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def function_code(fn):
    """ returns the source code of the given function 
        by: Cody Kochmann 
      """
    from inspect import getsource
    assert callable(fn), 'function_source needs its argument to be callable'
    if '__source__' in dir(fn):
        return fn.__source__
    else:
        try:
            return getsource(fn)
        except OSError:
            return None