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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 _get_targets_by_declared_platform_with_placeholders(self, targets_by_platform):
 """ Aggregates a dict that maps a platform string to a list of targets that specify the platform. If no targets have platforms arguments, return a dict containing platforms inherited from the PythonSetup object. :param tgts: a list of :class:`Target` objects. :returns: a dict mapping a platform string to a list of targets that specify the platform. """


    if not targets_by_platform:
      for platform in self._python_setup.platforms:
        targets_by_platform[platform] = ['(No target) Platform inherited from either the '
                                          '--platforms option or a pants.ini file.']
    return targets_by_platform