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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 pip_search(self, search_string=None):
 """Search for pip packages in PyPI matching `search_string`."""

        extra_args = ['search', search_string]
        return self._call_pip(name='root', extra_args=extra_args,
                              callback=self._pip_search)