Freeze when comparing byte data with verbose flag

When comparing a lot of byte data (like two images), pytest will look frozen when using the -v flag. Not sure if this is a bug, but I at least wanted to document this behaviour.

To clarify this, here is the code making problems

def test_compare():
    with open('one.png', "rb") as f:
        actual_image = f.read()

    with open('two.png', "rb") as f:
        expected_image = f.read()

    assert actual_image == expected_image

Assuming that both images are different, running this code with “pytest -s” will take a view milliseconds, but running it with “pytest -s -v” took 12 minutes on my computer.

A workaround is to put the comparison in an own variable

def test_compare():
    with open('one.png', 'rb') as f:
        actual_image = f.read()

    with open('two.png', 'rb') as f:
        expected_image = f.read()

    result = actual_image == expected_image
    assert result

Running this will take a view milliseconds only, no matter if the assertion fails or succeeds.

I have attached an example workspace.

By the way you will also run into this issue when executing the tests using the pytest runner of PyCharm.

freezing_assertion.zip

[The-Compiler]

Might be a duplicate of #7206?

[Zac-HD]

Closing as duplicate; per https://github.com/pytest-dev/pytest/issues/8998#issuecomment-900389987 we should try adding a heuristic that skips difflib.ndiff() for sequences above a fixed length (tbd based on some benchmarks).