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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_method_repo(self, namespace=None):
 """ Returns the method repository for the specified CIM namespace within the mock repository. This is the original instance variable, so any modifications will change the mock repository. Validates that the namespace exists in the mock repository. If the method repository does not contain the namespace yet, it is added. Parameters: namespace(:term:`string`):
 Namespace name. Must not be `None`. Returns: dict of dict of method callback function: Method repository. Raises: :exc:`~pywbem.CIMError`: CIM_ERR_INVALID_NAMESPACE: Namespace does not exist. """

        self._validate_namespace(namespace)
        if namespace not in self.methods:
            self.methods[namespace] = NocaseDict()
        return self.methods[namespace]