In Python 2.7, functools.wraps() does not provide the '__wrapped__'
attribute. This attribute is used by
oslo_utils.reflection.get_signature() when getting the signature of a
function. If a function is decorated without the '__wrapped__'
attribute then the signature will be of the decorator rather than the
underlying function.
From the six documentation for six.wraps():
This is exactly the functools.wraps() decorator, but it sets the
__wrapped__ attribute on what it decorates as functools.wraps()
does on Python versions after 3.2.
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