def patch_func(replacement, target_mod, func_name):
    """
    Patch func_name in target_mod with replacement

    Important - original must be resolved by name to avoid
    patching an already patched function.
    """
    original = getattr(target_mod, func_name)

    # set the 'unpatched' attribute on the replacement to
    # point to the original.
    vars(replacement).setdefault('unpatched', original)

    # replace the function in the original module
    setattr(target_mod, func_name, replacement)
Patch func_name in target_mod with replacement

    Important - original must be resolved by name to avoid
    patching an already patched function.