tensorflow_federated/python/learning/templates/client_works.py [90:112]:
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  def __init__(self, initialize_fn, next_fn):
    super().__init__(initialize_fn, next_fn, next_is_multi_arg=True)

    if not initialize_fn.type_signature.result.is_federated():
      raise errors.TemplateNotFederatedError(
          f'Provided `initialize_fn` must return a federated type, but found '
          f'return type:\n{initialize_fn.type_signature.result}\nTip: If you '
          f'see a collection of federated types, try wrapping the returned '
          f'value in `tff.federated_zip` before returning.')
    next_types = (
        structure.flatten(next_fn.type_signature.parameter) +
        structure.flatten(next_fn.type_signature.result))
    if not all([t.is_federated() for t in next_types]):
      offending_types = '\n- '.join(
          [t for t in next_types if not t.is_federated()])
      raise errors.TemplateNotFederatedError(
          f'Provided `next_fn` must be a *federated* computation, that is, '
          f'operate on `tff.FederatedType`s, but found\n'
          f'next_fn with type signature:\n{next_fn.type_signature}\n'
          f'The non-federated types are:\n {offending_types}.')

    if initialize_fn.type_signature.result.placement != placements.SERVER:
      raise errors.TemplatePlacementError(
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tensorflow_federated/python/learning/templates/finalizers.py [72:94]:
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  def __init__(self, initialize_fn, next_fn):
    super().__init__(initialize_fn, next_fn, next_is_multi_arg=True)

    if not initialize_fn.type_signature.result.is_federated():
      raise errors.TemplateNotFederatedError(
          f'Provided `initialize_fn` must return a federated type, but found '
          f'return type:\n{initialize_fn.type_signature.result}\nTip: If you '
          f'see a collection of federated types, try wrapping the returned '
          f'value in `tff.federated_zip` before returning.')
    next_types = (
        structure.flatten(next_fn.type_signature.parameter) +
        structure.flatten(next_fn.type_signature.result))
    if not all([t.is_federated() for t in next_types]):
      offending_types = '\n- '.join(
          [t for t in next_types if not t.is_federated()])
      raise errors.TemplateNotFederatedError(
          f'Provided `next_fn` must be a *federated* computation, that is, '
          f'operate on `tff.FederatedType`s, but found\n'
          f'next_fn with type signature:\n{next_fn.type_signature}\n'
          f'The non-federated types are:\n {offending_types}.')

    if initialize_fn.type_signature.result.placement != placements.SERVER:
      raise errors.TemplatePlacementError(
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