how to pass values

One of the things that happens when testing a program, is I send things (data) to the program and I check if what I think will happen is the same as the results I get back. Testing helps me answer 2 questions

  • what is the same?

  • what is different?

The difference helps me know what to change to get what I want.

This is the job of the assertEqual method, which takes 2 inputs and checks if they are the same

self.assertEqual(reality, my_expectation)

where

  • reality is what happens when I do something with code

  • my_expectation is what I think will happen when I do something with code

I use the identity function in this chapter to show how something is passed from a test to a function in a module

preview

These are the tests I have by the end of the chapter

 1import src.telephone
 2import unittest
 3
 4
 5class TestTelephone(unittest.TestCase):
 6
 7    def test_passing_a_string(self):
 8        self.assertEqual(
 9            src.telephone.text("hello"),
10            "I got: hello"
11        )
12        self.assertEqual(
13            src.telephone.text("yes"),
14            "I got: yes"
15        )
16
17    def test_passing_a_class(self):
18        self.assertEqual(
19            src.telephone.text(object),
20            "I got: <class 'object'>"
21        )
22
23    def test_passing_none(self):
24        self.assertEqual(
25            src.telephone.text(None),
26            "I got: None"
27        )
28
29    def test_passing_a_boolean(self):
30        self.assertEqual(
31            src.telephone.text(True),
32            "I got: True"
33        )
34        self.assertEqual(
35            src.telephone.text(False),
36            "I got: False"
37        )
38
39    def test_passing_an_integer(self):
40        self.assertEqual(
41            src.telephone.text(1234),
42            "I got: 1234"
43        )
44
45    def test_passing_a_float(self):
46        self.assertEqual(
47            src.telephone.text(1.234),
48            "I got: 1.234"
49        )
50
51    def test_passing_a_tuple(self):
52        self.assertEqual(
53            src.telephone.text((1, 2, 3, "n")),
54            "I got: (1, 2, 3, 'n')"
55        )
56
57    def test_passing_a_list(self):
58        self.assertEqual(
59            src.telephone.text([1, 2, 3, "n"]),
60            "I got: [1, 2, 3, 'n']"
61        )
62
63    def test_passing_a_dictionary(self):
64        self.assertEqual(
65            src.telephone.text({
66                'key1': 'value1',
67                'keyN': [1, 2, 3, 'n'],
68            }),
69            "I got: {'key1': 'value1', 'keyN': [0, 1, 2, 'n']}"
70        )
71
72
73# Exceptions seen
74# AssertionError
75# NameError
76# AttributeError
77# TypeError

start the project

  • I name this project telephone

  • I open a terminal

  • I make a directory for the project

    mkdir telephone

    the terminal goes back to the command line

  • I change directory to the project

    cd telephone

    the terminal shows I am in the telephone folder

    .../pumping_python/telephone

  • I make a directory for the source code

    mkdir src

    the terminal goes back to the command line

  • I make a Python file to hold the source code in the src directory

    touch src/telephone.py

    Note

    on Windows without Windows Subsystem for Linux use New-Item src/telephone.py instead of touch src/telephone.py

    New-Item src/telephone.py

    the terminal goes back to the command line

  • 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

    Note

    on Windows without Windows Subsystem for Linux use New-Item tests/__init__.py instead of touch tests/__init__.py

    New-Item tests/__init__.py

    the terminal goes back to the command line

  • I make a Python file for the tests in the tests directory

    touch tests/test_telephone.py

    Note

    on Windows without Windows Subsystem for Linux use New-Item tests/test_telephone.py instead of touch tests/test_telephone.py

    New-Item tests/test_telephone.py

    the terminal goes back to the command line

  • I open test_telephone.py in the editor of the Integrated Development Environment (IDE)

    Tip

    I can use the terminal to open a file in the Integrated Development Environment (IDE) by typing the name of the program and the name of the file. That means when I type this in the terminal

    code tests/test_telephone.py

    Visual Studio Code opens test_telephone.py in the editor

  • I add the first failing test to test_telephone.py

    1import unittest
2
3
4class TestTelephone(unittest.TestCase):
5
6    def test_failure(self):
7        self.assertFalse(True)

  • I make a requirements file for the Python packages I need, in the terminal

    echo "pytest" > requirements.txt

    the terminal goes back to the command line

  • I add pytest-watcher to the file

    echo "pytest-watcher" >> requirements.txt

    the terminal goes back to the command line

  • I setup the project with uv

    uv init

    the terminal shows

    Initialized project `telephone`

    then goes back to the command line

  • I remove main.py from the project because I do not use it

    rm main.py

    the terminal goes back to the command line

  • I install the Python packages I gave in the requirements file

    uv add --requirement requirements.txt

    the terminal shows it installed the Python packages

  • I use pytest-watcher to run the tests automatically

    uv run pytest-watcher . --now

    the terminal shows

    ================================ FAILURES ================================
______________________ TestTelephone.test_failure ________________________

self = <tests.test_telephone.TestTelephone testMethod=test_failure>

    def test_failure(self):
>       self.assertFalse(True)
E       AssertionError: True is not false

tests/test_telephone.py:7: AssertionError
======================== short test summary info =========================
FAILED tests/test_telephone.py::TestTelephone::test_failure - AssertionError: True is not false
=========================== 1 failed in X.YZs ============================

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

     4class TestTelephone(unittest.TestCase):
 5
 6    def test_failure(self):
 7        self.assertFalse(True)
 8
 9
10# Exceptions seen
11# AssertionError

  • then I change True to False in the assertion

    7        self.assertFalse(False)

    the test passes

test_passing_a_string

RED: make it fail

I change test_failure to test_passing_a_string

 1import unittest
 2
 3
 4class TestTelephone(unittest.TestCase):
 5
 6    def test_passing_a_string(self):
 7        self.assertEqual(
 8            src.telephone.text("hello"),
 9            "I got: hello"
10        )
11
12
13# Exceptions seen

the terminal shows NameError

NameError: name 'src' is not defined

there is no definition for src in test_telephone.py

GREEN: make it pass

  • I add NameError to the list of Exceptions seen

    13# Exceptions seen
14# AssertionError
15# NameError

  • I add an import statement for the telephone module which is in the src folder, at the top of the file

    1import src.telephone
2import unittest
3
4
5class TestTelephone(unittest.TestCase):

    the terminal shows AttributeError

    AttributeError: module 'src.telephone' has no attribute 'text'

    there is no definition for text in telephone.py in the src folder

  • I AttributeError to the list of Exceptions seen in test_telephone.py

    14# Exceptions seen
15# AssertionError
16# NameError
17# AttributeError

  • I open telephone.py from the src folder in the editor, then add a name

    1text

    the terminal shows NameError

    NameError: name 'text' is not defined

    the name is in the file but I have not told Python what it means

  • I point text to None

    1text = None

    the terminal shows TypeError

    TypeError: 'NoneType' object is not callable

    I cannot call None the way I can call a function

  • I add TypeError to the list of Exceptions seen in test_telephone.py

    14# Exceptions seen
15# AssertionError
16# NameError
17# AttributeError
18# TypeError

  • I change text to a function in telephone.py to make it callable

    1def text():
2    return None

    the terminal shows TypeError

    TypeError: text() takes 0 positional arguments but 1 was given

    src.telephone.text was called with "hello" as input but the definition of the function does not take any input - the parentheses are empty

  • I make the function take input and call it the_input

    1def text(the_input):
2    return None

    the terminal shows AssertionError

    AssertionError: None != 'I got: hello'

    the test expects 'I got: hello' and the text function returns None

    Note

    the_input is just the name I used for the input, I can use any name I want

  • I copy the string from the terminal and paste it in the return statement to replace None

    1def text(the_input):
2    return 'I got: hello'

    the test passes!

REFACTOR: make it better

The problem with this solution is that the text function does not care about what it gets, it always returns 'I got: hello' when called. I want it to return the value it gets as part of the message.

  • I add a new assertion to test_passing_a_string in test_telephone.py

     7    def test_passing_a_string(self):
 8        self.assertEqual(
 9            src.telephone.text("hello"),
10            "I got: hello"
11        )
12        self.assertEqual(
13            src.telephone.text("yes"),
14            "I got: yes"
15        )
16
17
18# Exceptions seen

    the terminal shows AssertionError

    AssertionError: 'I got: hello' != 'I got: yes'

    the text function always returns 'I got: hello', the test expects 'I got: yes'

  • I change the return statement in telephone.py to match

    1def text(the_input):
2    return 'I got: yes'

    the terminal shows AssertionError

    AssertionError: 'I got: yes' != 'I got: hello'

    it did not work, my change broke the test that was passing before. The return statement has to use the input

string interpolation

I use an f-string which lets me add any values I want to a string

1def text(the_input):
2    return f'I got: {the_input}'

the test passes.

This is called string interpolation, I can use it to put values in strings.

A string is any characters inside quotes e.g.

  • 'single quotes'

  • '''triple single quotes'''

  • "double quotes"

  • """triple double quotes"""

test_passing_a_class

RED: make it fail

I add a failing test in test_telephone.py to see what happens when I pass a class from a test to the text function

 7    def test_passing_a_string(self):
 8        self.assertEqual(
 9            src.telephone.text("hello"),
10            "I got: hello"
11        )
12        self.assertEqual(
13            src.telephone.text("yes"),
14            "I got: yes"
15        )
16
17    def test_passing_a_class(self):
18        self.assertEqual(
19            src.telephone.text(object),
20            "I got: object"
21        )
22
23
24# Exceptions seen

the terminal shows AssertionError

AssertionError: "I got: <class 'object'>" != 'I got: object'

object is the mother class that all Python classes come from

GREEN: make it pass

I change the expectation in the test to match the result

17    def test_passing_a_class(self):
18        self.assertEqual(
19            src.telephone.text(object),
20            "I got: <class 'object'>"
21        )

the test passes

REFACTOR: make it better

  • I add another assertion with the TestTelephone class to test_passing_a_class in test_telephone.py

    17    def test_passing_a_class(self):
18        self.assertEqual(
19            src.telephone.text(object),
20            "I got: <class 'object'>"
21        )
22        self.assertEqual(
23            src.telephone.text(TestTelephone),
24            "I got: <class 'object'>"
25        )

    the terminal shows AssertionError

    AssertionError: "I got: <class 'tests.test_telephone.TestTelephone'>" != "I got: <class 'object'>"

    even though they are both classes, object and TestTelephone are different

  • I change the expectation

    17    def test_passing_a_class(self):
18        self.assertEqual(
19            src.telephone.text(object),
20            "I got: <class 'object'>"
21        )
22        self.assertEqual(
23            src.telephone.text(TestTelephone),
24            "I got: <class 'tests.test_telephone.TestTelephone'>"
25        )

    the test passes. What does tests.test_telephone.TestTelephone point to?

test_passing_none

RED: make it fail

I add a new failing test for None (it is the simplest Python data structure) in test_telephone.py

17    def test_passing_a_class(self):
18        self.assertEqual(
19            src.telephone.text(object),
20            "I got: <class 'object'>"
21        )
22        self.assertEqual(
23            src.telephone.text(TestTelephone),
24            "I got: <class 'tests.test_telephone.TestTelephone'>"
25        )
26
27    def test_passing_none(self):
28        self.assertEqual(
29            src.telephone.text(None),
30            "I got: 'None'"
31        )
32
33
34# Exceptions seen

the terminal shows AssertionError

AssertionError: 'I got: None' != "I got: 'None'"

GREEN: make it pass

I remove the quotes from around None in the expectation

27    def test_passing_none(self):
28        self.assertEqual(
29            src.telephone.text(None),
30            "I got: None"
31        )
32
33
34# Exceptions seen

the test passes.

test_passing_a_boolean

RED: make it fail

I add a test for booleans, first with an assertion for True

27    def test_passing_none(self):
28        self.assertEqual(
29            src.telephone.text(None),
30            "I got: None"
31        )
32
33    def test_passing_a_boolean(self):
34        self.assertEqual(
35            src.telephone.text(True),
36            "I got: 'True'"
37        )
38
39
40# Exceptions seen

the terminal shows AssertionError

AssertionError: "I got: True" != "I got: 'True'"

GREEN: make it pass

  • I change the expectation

    33    def test_passing_a_boolean(self):
34        self.assertEqual(
35            src.telephone.text(True),
36            "I got: True"
37        )

    the test passes

  • I add an assertion for False

    33    def test_passing_a_boolean(self):
34        self.assertEqual(
35            src.telephone.text(True),
36            "I got: True"
37        )
38        self.assertEqual(
39            src.telephone.text(False),
40            "I got: 'False'"
41        )

    the terminal shows AssertionError

    AssertionError: "I got: False" != "I got: 'False'"

  • I change the expectation

    33    def test_passing_a_boolean(self):
34        self.assertEqual(
35            src.telephone.text(True),
36            "I got: True"
37        )
38        self.assertEqual(
39            src.telephone.text(False),
40            "I got: False"
41        )
42
43
44# Exceptions seen

    the test passes.

test_passing_an_integer

RED: make it fail

I add a test for an integer (a whole number)

33    def test_passing_a_boolean(self):
34        self.assertEqual(
35            src.telephone.text(True),
36            "I got: True"
37        )
38        self.assertEqual(
39            src.telephone.text(False),
40            "I got: False"
41        )
42
43    def test_passing_an_integer(self):
44        self.assertEqual(
45            src.telephone.text(1234),
46            "I got: '1234'"
47        )
48
49
50# Exceptions seen

the terminal shows AssertionError

AssertionError: 'I got: 1234' != "I got: '1234'"

GREEN: make it pass

I remove the quotes from the expectation

43    def test_passing_an_integer(self):
44        self.assertEqual(
45            src.telephone.text(1234),
46            "I got: 1234"
47        )
48
49
50# Exceptions seen

the test passes.

test_passing_a_float

RED: make it fail

I add a test for a float (binary floating point decimal numbers)

43    def test_passing_an_integer(self):
44        self.assertEqual(
45            src.telephone.text(1234),
46            "I got: 1234"
47        )
48
49    def test_passing_a_float(self):
50        self.assertEqual(
51            src.telephone.text(1.234),
52            "I got: '1.234'"
53        )
54
55
56# Exceptions seen

the terminal shows AssertionError

AssertionError: 'I got: 1.234' != "I got: '1.234'"

GREEN: make it pass

I remove the quotes from the number

49    def test_passing_a_float(self):
50        self.assertEqual(
51            src.telephone.text(1.234),
52            "I got: 1.234"
53        )
54
55
56# Exceptions seen

the test passes.

test_passing_a_tuple

RED: make it fail

I add a test for a tuple (things in parentheses (()), separated by a comma)

49    def test_passing_a_float(self):
50        self.assertEqual(
51            src.telephone.text(1.234),
52            "I got: 1.234"
53        )
54
55    def test_passing_a_tuple(self):
56        self.assertEqual(
57            src.telephone.text((1, 2, 3, "n")),
58            "I got: '(1, 2, 3, n)'"
59        )
60
61
62# Exceptions seen

the terminal shows AssertionError

AssertionError: "I got: (1, 2, 3, 'n')" != "I got: '(1, 2, 3, n)'"

GREEN: make it pass

I change the expectation

55    def test_passing_a_tuple(self):
56        self.assertEqual(
57            src.telephone.text((1, 2, 3, "n")),
58            "I got: (1, 2, 3, 'n')"
59        )

the test passes.

test_passing_a_list

RED: make it fail

I add a test for a list (things in square brackets ([]), separated by a comma)

55    def test_passing_a_tuple(self):
56        self.assertEqual(
57            src.telephone.text((1, 2, 3, "n")),
58            "I got: (1, 2, 3, 'n')"
59        )
60
61    def test_passing_a_list(self):
62        self.assertEqual(
63            src.telephone.text([1, 2, 3, "n"]),
64            "I got: '[1, 2, 3, n]'"
65        )
66
67
68# Exceptions seen

the terminal shows AssertionError

AssertionError: "I got: [1, 2, 3, 'n']" != "I got: '[1, 2, 3, n]'"

GREEN: make it pass

I change the expectation to match reality

61    def test_passing_a_list(self):
62        self.assertEqual(
63            src.telephone.text([1, 2, 3, "n"]),
64            "I got: [1, 2, 3, 'n']"
65        )

the test passes.

test_passing_a_dictionary

RED: make it fail

I add a test for a dictionary (key-value pairs in curly braces ({}), separated by a comma)

61    def test_passing_a_list(self):
62        self.assertEqual(
63            src.telephone.text([1, 2, 3, "n"]),
64            "I got: [1, 2, 3, 'n']"
65        )
66
67    def test_passing_a_dictionary(self):
68        self.assertEqual(
69            src.telephone.text({
70                "key1": "value1",
71                "keyN": [0, 1, 2, "n"],
72            }),
73            "I got: '{key1: value1, keyN: [0, 1, 2, n]}'"
74        )
75
76
77# Exceptions seen

the terminal shows AssertionError

AssertionError: "I got: {'key1': 'value1', 'keyN': [0, 1, 2, 'n']}" != "I got: '{key1: value1, keyN: [0, 1, 2, 'n']}'"

GREEN: make it pass

I change the expectation

67    def test_passing_a_dictionary(self):
68        self.assertEqual(
69            src.telephone.text({
70                "key1": "value1",
71                "keyN": [0, 1, 2, "n"],
72            }),
73            "I got: {'key1': 'value1', 'keyN': [0, 1, 2, 'n']}"
74        )
75
76
77# Exceptions seen

the terminal shows all tests are passing.

test_telephone

Time to write the program that makes the tests pass without looking at test_telephone.py

RED: make it fail

  • I close test_telephone.py

  • then delete all the text in telephone.py, the terminal shows 9 failures, I start with the last AttributeError

    AttributeError: module 'src.telephone' has no attribute 'text'

GREEN: make it pass

  • I add the name to telephone.py

    1text

    the terminal shows NameError

    NameError: name 'text' is not defined

  • I point it to None to define it

    1text = None

    the terminal shows TypeError

    TypeError: 'NoneType' object is not callable

  • I make text a function

    1def text():
2    return None

    the terminal shows TypeError

    TypeError: text() takes 0 positional arguments but 1 was given

  • I make the function take input

    1def text(value):
2    return None

    the terminal shows AssertionError

    AssertionError: None != 'I got: None'

  • I copy the string from the terminal and paste it in the return statement to match the expectation of the test

    1def text(value):
2    return 'I got: None'

    the terminal shows AssertionError

    AssertionError: 'I got: None' != 'I got: 1234'

  • I add a return statement to see the difference between the input and the expected output

    1def text(value):
2    return value
3    return 'I got: None'

    the test summary info shows that every test has AssertionError

    AssertionError: True != 'I got: True'
AssertionError: <class 'object'> != "I got: <class 'object'>"
AssertionError: {'key1': 'value1', 'keyN': [0, 1, 2, 'n']} != "I got: {'key1': 'value1', 'keyN': [0, 1, 2, 'n']}"
AssertionError: 1.234 != 'I got: 1.234'
AssertionError: [1, 2, 3, 'n'] != "I got: [1, 2, 3, 'n']"
AssertionError: "hello" != 'I got: hello'
AssertionError: (1, 2, 3, 'n') != "I got: (1, 2, 3, 'n')"
AssertionError: 1234 != 'I got: 1234'
AssertionError: None != 'I got: None'

    they all expect the input as part of the message

  • I remove the first return statement then make the second one use an f-string

    1def text(value):
2    return f'I got: {value}'

    and all the tests are passing! I am a programmer!!

close the project

  • I close telephone.py in the editor

  • I click in the terminal and use q on the keyboard to leave the tests and the terminal goes back to the command line

  • I change directory to the parent of telephone

    cd ..

    the terminal shows

    .../pumping_python

    I am back in the pumping_python directory

review

Here are the tests I ran to see what happens when I pass Python basic data structures from a test to a program and place them in an f-string which is one way to do string interpolation

I also saw the following Exceptions

code from the chapter

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

what is next?

you have covered a bit so far and know

Would you like to use some of the assert methods from AssertionError to test Python’s data structures?

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