back to the terminal to make a requirements file for the Python packages I need

    echo "pytest" > requirements.txt

    the terminal goes back to the command line.

  • I add pytest-watcher to the requirements file

    echo "pytest-watcher" >> requirements.txt

    the terminal goes back to the command line.

  • I use uv to install pytest-watcher with the requirements file

    uv add --requirement requirements.txt

    the terminal shows that it installed pytest-watcher and its dependencies.

  • I add the new files and folder to git for tracking

    git add .

    the terminal goes back to the command line.

  • I add a git commit message

    git commit --all --message 'setup project'

    the terminal shows

    [main (root-commit) a0b12c3] setup project
 8 files changed, X insertions(+)
 create mode 100644 .gitignore
 create mode 100644 .python-version
 create mode 100644 README.md
 create mode 100644 pyproject.toml
 create mode 100644 requirements.txt
 create mode 100644 tests/__init__.py
 create mode 100644 tests/test_assertion_error.py
 create mode 100644 uv.lock

    then goes back to the command line.

  • I use pytest-watcher to run the tests automatically

    uv run pytest-watcher . --now

    the terminal is my friend, and shows AssertionError

    ========================== ERRORS ==========================
______ ERROR collecting tests/test_assertion_error.py ______
tests/test_assertion_error.py:2: in <module>
    assert False is True
E   assert False is True
================= short test summary info ==================
ERROR tests/test_assertion_error.py - assert False is True
!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!
===================== 1 error in I.JKs =====================

    because False is NOT True.

    if the terminal does not show the same error, then check if

    • your tests/__init__.py has two underscores (__) before and after init for __init__.py not _init_.py

    • you ran echo "pytest-watcher" >> requirements.txt, to add pytest-watcher to the requirements file

    and try uv run pytest-watcher . --now again

  • I add AssertionError to the list of Exceptions seen in test_assertion_error.py

    1assert False is True
2
3
4# Exceptions seen
5# AssertionError

  • I change True to False in the assertion

    1# assert False is True
2assert False is False
3
4
5# Exceptions seen
6# AssertionError

    the test passes.

the assert keyword

We know that the result of 1 + 1 is 2. What if I said that the result of '1' + '1' is '11', would you agree?

I can use assertions to make the computer check if these statements are True.

RED: make it fail

  • I remove the first failing assertion then add the first statement

    11 + 1 == 2
2
3
4# Exceptions seen
5# AssertionError

    • the test is still passing

    • == is two equal signs on the keyboard (=+=) and means is equal which makes this statement read as 1 + 1 is equal to 2

  • I change the result of the equation

    1# 1 + 1 == 2
21 + 1 == 11
3
4
5# Exceptions seen
6# AssertionError

    the test is still passing. Why is it still passing? 1 + 1 cannot be both 2 and 11, is Python broken?

  • I want the test to fail if I write a statement that is NOT True. I change it to an assert statement

    1# 1 + 1 == 2
2# 1 + 1 == 11
3assert 1 + 1 == 11
4
5
6# Exceptions seen
7# AssertionError

    the terminal is my friend, and shows

    E   assert (1 + 1) == 11

    because Python does not care if the statement is False or True as long as it follows the rules. Once I make it an assertion with the assert keyword it tells the computer DO NOT CONTINUE, if "1 + 1 == 11" is False

GREEN: make it pass

I change the result back to make the statement True

1# 1 + 1 == 2
2# 1 + 1 == 11
3# assert 1 + 1 == 11
4assert 1 + 1 == 2
5
6
7# Exceptions seen

the test passes.

REFACTOR: make it better

  • I add the other statement ('1' + '1' == '11')

    1# 1 + 1 == 2
2# 1 + 1 == 11
3# assert 1 + 1 == 11
4assert 1 + 1 == 2
5
6'1' + '1' == '2'
7
8
9# Exceptions seen

    the terminal still shows no tests ran because the statement is not an assertion.

  • I add assert before the statement to make it an assertion

     1# 1 + 1 == 2
 2# 1 + 1 == 11
 3# assert 1 + 1 == 11
 4assert 1 + 1 == 2
 5
 6# '1' + '1' == '2'
 7assert '1' + '1' == '2'
 8
 9
10# Exceptions seen

    the terminal is my friend, and shows AssertionError

    E   AssertionError: assert ('1' + '1') == '2'

    because the assert before the statement makes it a command to Python - DO NOT CONTINUE if "'1' + '1' == '2'" is False

what causes AssertionError?

AssertionError is raised if the statement after the assert keyword is False. It was in the first failing assertion

assert False is True

With this statement, I tell Python - “DO NOT CONTINUE, if False is the same object as True”, or said a different way “GO TO THE NEXT LINE, ONLY if False is the same object as True”.

  • I change the statement to make it True

     6# '1' + '1' == '2'
 7# assert '1' + '1' == '2'
 8assert '1' + '1' == '11'
 9
10
11# Exceptions seen

    the test passes because the statement is now True, '1' + '1' is equal to '11'

  • These two statements are NOT the same

    • 1 + 1 == 2 checks if the result of adding two numbers is equal to the number on the right side of the equality symbol (==).

    • '1' + '1' == '11' checks if the result of “adding” 2 strings is equal to the string on the right side of the equality symbol (==). A string is anything inside quotes.

    I add another statement to show how '1' + '1' == '11'

     6# '1' + '1' == '2'
 7# assert '1' + '1' == '2'
 8assert '1' + '1' == '11'
 9
10'I am' + ' alive' == '11'
11
12
13# Exceptions seen

    the test is still green because 'I am' + ' alive' == '11' is just a statement. Python does not care whether it is False or True and the statement follows Python rules.

  • I change the statement to an assertion with the assert keyword

     6# '1' + '1' == '2'
 7# assert '1' + '1' == '2'
 8assert '1' + '1' == '11'
 9
10# 'I am' + ' alive' == '11'
11assert 'I am' + ' alive' == '11'
12
13
14# Exceptions seen

    the terminal is my friend, and shows AssertionError

    E   AssertionError: assert ('I am' + ' alive') == '11'

    because the assert before the statement makes it a command to Python - DO NOT CONTINUE if "'I am' + ' alive' == '11'" is False

  • I change the statement to make it True

    10# 'I am' + ' alive' == '11'
11# assert 'I am' + ' alive' == '11'
12assert 'I am' + ' alive' == 'I am alive'
13
14
15# Exceptions seen

  • I remove the commented lines

    1assert 1 + 1 == 2
2
3assert '1' + '1' == '11'
4
5assert 'I am' + ' alive' == 'I am alive'
6
7
8# Exceptions seen
9# AssertionError

  • I open a new terminal then change directories to assertion_error

    cd assertion_error

    the terminal shows I am in the assertion_error folder

    .../pumping_python/assertion_error

  • I add a git commit message

    git commit --all --message \
'test the assert keyword'

    the terminal shows a summary of the changes then goes back to the command line.

test AssertionError with None

None is used when there is no value. I can use assertions to test if something is the same object as None, this is useful if I want to check what value I get from a process.

For example, if I have people fill a form and I want a test for when they leave something blank, I can use an assertion to make sure that the value is None.

RED: make it fail

  • I go back to the terminal where the tests are running

  • I add a new statement to test_assertion_error.py

    5assert 'I am' + ' alive' == 'I am alive'
6
7None is not None
8
9# Exceptions seen

    the test is still passing

  • I add the assert keyword

     5assert 'I am' + ' alive' == 'I am alive'
 6
 7# None is not None
 8assert None is not None
 9
10# Exceptions seen

    the terminal is my friend, and shows AssertionError

    E       assert None is not None

    because None is not None is False.

GREEN: make it pass

I change the statement to make it True

 7  # None is not None
 8  # assert None is not None
 9  assert None is None
10
11  # Exceptions seen

the test passes.

REFACTOR: make it better

  • I add a note with what I learned from the experiment

     7# None is not None
 8# assert None is not None
 9assert None is None
10
11
12# NOTES
13# None is None
14
15# Exceptions seen
16# AssertionError

  • I add a new assertion to compare None with False.

     7# None is not None
 8# assert None is not None
 9assert None is None
10
11assert False is None
12
13
14# NOTES

    the terminal shows

    E    assert False is None

  • I change the assert statement to make it True

     7# None is not None
 8# assert None is not None
 9assert None is None
10
11# assert False is None
12assert False is not None
13
14
15# NOTES

    the test passes.

  • I add a note about False

    11# assert False is None
12assert False is not None
13
14
15# NOTES
16# False is not None
17# None is None
18
19# Exceptions seen

  • I add an assertion to compare None with True.

    11# assert False is None
12assert False is not None
13
14assert True is None
15
16
17# NOTES

    the terminal shows

    E    assert True is None

  • I change the assert statement to make it True

    11# assert False is None
12assert False is not None
13
14# assert True is None
15assert True is not None
16
17
18# NOTES

    the test passes.

  • I add a note about True

    18# NOTES
19# True is not None
20# False is not None
21# None is None
22
23# Exceptions seen
24# AssertionError

  • I add an assert statement for an integer (a whole number without decimals)

    14# assert True is None
15assert True is not None
16
17assert 0 is None
18
19
20# NOTES

    the terminal shows

    E       assert 0 is None

  • I change the statement to make it True

    14# assert True is None
15assert True is not None
16
17# assert 0 is None
18assert 0 is not None
19
20
21# NOTES

    the test passes.

  • I add a note about integers

    21# NOTES
22# an integer is not None
23# True is not None
24# False is not None
25# None is None
26
27
28# Exceptions seen
29# AssertionError

  • I add an assert statement for a float (binary floating point decimal number)

    17# assert 0 is None
18assert 0 is not None
19
20assert 0.0 is None
21
22
23# NOTES

    the terminal shows

    E       assert 0.0 is None

  • I change the statement to make it True

    17# assert 0 is None
18assert 0 is not None
19
20# assert 0.0 is None
21assert 0.0 is not None
22
23
24# NOTES

    the test passes.

  • I add a note about floats

    24# NOTES
25# a float is not None
26# an integer is not None
27# True is not None
28# False is not None
29# None is None
30
31
32# Exceptions seen
33# AssertionError

  • I add an assert statement for a string (anything in quotes)

    20# assert 0.0 is None
21assert 0.0 is not None
22
23assert '' is None
24
25
26# NOTES

    the terminal is my friend, and shows AssertionError

    E   AssertionError: assert '' is None

    because a string is not the same object as None.

  • I change the statement to make it True

    20# assert 0.0 is None
21assert 0.0 is not None
22
23# assert '' is None
24assert '' is not None
25
26
27# NOTES

    the test passes because a string is not the same object as None.

  • I add a note about strings

    27# NOTES
28# a string is not None
29# a float is not None
30# an integer is not None
31# True is not None
32# False is not None
33# None is None
34
35
36# Exceptions seen
37# AssertionError

  • I add an assert statement for a tuple (anything in parentheses ( ) separated by a comma)

    26# assert '' is None
27assert '' is not None
28
29assert () is None
30
31
32# NOTES

    the terminal shows

    E   assert {} is None

    because a tuple is not the same object None.

  • I change the statement to make it True

    26# assert '' is None
27assert '' is not None
28
29# assert () is None
30assert () is not None
31
32
33# NOTES

    the test passes because a tuple is not the same object None.

  • I add a note about tuples

    30# NOTES
31# a tuple is not None
32# a string is not None
33# a float is not None
34# an integer is not None
35# True is not None
36# False is not None
37# None is None
38
39
40# Exceptions seen
41# AssertionError

  • I add an assert statement for a list (anything in square brackets [ ])

    26# assert () is None
27assert () is not None
28
29assert [] is None
30
31
32# NOTES

    the terminal shows

    E   assert [] is None

    because a list is not the same object as None.

  • I change the statement to make it True

    26# assert () is None
27assert () is not None
28
29# assert [] is None
30assert [] is not None
31
32
33# NOTES

    the test passes because a list is not the same object as None.

  • I add a note about lists

    33# NOTES
34# a list is not None
35# a tuple is not None
36# a string is not None
37# a float is not None
38# an integer is not None
39# True is not None
40# False is not None
41# None is None
42
43
44# Exceptions seen
45# AssertionError

  • I add an assert statement for a set

    29# assert [] is None
30assert [] is not None
31
32assert set() is None
33
34
35# NOTES

    the terminal is my friend, and shows AssertionError

    E   assert set() is None

  • I change the statement to make it True

    29# assert [] is None
30assert [] is not None
31
32# assert set() is None
33assert set() is not None
34
35
36# NOTES

    the test passes.

  • I add a note about sets

    36# NOTES
37# a set is not None
38# a list is not None
39# a tuple is not None
40# a string is not None
41# a float is not None
42# an integer is not None
43# True is not None
44# False is not None
45# None is None
46
47
48# Exceptions seen
49# AssertionError

  • I add an assert statement for a dictionary

    32# assert set() is None
33assert set() is not None
34
35assert {} is None
36
37
38# NOTES

    the terminal shows

    E   assert {} is None

  • I change the statement to make it True

    32# assert set() is None
33assert set() is not None
34
35# assert {} is None
36assert {} is not None
37
38
39# NOTES

    the test passes.

  • I add a note about dictionaries

    51# NOTES
52# a dictionary is not None
53# a set is not None
54# a list is not None
55# a tuple is not None
56# a string is not None
57# a float is not None
58# an integer is not None
59# True is not None
60# False is not None
61# None is None
62
63
64# Exceptions seen
65# AssertionError

  • I remove the commented lines

     5assert 'I am' + ' alive' == 'I am alive'
 6
 7assert None is None
 8
 9assert False is not None
10
11assert True is not None
12
13assert 0 is not None
14
15assert 0.0 is not None
16
17assert '' is not None
18
19assert () is not None
20
21assert [] is not None
22
23assert set() is not None
24
25assert {} is not None
26
27
28# NOTES

  • I add comments to group the assertions

     1# test assert keyword
 2assert 1 + 1 == 2
 3
 4assert '1' + '1' == '11'
 5
 6assert 'I am' + ' alive' == 'I am alive'
 7
 8
 9# test AssertionError with None
10assert None is None

  • I add a git commit message in the other terminal

    git commit --all --message \
'add test AssertionError with None'

    the terminal shows a summary of the changes then goes back to the command line.

I can use assertions to test if something is None.

test AssertionError with False

False is one of the two booleans and assert False is not None shows that False is NOT None. is None False?

I can use assertions to test if something is the same object as False or not.

RED: make it fail

  • I go back to the terminal where the tests are running

  • I add a new statement to test_assertion_error.py with a comment to group these new statements

    28assert {} is not None
29
30
31# test AssertionError with False
32None is False
33
34
35# NOTES

    the test is still passing.

  • I add the assert keyword

    31# test AssertionError with False
32# None is False
33assert None is False
34
35
36# NOTES

    the terminal is my friend, and shows

    E       assert None is False

    because None is not the same object as False.

GREEN: make it pass

I change the assertion to make it True

31  # test AssertionError with False
32  # None is False
33  # assert None is False
34  assert None is not False
35
36
37  # NOTES

the test passes because None is not the same object as False.

REFACTOR: make it better

  • I add a note about None

    37# NOTES
38# a dictionary is not None
39# a set is not None
40# a list is not None
41# a tuple is not None
42# a string is not None
43# a float is not None
44# an integer is not None
45# True is not None
46# False is not None
47# None is not False
48# None is None

  • I add a failing assert statement about False

    31# test AssertionError with False
32# None is False
33# assert None is False
34assert None is not False
35
36assert False is not False
37
38
39# NOTES

    the terminal shows

    E       assert False is not False

    because False is False.

  • I change the statement to make it True

    31# test AssertionError with False
32# None is False
33# assert None is False
34assert None is not False
35
36# assert False is not False
37assert False is False
38
39
40# NOTES

    the test passes because False is False.

  • I add a note about False

    40# NOTES
41# a dictionary is not None
42# a set is not None
43# a list is not None
44# a tuple is not None
45# a string is not None
46# a float is not None
47# an integer is not None
48# True is not None
49# False is False
50# False is not None
51# None is not False
52# None is None

  • I add an assert statement to see if True is the same object as False

    36# assert False is not False
37assert False is False
38
39assert True is False
40
41
42# NOTES

    the terminal

    E       assert True is False

    because True is not the same object as False.

  • I change the statement to make it True

    36# assert False is not False
37assert False is False
38
39# assert True is False
40assert True is not False
41
42
43# NOTES

    the test passes because True is not the same object as False.

  • I add a note about True

    43# NOTES
44# a dictionary is not None
45# a set is not None
46# a list is not None
47# a tuple is not None
48# a string is not None
49# a float is not None
50# an integer is not None
51# True is not False
52# True is not None
53# False is False
54# False is not None
55# None is not False
56# None is None

  • I add an assert statement to see if an integer (a whole number without decimals) is the same object as False

    39# assert True is False
40assert True is not False
41
42assert 0 is False
43
44
45# NOTES

    the terminal shows

    E       assert 0 is False

    because an integer is not the same object as False.

  • I change the statement to make it True

    39# assert True is False
40assert True is not False
41
42# assert 0 is False
43assert 0 is not False
44
45
46# NOTES

    the test passes because an integer is not the same object as False.

  • I add a note about integers

    46# NOTES
47# a dictionary is not None
48# a set is not None
49# a list is not None
50# a tuple is not None
51# a string is not None
52# a float is not None
53# an integer is not False
54# an integer is not None
55# True is not False
56# True is not None
57# False is False
58# False is not None
59# None is not False
60# None is None

  • I add an assert statement to see if a float (binary floating point decimal number) is the same object as False

    42# assert 0 is False
43assert 0 is not False
44
45assert 0.0 is False
46
47
48# NOTES

    the terminal shows

    E       assert 0.0 is False

    because a float is not the same object as False.

  • I change the statement to make it True

    42# assert 0 is False
43assert 0 is not False
44
45# assert 0.0 is False
46assert 0.0 is not False
47
48
49# NOTES

    the test passes because a float is not the same object as False.

  • I add a note about floats

    49# NOTES
50# a dictionary is not None
51# a set is not None
52# a list is not None
53# a tuple is not None
54# a string is not None
55# a float is not False
56# a float is not None
57# an integer is not False
58# an integer is not None
59# True is not False
60# True is not None
61# False is False
62# False is not None
63# None is not False
64# None is None

  • I add an assert statement to see if a string (anything in quotes) is the same object as False

    45# assert 0.0 is False
46assert 0.0 is not False
47
48assert '' is False
49
50
51# NOTES

    the terminal shows AssertionError

    E       AssertionError: assert '' is False

    because a string is not the same object as False.

  • I change the statement to make it True

    45# assert 0.0 is False
46assert 0.0 is not False
47
48# assert '' is False
49assert '' is not False
50
51
52# NOTES

    the test passes because a string is not the same object as False.

  • I add a note about strings

    52# NOTES
53# a dictionary is not None
54# a set is not None
55# a list is not None
56# a tuple is not None
57# a string is not False
58# a string is not None
59# a float is not False
60# a float is not None
61# an integer is not False
62# an integer is not None
63# True is not False
64# True is not None
65# False is False
66# False is not None
67# None is not False
68# None is None

  • I add an assert statement to see if a tuple (anything in parentheses ( ) separated by a comma) is the same object as False

    48# assert '' is False
49assert '' is not False
50
51assert () is False
52
53
54# NOTES

    the terminal shows

    E   assert () is False

    because a tuple is not the same object as False.

  • I change the statement to make it True

    48# assert '' is False
49assert '' is not False
50
51# assert () is False
52assert () is not False
53
54
55# NOTES

    the test passes because a tuple is not the same object as False.

  • I add a note about tuples

    55# NOTES
56# a dictionary is not None
57# a set is not None
58# a list is not None
59# a tuple is not False
60# a tuple is not None
61# a string is not False
62# a string is not None
63# a float is not False
64# a float is not None
65# an integer is not False
66# an integer is not None
67# True is not False
68# True is not None
69# False is False
70# False is not None
71# None is not False
72# None is None

  • I add an assert statement to see if a list (anything in square brackets [ ]) is the same object as False

    51# assert () is False
52assert () is not False
53
54assert [] is False
55
56
57# NOTES

    the terminal shows

    E   assert [] is False

    because a list is not the same object as False.

  • I change the statement to make it True

    51# assert () is False
52assert () is not False
53
54# assert [] is False
55assert [] is not False
56
57
58# NOTES

    the test passes because a list is not the same object as False.

  • I add a note about lists

    58# NOTES
59# a dictionary is not None
60# a set is not None
61# a list is not False
62# a list is not None
63# a tuple is not False
64# a tuple is not None
65# a string is not False
66# a string is not None
67# a float is not False
68# a float is not None
69# an integer is not False
70# an integer is not None
71# True is not False
72# True is not None
73# False is False
74# False is not None
75# None is not False
76# None is None

  • I add an assert statement to see if a set is the same object as False

    54# assert [] is False
55assert [] is not False
56
57assert set() is False
58
59
60# NOTES

    the terminal shows

    E   assert set() is False

    because a set is not the same object as False.

  • I change the statement to make it True

    54# assert [] is False
55assert [] is not False
56
57# assert set() is False
58assert set() is not False
59
60
61# NOTES

    the test passes because a set is not the same object as False.

  • I add a note about sets

    61# NOTES
62# a dictionary is not None
63# a set is not False
64# a set is not None
65# a list is not False
66# a list is not None
67# a tuple is not False
68# a tuple is not None
69# a string is not False
70# a string is not None
71# a float is not False
72# a float is not None
73# an integer is not False
74# an integer is not None
75# True is not False
76# True is not None
77# False is False
78# False is not None
79# None is not False
80# None is None

  • I add an assert statement to see if a dictionary is the same object as False

    57# assert set() is False
58assert set() is not False
59
60assert {} is False
61
62
63# NOTES

    the terminal shows

    E   assert {} is False

    because a dictionary is not the same object as False.

  • I change the statement to make it True

    51# assert set() is False
52assert set() is not False
53
54# assert {} is False
55assert {} is not False
56
57
58# NOTES

    the test passes because a dictionary is not the same object as False.

  • I add a note about dictionaries

    64# NOTES
65# a dictionary is not False
66# a dictionary is not None
67# a set is not False
68# a set is not None
69# a list is not False
70# a list is not None
71# a tuple is not False
72# a tuple is not None
73# a string is not False
74# a string is not None
75# a float is not False
76# a float is not None
77# an integer is not False
78# an integer is not None
79# True is not False
80# True is not None
81# False is False
82# False is not None
83# None is not False
84# None is None
85
86
87# Exceptions seen
88# AssertionError

  • I remove the commented assertions

    31# test AssertionError with False
32assert None is not False
33
34assert False is False
35
36assert True is not False
37
38assert 0 is not False
39
40assert 0.0 is not False
41
42assert '' is not False
43
44assert () is not False
45
46assert [] is not False
47
48assert set() is not False
49
50assert {} is not False
51
52
53# NOTES

  • I add a git commit message in the other terminal

    git commit --all --message \
'add test AssertionError with False'

    the terminal shows a summary of the changes then goes back to the command line.

test AssertionError with True

True is the other boolean and is NOT None. I can use assertions to test if something is the same object as True or not.

RED: make it fail

  • I go back to the terminal where the tests are running

  • I add a new statement to test_assertion_error.py with a comment to group these new statements

    50assert {} is not False
51
52
53# test AssertionError with True
54None is True
55
56
57# NOTES

    the test is still passing.

  • I add the assert keyword

    53# test AssertionError with True
54# None is True
55assert None is True
56
57
58# NOTES

    the terminal is my friend, and shows

    E       assert None is True

    because None is not the same object as True.

GREEN: make it pass

I change the statement to make it True

53# test AssertionError with True
54# None is True
55# assert None is True
56assert None is not True
57
58
59# NOTES

the test passes because None is not the same object as True.

REFACTOR: make it better

  • I add a note about None

    59# NOTES
60# a dictionary is not False
61# a dictionary is not None
62# a set is not False
63# a set is not None
64# a list is not False
65# a list is not None
66# a tuple is not False
67# a tuple is not None
68# a string is not False
69# a string is not None
70# a float is not False
71# a float is not None
72# an integer is not False
73# an integer is not None
74# True is not False
75# True is not None
76# False is False
77# False is not None
78# None is not True
79# None is not False
80# None is None

  • I add an assert statement to see if False is the same object as True

    53# test AssertionError with True
54# None is True
55# assert None is True
56assert None is not True
57
58assert False is True
59
60
61# NOTES

    the terminal shows

    E       assert False is True

    because False is not the same object as True.

  • I change the statement to make it True

    53# test AssertionError with True
54# None is True
55# assert None is True
56assert None is not True
57
58# assert False is True
59assert False is not True
60
61
62# NOTES

    the test passes because False is not the same object as True.

  • I add a note about False

    62# NOTES
63# a dictionary is not False
64# a dictionary is not None
65# a set is not False
66# a set is not None
67# a list is not False
68# a list is not None
69# a tuple is not False
70# a tuple is not None
71# a string is not False
72# a string is not None
73# a float is not False
74# a float is not None
75# an integer is not False
76# an integer is not None
77# True is not False
78# True is not None
79# False is not True
80# False is False
81# False is not None
82# None is not True
83# None is not False
84# None is None

  • I add an assert statement about True, that will fail

    58# assert False is True
59assert False is not True
60
61assert True is not True
62
63
64# NOTES

    the terminal shows

    E       assert True is not True

    because True is True.

    58# assert False is True
59assert False is not True
60
61# assert True is not True
62assert True is True
63
64
65# NOTES

    the test passes because True is True.

  • I add a note about True

    65# NOTES
66# a dictionary is not False
67# a dictionary is not None
68# a set is not False
69# a set is not None
70# a list is not False
71# a list is not None
72# a tuple is not False
73# a tuple is not None
74# a string is not False
75# a string is not None
76# a float is not False
77# a float is not None
78# an integer is not False
79# an integer is not None
80# True is True
81# True is not False
82# True is not None
83# False is not True
84# False is False
85# False is not None
86# None is not True
87# None is not False
88# None is None

  • I add an assert statement to see if an integer (a whole number without decimals) is the same object as True

    61# assert True is not True
62assert True is True
63
64assert 0 is True
65
66
67# NOTES

    the terminal shows

    E       assert 0 is True

    because an integer is not the same object as True.

  • I change the statement to make it True

    61# assert True is not True
62assert True is True
63
64# assert 0 is True
65assert 0 is not True
66
67
68# NOTES

    the test passes because an integer is not the same object as True.

  • I add a note about integers

    68# NOTES
69# a dictionary is not False
70# a dictionary is not None
71# a set is not False
72# a set is not None
73# a list is not False
74# a list is not None
75# a tuple is not False
76# a tuple is not None
77# a string is not False
78# a string is not None
79# a float is not False
80# a float is not None
81# an integer is not True
82# an integer is not False
83# an integer is not None
84# True is True
85# True is not False
86# True is not None
87# False is not True
88# False is False
89# False is not None
90# None is not True
91# None is not False
92# None is None

  • I add an assert statement to see if a float (binary floating point decimal number) is the same object as True

    64# assert 0 is True
65assert 0 is not True
66
67assert 0.0 is True
68
69
70# NOTES

    the terminal shows

    E       assert 0.0 is True

    because a float is not the same object as True.

  • I change the statement to make it True

    64# assert 0 is True
65assert 0 is not True
66
67# assert 0.0 is True
68assert 0.0 is not True
69
70
71# NOTES

    the test passes because a float is not the same object as True.

  • I add a note about floats

    71# NOTES
72# a dictionary is not False
73# a dictionary is not None
74# a set is not False
75# a set is not None
76# a list is not False
77# a list is not None
78# a tuple is not False
79# a tuple is not None
80# a string is not False
81# a string is not None
82# a float is not True
83# a float is not False
84# a float is not None
85# an integer is not True
86# an integer is not False
87# an integer is not None
88# True is True
89# True is not False
90# True is not None
91# False is not True
92# False is False
93# False is not None
94# None is not True
95# None is not False
96# None is None

  • I add an assert statement to see if a string (anything in quotes) is the same object as True

    67# assert 0.0 is True
68assert 0.0 is not True
69
70assert '' is True
71
72
73# NOTES

    the terminal shows AssertionError

    E       AssertionError: assert '' is True

    because a string is not the same object as True.

  • I change the statement to make it True

    67# assert 0.0 is True
68assert 0.0 is not True
69
70# assert '' is True
71assert '' is not True
72
73
74# NOTES

    the test passes because a string is not the same object as True.

  • I add a note about strings

     74# NOTES
 75# a dictionary is not False
 76# a dictionary is not None
 77# a set is not False
 78# a set is not None
 79# a list is not False
 80# a list is not None
 81# a tuple is not False
 82# a tuple is not None
 83# a string is not True
 84# a string is not False
 85# a string is not None
 86# a float is not True
 87# a float is not False
 88# a float is not None
 89# an integer is not True
 90# an integer is not False
 91# an integer is not None
 92# True is True
 93# True is not False
 94# True is not None
 95# False is not True
 96# False is False
 97# False is not None
 98# None is not True
 99# None is not False
100# None is None

  • I add an assert statement to see if a tuple (anything in parentheses ( ) separated by a comma) is the same object as True

    the terminal shows

    E   assert () is True

  • I change the statement to make it True

    70# assert '' is True
71assert '' is not True
72
73# assert () is True
74assert () is not True
75
76
77# NOTES

    the test passes.

  • I add a note about tuples

     77# NOTES
 78# a dictionary is not False
 79# a dictionary is not None
 80# a set is not False
 81# a set is not None
 82# a list is not False
 83# a list is not None
 84# a tuple is not True
 85# a tuple is not False
 86# a tuple is not None
 87# a string is not True
 88# a string is not False
 89# a string is not None
 90# a float is not True
 91# a float is not False
 92# a float is not None
 93# an integer is not True
 94# an integer is not False
 95# an integer is not None
 96# True is True
 97# True is not False
 98# True is not None
 99# False is not True
100# False is False
101# False is not None
102# None is not True
103# None is not False
104# None is None

  • I add an assert statement to see if a list (anything in square brackets [ ]) is the same object as True

    73# assert () is True
74assert () is not True
75
76assert [] is True
77
78
79# NOTES

    the terminal shows

    E   assert [] is True

    because a list is not the same object as True.

  • I change the statement to make it True

    73# assert () is True
74assert () is not True
75
76# assert [] is True
77assert [] is not True
78
79
80# NOTES

    the test passes because a list is not the same object as True.

  • I add a note about lists

     80# NOTES
 81# a dictionary is not False
 82# a dictionary is not None
 83# a set is not False
 84# a set is not None
 85# a list is not True
 86# a list is not False
 87# a list is not None
 88# a tuple is not True
 89# a tuple is not False
 90# a tuple is not None
 91# a string is not True
 92# a string is not False
 93# a string is not None
 94# a float is not True
 95# a float is not False
 96# a float is not None
 97# an integer is not True
 98# an integer is not False
 99# an integer is not None
100# True is True
101# True is not False
102# True is not None
103# False is not True
104# False is False
105# False is not None
106# None is not True
107# None is not False
108# None is None

  • I add an assert statement to see if a set is the same object as True

    76# assert [] is True
77assert [] is not True
78
79assert set() is True
80
81
82# NOTES

    the terminal shows

    E   assert set() is True

    because a set is not the same object as True.

  • I change the statement to make it True

    76# assert [] is True
77assert [] is not True
78
79# assert set() is True
80assert set() is not True
81
82
83# NOTES

    the test passes because a set is not the same object as True.

  • I add a note about sets

     83# NOTES
 84# a dictionary is not False
 85# a dictionary is not None
 86# a set is not True
 87# a set is not False
 88# a set is not None
 89# a list is not True
 90# a list is not False
 91# a list is not None
 92# a tuple is not True
 93# a tuple is not False
 94# a tuple is not None
 95# a string is not True
 96# a string is not False
 97# a string is not None
 98# a float is not True
 99# a float is not False
100# a float is not None
101# an integer is not True
102# an integer is not False
103# an integer is not None
104# True is True
105# True is not False
106# True is not None
107# False is not True
108# False is False
109# False is not None
110# None is not True
111# None is not False
112# None is None

  • I add an assert statement to see if a dictionary is the same object as True

    79# assert set() is True
80assert set() is not True
81
82assert {} is True
83
84
85# NOTES

    the terminal shows

    E   assert {} is True

    because a dictionary is not the same object as True.

  • I change the statement to make it True

    79# assert set() is True
80assert set() is not True
81
82# assert {} is True
83assert {} is not True
84
85
86# NOTES

    the test passes because a dictionary is not the same object as True.

  • I add a note about dictionaries

     86# NOTES
 87# a dictionary is not True
 88# a dictionary is not False
 89# a dictionary is not None
 90# a set is not True
 91# a set is not False
 92# a set is not None
 93# a list is not True
 94# a list is not False
 95# a list is not None
 96# a tuple is not True
 97# a tuple is not False
 98# a tuple is not None
 99# a string is not True
100# a string is not False
101# a string is not None
102# a float is not True
103# a float is not False
104# a float is not None
105# an integer is not True
106# an integer is not False
107# an integer is not None
108# True is True
109# True is not False
110# True is not None
111# False is not True
112# False is False
113# False is not None
114# None is not True
115# None is not False
116# None is None
117
118
119# Exceptions seen
120# AssertionError

  • I remove the commented assertions

    53# test AssertionError with True
54assert None is not True
55
56assert False is not True
57
58assert True is True
59
60assert 0 is not True
61
62assert 0.0 is not True
63
64assert '' is not True
65
66assert () is not True
67
68assert [] is not True
69
70assert set() is not True
71
72assert {} is not True
73
74
75# NOTES

  • I add a git commit message in the other terminal

    git commit --all --message \
'test AssertionError with True'

    the terminal shows a summary of the changes then goes back to the command line.

I can use assertions to test if something is the same object as True or NOT.

test AssertionError with equality

All the assertions I have typed so far show that True, False and None are different. They give me a basic expectation of Python because I can compare things with them.

I can also use assertions to test if two things are equal, like I did when I test the assert keyword.

RED: make it fail

  • I go back to the terminal where the tests are running

  • I add a new statement to test_assertion_error.py with a comment to group these new statements

    72assert {} is not True
73
74
75# test AssertionError with equality
76None != None
77
78
79# NOTES

    the test is still passing.

  • I add the assert keyword

    75# test AssertionError with equality
76# None != None
77assert None != None
78
79
80# NOTES

    the terminal is my friend, and shows

    E   assert None != None

    • because None is equal to None.

    • == is two equal signs on the keyboard (=+=) and means is equal which makes this statement read as None is equal to None

GREEN: make it pass

I change the statement to make it True

75# test AssertionError with equality
76# None != None
77# assert None != None
78assert None == None
79
80
81# NOTES

  • the test passes because None is equal to None

  • == is two equal signs on the keyboard (=+=) and means is equal which makes this statement read as 1 + 1 is equal to 2

REFACTOR: make it better

  • I add to the None is None note

     81# NOTES
 82# a dictionary is not True
 83# a dictionary is not False
 84# a dictionary is not None
 85# a set is not True
 86# a set is not False
 87# a set is not None
 88# a list is not True
 89# a list is not False
 90# a list is not None
 91# a tuple is not True
 92# a tuple is not False
 93# a tuple is not None
 94# a string is not True
 95# a string is not False
 96# a string is not None
 97# a float is not True
 98# a float is not False
 99# a float is not None
100# an integer is not True
101# an integer is not False
102# an integer is not None
103# True is True
104# True is not False
105# True is not None
106# False is not True
107# False is False
108# False is not None
109# None is not True
110# None is not False
111# None is None and equal to None

  • I add an assertion to compare False with None

    75# test AssertionError with equality
76# None != None
77# assert None != None
78assert None == None
79
80assert False == None
81
82
83# NOTES

    the terminal is my friend, and shows

    E    assert False == None

    because False is NOT equal to None and None is NOT equal to False.

  • I change the assert statement to make it True

    75# test AssertionError with equality
76# None != None
77# assert None != None
78assert None == None
79
80# assert False == None
81assert False != None
82
83
84# NOTES

    the test passes because False is NOT equal to None and None is NOT equal to False.

  • I add to the False is not None and None is not False notes because equality goes both ways

     84# NOTES
 85# a dictionary is not True
 86# a dictionary is not False
 87# a dictionary is not None
 88# a set is not True
 89# a set is not False
 90# a set is not None
 91# a list is not True
 92# a list is not False
 93# a list is not None
 94# a tuple is not True
 95# a tuple is not False
 96# a tuple is not None
 97# a string is not True
 98# a string is not False
 99# a string is not None
100# a float is not True
101# a float is not False
102# a float is not None
103# an integer is not True
104# an integer is not False
105# an integer is not None
106# True is True
107# True is not False
108# True is not None
109# False is not True
110# False is False
111# False is not None and NOT equal to None
112# None is not True
113# None is not False and NOT equal to False
114# None is None and equal to None

  • I add an assertion to compare False with True

    80# assert False == None
81assert False != None
82
83assert False == True
84
85
86# NOTES

    the terminal is my friend, and shows

    E       assert False == True

    because False is NOT equal to True and True is NOT equal to False.

  • I change the assert statement to make it True

    80# assert False == None
81assert False != None
82
83# assert False == True
84assert False != True
85
86
87# NOTES

    the test passes because False is NOT equal to True and True is NOT equal to False.

  • I add to the False is not True and True is not False notes because equality goes both ways

     87# NOTES
 88# a dictionary is not True
 89# a dictionary is not False
 90# a dictionary is not None
 91# a set is not True
 92# a set is not False
 93# a set is not None
 94# a list is not True
 95# a list is not False
 96# a list is not None
 97# a tuple is not True
 98# a tuple is not False
 99# a tuple is not None
100# a string is not True
101# a string is not False
102# a string is not None
103# a float is not True
104# a float is not False
105# a float is not None
106# an integer is not True
107# an integer is not False
108# an integer is not None
109# True is True
110# True is not False and NOT equal to False
111# True is not None
112# False is not True and NOT equal to True
113# False is False
114# False is not None and NOT equal to None
115# None is not True
116# None is not False and NOT equal to False
117# None is None and equal to None

  • I add an assertion to compare False with itself

    83# assert False == True
84assert False != True
85
86assert False != False
87
88
89# NOTES

    the terminal is my friend, and shows

    E    assert False != False

    because False is equal to False.

  • I make the statement True

    83# assert False == True
84assert False != True
85
86# assert False != False
87assert False == False
88
89
90# NOTES

    the test passes because False is equal to False.

  • I add to the False is False note

     90# NOTES
 91# a dictionary is not True
 92# a dictionary is not False
 93# a dictionary is not None
 94# a set is not True
 95# a set is not False
 96# a set is not None
 97# a list is not True
 98# a list is not False
 99# a list is not None
100# a tuple is not True
101# a tuple is not False
102# a tuple is not None
103# a string is not True
104# a string is not False
105# a string is not None
106# a float is not True
107# a float is not False
108# a float is not None
109# an integer is not True
110# an integer is not False
111# an integer is not None
112# True is True
113# True is not False and NOT equal to False
114# True is not None
115# False is not True and NOT equal to True
116# False is False and equal to False
117# False is not None and NOT equal to None
118# None is not True
119# None is not False and NOT equal to False
120# None is None and equal to None

  • I add a new failing assertion to compare True with None

    86# assert False != False
87assert False == False
88
89assert True == None
90
91
92# NOTES

    the terminal is my friend, and shows

    E    assert True == None

    because True is NOT equal None and None is NOT equal to True.

  • I change the assert statement to make it True

    86# assert False != False
87assert False == False
88
89# assert True == None
90assert True != None
91
92
93# NOTES

    the test passes because True is NOT equal None and None is NOT equal to True.

  • I add to the True is not None and None is not True notes because equality goes both ways

     93# NOTES
 94# a dictionary is not True
 95# a dictionary is not False
 96# a dictionary is not None
 97# a set is not True
 98# a set is not False
 99# a set is not None
100# a list is not True
101# a list is not False
102# a list is not None
103# a tuple is not True
104# a tuple is not False
105# a tuple is not None
106# a string is not True
107# a string is not False
108# a string is not None
109# a float is not True
110# a float is not False
111# a float is not None
112# an integer is not True
113# an integer is not False
114# an integer is not None
115# True is True
116# True is not False and NOT equal to False
117# True is not None and NOT equal to None
118# False is not True and NOT equal to True
119# False is False and equal to False
120# False is not None and NOT equal to None
121# None is not True and NOT equal to True
122# None is not False and NOT equal to False
123# None is None and equal to None

  • I add an assertion to compare True with itself

    89# assert True == None
90assert True != None
91
92assert True != True
93
94
95# NOTES

    the terminal is my friend, and shows

    E    assert True != True

    because True is equal to True.

  • I make the statement True

    89# assert True == None
90assert True != None
91
92# assert True != True
93assert True == True
94
95
96# NOTES

    the test passes because True is equal to True.

  • I add to the True is True note

     96# NOTES
 97# a dictionary is not True
 98# a dictionary is not False
 99# a dictionary is not None
100# a set is not True
101# a set is not False
102# a set is not None
103# a list is not True
104# a list is not False
105# a list is not None
106# a tuple is not True
107# a tuple is not False
108# a tuple is not None
109# a string is not True
110# a string is not False
111# a string is not None
112# a float is not True
113# a float is not False
114# a float is not None
115# an integer is not True
116# an integer is not False
117# an integer is not None
118# True is True and equal to True
119# True is not False and NOT equal to False
120# True is not None and NOT equal to None
121# False is not True and NOT equal to True
122# False is False and equal to False
123# False is not None and NOT equal to None
124# None is not True and NOT equal to True
125# None is not False and NOT equal to False
126# None is None and equal to None
127
128
129# Exceptions seen
130# AssertionError

  • I remove the commented assertions

    75# test AssertionError with equality
76assert None == None
77
78assert False != None
79
80assert False != True
81
82assert False == False
83
84assert True != None
85
86assert True == True
87
88
89# NOTES

  • I add a git commit message in the other terminal

    git commit --all --message \
'add test AssertionError with equality'

    the terminal shows a summary of the changes then goes back to the command line.

I can use assertions to test if two things are equal.

test AssertionError with is vs equal

Some of the tests I have written use is and some use ==, if they mean the same thing, why do I use different symbols? What is the difference between is and ==?

In Python they do not mean the same thing

  • x is y states that x is the same exact object as y

  • x == y states that x is equal to y according to a rule or instruction programmed in Python

  • x is not y states that x is NOT the same exact object as y

  • x != y states that x is NOT equal to y according to a rule or instruction programmed in Python

This means that things can be equal without being the same exact object. For example, an integer (a whole number without decimals) can be equal to a float (binary floating point decimal numbers) and they are NOT the same object.

RED: make it fail

  • I add statements to show this

    86assert True == True
87
88
89# test AssertionError with is vs equal
900 is 0.0
91
92
93# NOTES

    the test is still passing.

  • I add the assert keyword

    89# test AssertionError with is vs equal
90# 0 is 0.0
91assert 0 is 0.0
92
93
94# NOTES

    the terminal is my friend, and shows

    E   assert 0 is 0.0

    because an integer is not the same object as a float.

GREEN: make it pass

I change the assertion to make the statement True

89  # test AssertionError with is vs equal
90  # 0 is 0.0
91  # assert 0 is 0.0
92  assert 0 is not 0.0
93
94
95  # NOTES

the test passes because an integer is not the same object as a float.

REFACTOR: make it better

  • I add another assertion, this time with !=

    89# test AssertionError with is vs equal
90# 0 is 0.0
91# assert 0 is 0.0
92assert 0 is not 0.0
93
94assert 0 != 0.0
95
96
97# NOTES

    the terminal is my friend, and shows

    E       assert 0 != 0.0

    because 0 and 0.0 are equal, they are both zero.

  • I change the assertion to make the statement True

    89# test AssertionError with is vs equal
90# 0 is 0.0
91# assert 0 is 0.0
92assert 0 is not 0.0
93
94# assert 0 != 0.0
95assert 0 == 0.0
96
97
98# NOTES

    the test passes because 0 and 0.0 are equal, they are both zero.

  • I remove the commented assertions

    89# test AssertionError with is vs equal
90assert 0 is not 0.0
91
92assert 0 == 0.0
93
94
95# NOTES

  • I add a git commit message in the other terminal

    git commit --all --message \
'test AssertionError with is vs equal'

    the terminal shows a summary of the changes then goes back to the command line.

The tests show that an integer can be EQUAL to a float but an integer IS NOT a float.

a better way to organize tests

I used comments to group the assertions by what I was testing, and each time I made an assertion True, the terminal showed a confusing message (no tests ran).

Since I am using pytest-watcher which uses pytest to run the test, I can group the assertions with functions instead.

A function is code that is callable, this means I can write code to do something one time, and call the name for it to do that thing at a different time from when I write it.

functions are made with

  • the def keyword

  • a name

  • parentheses and a colon at the end

  • the code that makes up the function (its body) comes after the colon

def name_of_function():
    the body of the function
    ...

test_assertion_error_w_is_vs_equal

  • I add a function for the # test AssertionError with is vs equal group of assertions

    86assert True == True
87
88
89def assertion_error_w_is_vs_equal():
90# test AssertionError with is vs equal
91assert 0 is not 0.0
92
93assert 0 == 0.0

    the terminal is my friend, and shows IndentationError

    IndentationError: expected an indented block
                  after function definition on line 89

    because in Python the body of a function is indented under the name.

  • I indent the assertions with four spaces to make them the body of the function

    86assert True == True
87
88
89def assertion_error_w_is_vs_equal():
90    # test AssertionError with is vs equal
91    assert 0 is not 0.0
92
93    assert 0 == 0.0
94
95
96# NOTES

    the test passes and the terminal still shows no tests ran

  • I add test_ to the name of the function

    86assert True == True
87
88
89# def assertion_error_w_is_vs_equal():
90def test_assertion_error_w_is_vs_equal():
91    # test AssertionError with is vs equal
92    assert 0 is not 0.0
93
94    assert 0 == 0.0
95
96
97# NOTES

    the terminal shows green and the name of the test file with a better message

    ================== test session starts ===================
platform linux -- Python 3.X.Y, pytest-Z.A.B, pluggy-C.D.E
rootdir: /.../pumping_python/assertion_error
configfile: pyproject.toml
collected 1 item

tests/test_assertion_error.py .                     [100%]

=================== 1 passed in F.GHs ====================
[pytest-watcher]
Current runner args: []
Press w to show menu

    fantastic! pytest only calls the function if the name starts with test.

  • I remove the commented lines

    86assert True == True
87
88
89def test_assertion_error_w_is_vs_equal():
90    assert 0 is not 0.0
91
92    assert 0 == 0.0
93
94
95# NOTES

  • I add a git commit message

    git commit --all --message \
'add test_assertion_error_w_is_vs_equal'

    the terminal shows a summary of the changes then goes back to the command line.

test_assertion_error_w_equality

  • I add a function for the # test AssertionError with equality group of assertions

    72assert {} is not True
73
74
75def assertion_error_w_equality():
76# test AssertionError with equality
77assert None == None

    the terminal is my friend, and shows IndentationError

    IndentationError: expected an indented block
                  after function definition on line 75

    because in Python the body of a function is indented under the name.

  • I indent the assertions with four spaces to make them the body of the function

    75assert {} is not True
76
77
78def assertion_error_w_equality():
79    # test AssertionError with equality
80    assert None == None
81
82    assert False != None
83
84    assert False != True
85
86    assert False == False
87
88    assert True != None
89
90    assert True == True
91
92
93def test_assertion_error_w_is_vs_equal():

    the terminal shows

    =================== 1 passed in L.MNs ====================

    I have two functions with assertions and it only recognizes one.

  • I add test_ to the name of the function

    72assert {} is not True
73
74
75# def assertion_error_w_equality():
76def test_assertion_error_w_equality():
77    # test AssertionError with equality
78    assert None == None
79
80    assert False != None

    the terminal shows

    =================== 2 passed in O.PQs ====================

    because pytest only calls the function if the name starts with test

  • I remove the commented lines

    72assert {} is not True
73
74
75def test_assertion_error_w_equality():
76    assert None == None
77
78    assert False != None

  • I add a git commit message

    git commit --all --message \
'add test_assertion_error_w_equality'

    the terminal shows a summary of the changes then goes back to the command line.

test_assertion_error_w_true

  • I add a function for the # test AssertionError with True group of assertions

    50assert {} is not False
51
52
53def assertion_error_w_true():
54# test AssertionError with True
55assert None is not True

    the terminal is my friend, and shows IndentationError

    IndentationError: expected an indented block
                  after function definition on line 53

    because in Python the body of a function is indented under the name.

  • I indent the assertions with four spaces to make them the body of the function

    50assert {} is not False
51
52
53def assertion_error_w_true():
54    # test AssertionError with True
55    assert None is not True
56
57    assert False is not True
58
59    assert True is True
60
61    assert 0 is not True
62
63    assert 0.0 is not True
64
65    assert '' is not True
66
67    assert () is not True
68
69    assert [] is not True
70
71    assert set() is not True
72
73    assert {} is not True
74
75
76def test_assertion_error_w_equality():

    the terminal shows

    =================== 2 passed in R.STs ====================

    I have three functions with assertions and it only recognizes two.

  • I add test_ to the name of the function

    50assert {} is not False
51
52
53# def assertion_error_w_true():
54def test_assertion_error_w_true():
55    # test AssertionError with True
56    assert None is not True
57
58    assert False is not True

    the terminal shows

    =================== 3 passed in U.VWs ====================

    because pytest only calls the function if the name starts with test

  • I remove the commented lines

    50assert {} is not False
51
52
53def test_assertion_error_w_true():
54    assert None is not True
55
56    assert False is not True

  • I add a git commit message

    git commit --all --message \
'add test_assertion_error_w_true'

    the terminal shows a summary of the changes then goes back to the command line.

test_assertion_error_w_false

  • I add a function for the # test AssertionError with False group of assertions

    28assert {} is not None
29
30
31def assertion_error_w_false():
32# test AssertionError with False
33assert None is not False

    the terminal is my friend, and shows IndentationError

    IndentationError: expected an indented block
                  after function definition on line 31

    because in Python the body of a function is indented under the name.

  • I indent the assertions with four spaces to make them the body of the function

    28assert {} is not None
29
30
31def assertion_error_w_false():
32    # test AssertionError with False
33    assert None is not False
34
35    assert False is False
36
37    assert True is not False
38
39    assert 0 is not False
40
41    assert 0.0 is not False
42
43    assert '' is not False
44
45    assert () is not False
46
47    assert [] is not False
48
49    assert set() is not False
50
51    assert {} is not False
52
53
54def test_assertion_error_w_true():

    the terminal shows

    =================== 3 passed in X.YZs ====================

    I have four functions with assertions and it only recognizes three.

  • I add test_ to the name of the function

    28assert {} is not None
29
30
31# def assertion_error_w_false():
32def test_assertion_error_w_false():
33    # test AssertionError with False
34    assert None is not False
35
36    assert False is False

    the terminal shows

    =================== 4 passed in A.BCs ====================

    because pytest only calls the function if the name starts with test

  • I remove the commented lines

    28assert {} is not None
29
30
31def test_assertion_error_w_false():
32    assert None is not False
33
34    assert False is False

  • I add a git commit message

    git commit --all --message \
'add test_assertion_error_w_false'

    the terminal shows a summary of the changes then goes back to the command line.

test_assertion_error_w_none

  • I add a function for the # test AssertionError with None group of assertions

     6assert 'I am' + ' alive' == 'I am alive'
 7
 8
 9def assertion_error_w_none():
10# test AssertionError with None
11assert None is None

    the terminal is my friend, and shows IndentationError

    IndentationError: expected an indented block
                  after function definition on line 9

    because in Python the body of a function is indented under the name.

  • I indent the assertions with four spaces to make them the body of the function

     6assert 'I am' + ' alive' == 'I am alive'
 7
 8
 9def assertion_error_w_none():
10    # test AssertionError with None
11    assert None is None
12
13    assert False is not None
14
15    assert True is not None
16
17    assert 0 is not None
18
19    assert 0.0 is not None
20
21    assert '' is not None
22
23    assert () is not None
24
25    assert [] is not None
26
27    assert set() is not None
28
29    assert {} is not None
30
31
32def test_assertion_error_w_false():

    the terminal shows

    =================== 4 passed in D.EFs ====================

    I have five functions with assertions and it only recognizes four.

  • I add test_ to the name of the function

     6assert 'I am' + ' alive' == 'I am alive'
 7
 8
 9# def assertion_error_w_none():
10def test_assertion_error_w_none():
11    # test AssertionError with None
12    assert None is None
13
14    assert False is not None

    the terminal shows

    =================== 5 passed in G.HIs ====================

    because pytest only calls the function if the name starts with test

  • I remove the commented lines

     6assert 'I am' + ' alive' == 'I am alive'
 7
 8
 9def test_assertion_error_w_none():
10    assert None is None
11
12    assert False is not None

  • I add a git commit message

    git commit --all --message \
'add test_assertion_error_w_none'

    the terminal shows a summary of the changes then goes back to the command line.

test_assert_keyword

  • I add a function for the # test the assert keyword group of assertions

    1def assert_keyword():
2# test the assert keyword
3assert 1 + 1 == 2

    the terminal is my friend, and shows IndentationError

    IndentationError: expected an indented block
                  after function definition on line 1

    because in Python the body of a function is indented under the name.

  • I indent the assertions with four spaces to make them the body of the function

     1def assert_keyword():
 2    # test the assert keyword
 3    assert 1 + 1 == 2
 4
 5    assert '1' + '1' == '11'
 6
 7    assert 'I am' + ' alive' == 'I am alive'
 8
 9
10def test_assertion_error_w_none():

    the terminal shows

    =================== 5 passed in J.KLs ====================

    I have six functions with assertions and it only recognizes four.

  • I add test_ to the name of the function

     1# def assert_keyword():
 2def test_assert_keyword():
 3    # test the assert keyword
 4    assert 1 + 1 == 2
 5
 6    assert '1' + '1' == '11'
 7
 8    assert 'I am' + ' alive' == 'I am alive'
 9
10
11def test_assertion_error_w_none():

    the terminal shows

    =================== 6 passed in M.NOs ====================

    because pytest only calls the function if the name starts with test

  • I remove the commented lines

    1def test_assert_keyword():
2    assert 1 + 1 == 2
3
4    assert '1' + '1' == '11'
5
6    assert 'I am' + ' alive' == 'I am alive'
7
8
9def test_assertion_error_w_none():

  • I add a git commit message

    git commit --all --message \
'add test_assert_keyword'

    the terminal shows a summary of the changes then goes back to the command line.

pytest only calls the function if the name starts with test

  • I add a function to show that pytest only calls the function if the name starts with test

    89def test_assertion_error_w_is_vs_equal():
90    assert 0 is not 0.0
91
92    assert 0 == 0.0
93
94
95def will_not_run():
96    assert False == True
97
98
99# NOTES

    the terminal still shows

    =================== 6 passed in P.QRs ====================

  • I add a function that starts with test with the same assertion

     95def will_not_run():
 96    assert False == True
 97
 98
 99def test_failure():
100    assert False == True
101
102
103# NOTES

    the terminal is my friend, and shows AssertionError

  • I change the assertion to make it True

     99def test_failure():
100    # assert False == True
101    assert False == False
102
103
104# NOTES

    the test passes. I leave the commented line as a reminder of what will cause the failure.

  • I add a git commit message

    git commit --all --message \
'add test_failure'

    the terminal shows a summary of the changes then goes back to the command line.

close the project

  • I close test_assertion_error.py

  • I click in the terminal where the tests are running

  • I use q on the keyboard to leave the tests. The terminal goes back to the command line.

  • I change directory to the parent of assertion_error

    cd ..

    the terminal is my friend, and shows

    .../pumping_python

    I am back in the pumping_python directory.

review

The tests show that

How many questions can you answer about AssertionError?

code from the chapter

Do you want to see all the CODE I typed in this chapter?

what is next?

Would you like to test functions?

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