Why is pytest silencing nested equality assertion statements?

Question

I noticed that pytest is overriding custom assertion statements unless they're called directly in a test body or the statement is explicitly assert False. Here is some example code showing the interaction:

def assert_equality():
    assert 1 == 0, 'pytest will not display this message'


def assert_boolean():
    assert False, 'this message will be displayed'


def test_nested_assertion_equality():
    assert_equality()


def test_nested_assertion_boolean():
    assert_boolean()


def test_assertion_equality():
    assert 1 == 0, 'this message also will be displayed'

Output for these tests:

test_experiment.py:8 (test_nested_assertion_equality)
1 != 0

Expected :0
Actual   :1
<Click to see difference>

def test_nested_assertion_equality():
>       assert_equality()

test_experiment.py:10:

test_experiment.py:12 (test_nested_assertion_boolean)
def test_nested_assertion_boolean():
>       assert_boolean()

test_experiment.py:14: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def assert_boolean():
>       assert False, 'this message will be displayed'
E    AssertionError: this message will be displayed
E    assert False

test_experiment.py:6: AssertionError

test_experiment.py:16 (test_assertion_equality)
1 != 0

Expected :0
Actual   :1
<Click to see difference>

def test_assertion_equality():
>       assert 1 == 0, 'this message also will be displayed'
E    AssertionError: this message also will be displayed
E    assert 1 == 0

test_experiment.py:18: AssertionError

Why is this happening? Is there a way to stop it?

Answer 1

Pytest does override the asserts, it's called assert rewriting and is documented here: https://docs.pytest.org/en/7.1.x/how-to/assert.html

You can do a lot to modify its behavior and disable it by adding the command line argument --assert=plain.