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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 _alter_umask(self):
 """Temporarily alter umask to custom setting, if applicable"""

        if self.umask is None:
            yield  # nothing to do
        else:
            prev_umask = os.umask(self.umask)
            try:
                yield
            finally:
                os.umask(prev_umask)