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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 CountClientPlatformsByLabel(self, day_buckets):
 """Computes client-activity stats for all client platforms in the DB."""


    def ExtractPlatform(client_info):
      return client_info.last_snapshot.knowledge_base.os

    return self._CountClientStatisticByLabel(day_buckets, ExtractPlatform)