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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 parse_fingerprint(self, cmdline, key=None, sep=None):
 """Given a psutil.Process.cmdline, parse and return a fingerprint. :param list cmdline: The psutil.Process.cmdline of the current process. :param string key: The key for fingerprint discovery. :param string sep: The key/value separator for fingerprint discovery. :returns: The parsed fingerprint or `None`. :rtype: string or `None` """

    key = key or self.FINGERPRINT_CMD_KEY
    if key:
      sep = sep or self.FINGERPRINT_CMD_SEP
      cmdline = cmdline or []
      for cmd_part in cmdline:
        if cmd_part.startswith('{}{}'.format(key, sep)):
          return cmd_part.split(sep)[1]