AssertionError

AssertionError is raised to show that a statement is False. It was introduced in how to make a python test driven development environment with the first failing test

self.assertFalse(True)

which is like

assert True is False

I can use assertions when building a program to make sure something is True before it continues. I can also use them to test how the program behaves, for example when it is given inputs.

Assertions can help catch things that break passing tests when they are added. They also help me answer 2 questions

  • what is the same?

  • what is different?

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

requirements

  • I open a terminal to run makePythonTdd.sh with assertion_error as the name of the project

    ./makePythonTdd.sh assertion_error

    on Windows without Windows Subsystem Linux use makePythonTdd.ps1

    ./makePythonTdd.ps1 assertion_error

    it makes the folders and files that are needed, installs packages, runs the first test, and the terminal shows AssertionError

    E       AssertionError: True is not false

tests/test_assertion_error.py:7: AssertionError

  • I hold ctrl (windows/linux) or option (mac) on the keyboard and use the mouse to click on tests/test_assertion_error.py:7 to open it in the editor

  • then I change True to False to make the test pass

    7        self.assertFalse(False)

  • I change the name of the class to match the CapWords format

    4class TestAssertionError(unittest.TestCase):

test_assertion_error_w_none

red: make it fail

  • I change test_failure to test_assertion_error_w_none

    1import unittest
2
3
4class TestAssertionError(unittest.TestCase):
5
6    def test_assertion_error_w_none(self):
7        assert None is not None

    the terminal shows AssertionError

    E    assert None is not None

tests/test_assertion_error.py:7: AssertionError

    the assert at the beginning of the line makes the statement something like “DO NOT CONTINUE, UNLESS None is NOT None”. The AssertionError is raised to stop the program from running, because this statement is False

green: make it pass

I change the failing line to make it True

6    def test_assertion_error_w_none(self):
7        assert None is None

the test passes

refactor: make it better

I can also make assertions with some methods from the unittest.TestCase class

  • I add another failing line using the assertIsNotNone method which checks if something is NOT None

    6    def test_assertion_error_w_none(self):
7        assert None is None
8        self.assertIsNotNone(None)

    the terminal shows a more descriptive message for the AssertionError

    AssertionError: unexpectedly None

    I change the statement to use the assertIsNone method which checks if something is None

    6    def test_assertion_error_w_none(self):
7        assert None is None
8        self.assertIsNone(None)

    the test passes

  • I add a note

    11# NOTES
12# None is None
13
14
15# Exceptions Encountered
16# AssertionError

  • I add a new failing line

     6    def test_assertion_error_w_none(self):
 7        assert None is None
 8        self.assertIsNone(None)
 9
10        assert False is None

    the terminal shows AssertionError

    E    assert False is None

    I change the line to make it True

    10        assert False is not None

    the test passes.

  • I add another line with the assertIsNone method which checks if something is None

     6    def test_assertion_error_w_none(self):
 7        assert None is None
 8        self.assertIsNone(None)
 9
10        assert False is not None
11        self.assertIsNone(False)

    the terminal shows AssertionError

    AssertionError: False is not None

    I change the statement with assertIsNotNone to make it True

    10        assert False is None
11        self.assertIsNotNone(False)

    the test passes.

  • I add another note

    14# NOTES
15# False is not None
16# None is None

  • I add another assertion

     6    def test_assertion_error_w_none(self):
 7        assert None is None
 8        self.assertIsNone(None)
 9
10        assert False is not None
11        self.assertIsNotNone(False)
12
13        assert True is None

    the terminal shows AssertionError

    E    assert True is None

    I change the statement to make it True

    13        assert True is not None

    the test passes

  • I use the assertIsNone method

     6    def test_assertion_error_w_none(self):
 7        assert None is None
 8        self.assertIsNone(None)
 9
10        assert False is not None
11        self.assertIsNotNone(False)
12
13        assert True is not None
14        self.assertIsNone(True)

    the terminal shows AssertionError

    AssertionError: True is not None

    I make the statement True

    13        assert True is not None
14        self.assertIsNotNone(True)

    the test passes

  • I add a note

    17# NOTES
18# True is not None
19# False is not None
20# None is None

test_assertion_error_w_false

I can use assertions to test if something is False

red: make it fail

I add a failing test

13        assert True is not None
14        self.assertIsNotNone(True)
15
16    def test_assertion_error_w_false(self):
17        assert True is False

the terminal shows AssertionError

E    assert True is False

green: make it pass

I change the line to make it True

16    def test_assertion_error_w_false(self):
17        assert True is not False

the test passes

refactor: make it better

  • There is an assert method to check if something is False, it is the one from the first failing test

    16    def test_assertion_error_w_false(self):
17        assert True is not False
18        self.assertFalse(True)

    the terminal shows AssertionError

    AssertionError: True is not false

  • I make the statement True

    16    def test_assertion_error_w_false(self):
17        assert True is not False
18        self.assertFalse(False)

    the test passes

  • I add notes

    21# NOTES
22# False is False
23# True is not False
24# True is not None
25# False is not None
26# None is None

test_assertion_error_w_true

I can also use assertions to test if something is True

red: make it fail

I add a failing test

16    def test_assertion_error_w_false(self):
17        assert True is not False
18        self.assertFalse(False)
19
20    def test_assertion_error_w_true(self):
21        assert False is True

the terminal shows AssertionError

E    assert False is True

green: make it pass

I change the statement to make it True

20    def test_assertion_error_w_true(self):
21        assert False is not True

the test passes

refactor: make it better

  • there is an assert method to check if something is True

    20    def test_assertion_error_w_true(self):
21        assert False is not True
22        self.assertTrue(False)

    the terminal shows AssertionError

    AssertionError: False is not true

  • I change the failing line to make it True

    20    def test_assertion_error_w_true(self):
21        assert False is not True
22        self.assertTrue(True)

    the test passes

  • I add more notes

    25# NOTES
26# True is True
27# False is not True
28# False is False
29# True is not False
30# True is not None
31# False is not None
32# None is None

All these statements can also be said as - True, False and None are different. They set up a basic expectation of the language because I can compare things with them.

test_assertion_error_w_equality

I can use assertions to test if 2 things are equal

red: make it fail

I add a new failing test

20    def test_assertion_error_w_true(self):
21        assert False is not True
22        self.assertTrue(True)
23
24    def test_assertion_error_w_equality(self):
25        assert None != None

the terminal shows AssertionError

E    assert None != None

!= is the symbol for NOT equal which makes this statement read as assert None is NOT equal to None

green: make it pass

I change the statement to make it True

24    def test_assertion_error_w_equality(self):
25        assert None == None

the test passes. == is the symbol for is equal which makes this statement read as assert None is equal to None

refactor: make it better

  • there are assert methods to check for equality. I add assertNotEqual checks if the 2 things in the parentheses are NOT equal

    24    def test_assertion_error_w_equality(self):
25        assert None == None
26        self.assertNotEqual(None, None)

    the terminal shows AssertionError

    AssertionError: None == None

    I change the assertion to use the assertEqual method to make the failing line True

    24    def test_assertion_error_w_equality(self):
25        assert None == None
26        self.assertEqual(None, None)

    the test passes

  • I add to the notes

    29# NOTES
30# True is True
31# False is not True
32# False is False
33# True is not False
34# True is not None
35# False is not None
36# None is None and equal to None

  • I add a new failing line

    24    def test_assertion_error_w_equality(self):
25        assert None == None
26        self.assertEqual(None, None)
27
28        assert False == None

    the terminal shows AssertionError

    E    assert False == None

    I change the failing line to make it True

    28        assert False != None

    the test passes.

  • I add the assertEqual method

    28        assert False != None
29        self.assertEqual(False, None)

    the terminal shows AssertionError

    AssertionError: False != None

    I change the failing line to make it True

    28        assert False != None
29        self.assertNotEqual(False, None)

    the test passes

  • I add to the notes

    34# NOTES
35# True is True
36# False is not True
37# False is False
38# True is not False
39# True is not None
40# False is not None and not equal to None
41# None is None and equal to None

  • I add the next failing line

    24    def test_assertion_error_w_equality(self):
25        assert None == None
26        self.assertEqual(None, None)
27
28        assert False != None
29        self.assertNotEqual(False, None)
30
31        assert True == None

    the terminal shows AssertionError

    E    assert True == None

    I change the statement

    31        assert True != None

    the test passes

  • I add an assert method

    31        assert True != None
32        self.assertEqual(True, None)

    the terminal shows AssertionError

    AssertionError: True != None

    I change the method

    31        assert True != None
32        self.assertNotEqual(True, None)

    and the test passes

  • I add a note

    35# NOTES
36# True is True
37# False is not True
38# False is False
39# True is not False
40# True is not None and not equal to None
41# False is not None and not equal to None
42# None is None and equal to None

  • I add another failing line

    31    def test_assertion_error_w_equality(self):
32        assert None == None
33        self.assertEqual(None, None)
34
35        assert False != None
36        self.assertNotEqual(False, None)
37
38        assert True != None
39        self.assertNotEqual(True, None)
40
41        assert True == False

    the terminal shows AssertionError

    E    assert True == False

    I change the failing line

    34        assert True != False

    the test passes

  • I add assertEqual

    34        assert True != False
35        self.assertEqual(True, False)

    the terminal shows AssertionError

    AssertionError: True != False

    I change the assert method

    34        assert True != False
35        self.assertNotEqual(True, False)

    the test passes

  • I add a note

    38# NOTES
39# True is True
40# False is not True
41# False is False
42# True is not False and not equal to False
43# True is not None and not equal to None
44# False is not None and not equal to None
45# None is None and equal to None

  • on to the next line

    24    def test_assertion_error_w_equality(self):
25        assert None == None
26        self.assertEqual(None, None)
27
28        assert False != None
29        self.assertNotEqual(False, None)
30
31        assert True != None
32        self.assertNotEqual(True, None)
33
34        assert True != False
35        self.assertNotEqual(True, False)
36
37        assert False != False

    the terminal shows AssertionError

    E    assert False != False

    I make the line True

    37        assert False == False

    the test passes

  • I add another failing line with an assert method

    37        assert False == False
38        self.assertNotEqual(False, False)

    the terminal shows AssertionError

    AssertionError: False == False

    I change the method

    37        assert False == False
38        self.assertEqual(False, False)

    the test passes

  • I add a note

    41# NOTES
42# True is True
43# False is not True
44# False is False and equal to False
45# True is not False and not equal to False
46# True is not None and not equal to None
47# False is not None and not equal to None
48# None is None and equal to None

  • I add another failing line

    24    def test_assertion_error_w_equality(self):
25        assert None == None
26        self.assertEqual(None, None)
27
28        assert False != None
29        self.assertNotEqual(False, None)
30
31        assert True != None
32        self.assertNotEqual(True, None)
33
34        assert True != False
35        self.assertNotEqual(True, False)
36
37        assert False == False
38        self.assertEqual(False, False)
39
40        assert False == True

    the terminal shows AssertionError

    E    assert False == True

    I change the line

    40        assert False != True

    the test passes

  • I add another failing line

    40        assert False != True
41        self.assertEqual(False, True)

    the terminal shows AssertionError

    AssertionError: False != True

    I change the assert method

    40        assert False != True
41        self.assertNotEqual(False, True)

    the test passes

  • I add another note

    44# NOTES
45# True is True
46# False is not True and not equal to True
47# False is False and equal to False
48# True is not False and not equal to False
49# True is not None and not equal to None
50# False is not None and not equal to None
51# None is None and equal to None

  • time for the last statements

    24    def test_assertion_error_w_equality(self):
25        assert None == None
26        self.assertEqual(None, None)
27
28        assert False != None
29        self.assertNotEqual(False, None)
30
31        assert True != None
32        self.assertNotEqual(True, None)
33
34        assert True != False
35        self.assertNotEqual(True, False)
36
37        assert False == False
38        self.assertEqual(False, False)
39
40        assert False != True
41        self.assertNotEqual(False, True)
42
43        assert True != True

    the terminal shows AssertionError

    E    assert True != True

    I make the statement True

    43          assert True == True

    the test passes

  • I add a line with the assert method

    43          assert True == True
44          self.assertNotEqual(True, True)

    the terminal shows AssertionError

    AssertionError: True == True

    I change the method

    43          assert True == True
44          self.assertEqual(True, True)

    and all tests are passing!

  • I add a note for the last statement

    47# NOTES
48# True is True and equal to True
49# False is not True and not equal to True
50# False is False and equal to False
51# True is not False and not equal to False
52# True is not None and not equal to None
53# False is not None and not equal to None
54# None is None and equal to None

review

I can use assert statements and methods to test if something is

Would you like to test what is None?

Click here for all the tests from this chapter