dirty-equals

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Approach:

• new method on some types, e.g. __inequality_details__(op) should return a list of strings
• add a utility function to check for __inequality_details__ and call it if required,
• instructions on calling that function from pytest_assertrepr_compare

rough prototype for Contains:

def pytest_assertrepr_compare(config, op, left, right):
    if isinstance(right, Contains):
        word = 'not' if op == '==' else 'unexpectedly'
        if len(right.contained_values) == 1:
            descr = f'Container: {right.contained_values[0]!r} {word} found in'
        else:
            descr = f'Container: {right.contained_values!r} all {word} found in'
        return [
            f'[dirty-equals] "{op}" failed',
            descr,
            *pformat(left).split('\n'),
        ]

Any interest in me taking a crack at this?

I think best to leave it, I have some pretty strong ideas about how to go about it and it might require changes to other libraries to make it work.