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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def current_user(self):
 """Retrieve the user ID of the current user talking to your bot. This is mostly useful inside of a Python object macro to get the user ID of the person who caused the object macro to be invoked (i.e. to set a variable for that user from within the object). This will return ``None`` if used outside of the context of getting a reply (the value is unset at the end of the ``reply()`` method). """

        if self._brain._current_user is None:
            # They're doing it wrong.
            self._warn("current_user() is meant to be used from within a Python object macro!")
        return self._brain._current_user