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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_doc_from_docid(self, docid, doc_type_name=None, inst=True):
 """ Try to find a document based on its document id. if inst=True, if it hasn't been instantiated yet, it will be. """

        assert(docid is not None)
        if docid in self._docs_by_id:
            return self._docs_by_id[docid]
        if not inst:
            return None
        doc = self.__inst_doc(docid, doc_type_name)
        if doc is None:
            return None
        self._docs_by_id[docid] = doc
        return doc