How to Assert List Equality in pytest: Simple Guide

In pytest, you can assert that two lists are equal using the simple assert list1 == list2 statement. This checks if both lists have the same elements in the same order and will pass or fail the test accordingly.

Syntax

The basic syntax to assert list equality in pytest is:

assert list1 == list2

Here:

• list1 and list2 are the lists you want to compare.
• The assert keyword checks if the condition is true.
• If the lists are equal (same elements in the same order), the test passes.
• If not, pytest shows a detailed error message.

python

assert list1 == list2

Example

This example shows how to write a pytest test function that asserts two lists are equal. It demonstrates a passing test and what happens if the lists differ.

python

def test_lists_are_equal():
    list_a = [1, 2, 3]
    list_b = [1, 2, 3]
    assert list_a == list_b

def test_lists_are_not_equal():
    list_a = [1, 2, 3]
    list_b = [3, 2, 1]
    assert list_a == list_b

Output

============================= test session starts ==============================
collected 2 items

test_example.py .F                                                        [100%]

=================================== FAILURES ===================================
__________________________ test_lists_are_not_equal ___________________________

    def test_lists_are_not_equal():
        list_a = [1, 2, 3]
        list_b = [3, 2, 1]
>       assert list_a == list_b
E       AssertionError: assert [1, 2, 3] == [3, 2, 1]
E         At index 0 diff: 1 != 3
E         Full diff:
E         - [1, 2, 3]
E         + [3, 2, 1]

test_example.py:8: AssertionError
=========================== short test summary info ===========================
FAILED test_example.py::test_lists_are_not_equal - AssertionError: assert [1, 2, 3] == [3, 2, 1]
============================== 1 failed, 1 passed ==============================

Common Pitfalls

Common mistakes when asserting list equality in pytest include:

• Comparing lists with the same elements but different order, which will fail because order matters.
• Using assert list1 is list2 which checks if both variables point to the same object, not if their contents are equal.
• Not considering nested lists or complex objects where deep equality might be needed.

Correct way:

assert list1 == list2

Wrong way:

assert list1 is list2  # This checks identity, not equality

python

def test_wrong_identity_assertion():
    list_a = [1, 2, 3]
    list_b = [1, 2, 3]
    assert list_a is list_b  # This will fail

def test_correct_equality_assertion():
    list_a = [1, 2, 3]
    list_b = [1, 2, 3]
    assert list_a == list_b  # This will pass

Output

============================= test session starts ==============================
collected 2 items

test_pitfalls.py F.                                                        [100%]

=================================== FAILURES ===================================
__________________________ test_wrong_identity_assertion ______________________

    def test_wrong_identity_assertion():
        list_a = [1, 2, 3]
        list_b = [1, 2, 3]
>       assert list_a is list_b  # This will fail
E       AssertionError

test_pitfalls.py:4: AssertionError
=========================== short test summary info ===========================
FAILED test_pitfalls.py::test_wrong_identity_assertion - AssertionError
============================== 1 failed, 1 passed ==============================

Quick Reference

Tips for asserting list equality in pytest:

• Use assert list1 == list2 to check if lists have the same elements in order.
• Order matters: [1, 2, 3] is not equal to [3, 2, 1].
• For unordered comparison, consider sorting lists first: assert sorted(list1) == sorted(list2).
• Use pytest's detailed error messages to quickly find differences.

Key Takeaways

Use simple assert statements like assert list1 == list2 to check list equality in pytest.

List order matters; identical elements in different order will cause assertion failure.

Avoid using 'is' for list comparison as it checks identity, not content equality.

Sort lists before asserting if order does not matter for your test case.

Pytest shows clear error messages to help identify differences between lists.