numpy.all() and empty arrays

Question

I noticed that numpy.all(np.array([])) evaluates to True. I could not find any documentation for this particular case. Is this an undefined behavior or is all guaranteed to evaluate to True for empty arrays?

The Python list equivalent notes: If the iterable is empty, return True. — hpaulj
– hpaulj, Commented May 7, 2017 at 1:35

user6655984 · Accepted Answer · 2017-05-07 05:26:12Z

NumPy is consistent with the logic of Python lists:

>>> all([])
True
>>> any([])
False

And both follow the rules of formal logic, which might mean there is no need to document this as a special case. Example: unicorns do not exist, so the list of unicorns is []

The statement "all unicorns are red" is true. It spells out as "if X is a unicorn, then X is red", which is true for all objects since they are not unicorns.
The statement "some unicorns are red" is false, because it asserts the existence of at least one red unicorn.

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