numpy.nditer.enable_external_loop
method
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nditer.enable_external_loop()
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When the “external_loop” was not used during construction, but is desired, this modifies the iterator to behave as if the flag was specified. 
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Licensed under the 3-clause BSD License.
    https://docs.scipy.org/doc/numpy-1.16.1/reference/generated/numpy.nditer.enable_external_loop.html