Python unittest – assertEqual() function
Last Updated : 28 Mar, 2025
assertEqual() is a function from Python’s unittest library used for unit testing. It checks whether two values are equal and returns a boolean result based on the condition. If both values are equal, the test passes otherwise it fails and raises an AssertionError with an optional error message. For Example:
Python import unittest class TestNegative(unittest.TestCase): def test_fun(self): # Failing test: comparing different strings self.assertEqual("geeks", "gfg", "'geeks' is not equal to 'gfg'.") if __name__ == '__main__': unittest.main() Output:
Hangup (SIGHUP)
F
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FAIL: test_strings_not_equal (__main__.TestNegative)
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Traceback (most recent call last):
File "test.py", line 7, in test_strings_not_equal
self.assertEqual("geeks", "gfg", "'geeks' is not equal to 'gfg'.")
AssertionError: 'geeks' != 'gfg' : 'geeks' is not equal to 'gfg'.
Explanation: In this negative test case, assertEqual() compares two different strings, “geeks” and “gfg”, resulting in a test failure with an AssertionError.
Syntax of assertEqual()
assertEqual(firstValue, secondValue, message)
Parameter:
- firstValue: The first value to be compared.
- secondValue: The second value to be compared.
- message (optional): A custom message displayed when the test case fails.
Return Value:
- If firstValue == secondValue : The test passes and nothing is returned.
- If firstValue != secondValue : An AssertionError is raised with the provided message (if given).
Note: In the unittest module, test method names should begin with the prefix test_. This is necessary for the unittest framework to automatically discover and run the test methods.
Examples of assertEqual()
Example 1: This checks if assertEqual() works correctly by comparing two equal strings. If the strings match, the test passes, showing that the function is working as expected.
Python import unittest class TestPositive(unittest.TestCase): def test_fun(self): # Passing test: comparing identical strings self.assertEqual("hello", "hello", "Both values are equal.") if __name__ == '__main__': unittest.main() Output
.
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Ran 1 test in 0.000s
OK
Explanation: “hello” is compared with “hello” and since both values match, the test passes without errors, verifying that assertEqual() correctly identifies equal values.
Example 2: This test verifies that assertEqual() correctly compares identical integers, ensuring accurate numerical equality evaluation.
Python import unittest class TestIntegers(unittest.TestCase): def test_fun(self): # Passing test: comparing identical integers self.assertEqual(10, 10, "This test passes since 10 == 10.") if __name__ == '__main__': unittest.main()
.
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Ran 1 test in 0.000s
OK
Explanation: assertEqual() checks 10 against 10, confirming that it works consistently with numerical values as the test passes.
Example 3: This shows a failed test where assertEqual() compares two different integers, highlighting how the function detects mismatched values.
Python import unittest class TestFailure(unittest.TestCase): def test_fun(self): # Failing test: 5 is not equal to 10 self.assertEqual(5, 10, "This test fails since 5 != 10.") if __name__ == '__main__': unittest.main()
Hangup (SIGHUP)
F
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FAIL: test_fun (__main__.TestFailure)
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Traceback (most recent call last):
File "test.py", line 6, in test_fun
self.assertEqual(5, 10, "This test fails since 5 != 10.")
AssertionError: 5 != 10 : This test fails since 5 != 10.
Explanation: assertEqual() compares 5 and 10, leading to an AssertionError. This highlights how the function detects incorrect numerical comparisons, ensuring expected and actual outputs align.