Quick and dirty snapshot testing in python

mysnapshot.py

class MySnapshot:
    NO_ARGUMENT = object()

    def __init__(self, expected_value=NO_ARGUMENT):
        self.expected_value = expected_value

    def __eq__(self, other):
        if self.expected_value is self.NO_ARGUMENT:
            from inspect import currentframe, getframeinfo
            caller_frame = getframeinfo(currentframe().f_back)
            calling_filename = caller_frame.filename
            calling_line = caller_frame.lineno # calling line is 1 based
            replaced_contents = []
            with open(calling_filename) as f:
                for line_no, l in enumerate(f.readlines()):
                    if line_no+1 == calling_line:
                        l = l.replace("MySnapshot()", f"MySnapshot({repr(other)})")
                    replaced_contents.append(l)
            with open(calling_filename, 'w') as f:
                f.writelines(replaced_contents)

            return True

        else:
            return other == self.expected_value