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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 pid(self):
 """ The integer PID of the subprocess or None. """

        pf = self.path('cmd.pid')
        if not os.path.exists(pf):
            return None
        with open(pf, 'r') as f:
            return int(f.read())