subclassing file objects (to extend open and close operations) in python 3

Question

Suppose I want to extend the built-in file abstraction with extra operations at open and close time. In Python 2.7 this works:

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Answer 1

I had a similar problem, and a requirement of supporting both Python 2.x and 3.x. What I did was similar to the following (current full version):

class _file_obj(object):
    """Check if `f` is a file name and open the file in `mode`.
    A context manager."""
    def __init__(self, f, mode):
        if isinstance(f, str):
            self.file = open(f, mode)
        else:
            self.file = f
        self.close_file = (self.file is not f)
    def __enter__(self):
        return self
    def __exit__(self, *args, **kwargs):
        if (not self.close_file):
            return  # do nothing
        # clean up
        exit = getattr(self.file, '__exit__', None)
        if exit is not None:
            return exit(*args, **kwargs)
        else:
            exit = getattr(self.file, 'close', None)
            if exit is not None:
                exit()
    def __getattr__(self, attr):
        return getattr(self.file, attr)
    def __iter__(self):
        return iter(self.file)

It passes all calls to the underlying file objects and can be initialized from an open file or from a filename. Also works as a context manager. Inspired by this answer.