Check if numpy's array_like is an empty array

Question

Suppose a is an array_like and we want to check if it is empty. Two possible ways to accomplish this are:

if not a:
   pass

if numpy.array(a).size == 0:
   pass

The first solution would also evaluate to True if a=None. However I would like to only check for an empty array_like.

The second solution seems good enough for that. I was just wondering if there is a numpy built-in function for that or a better solution then to check for the size?

You definitely should not use the expression not a. If a is, in fact, a numpy array with size 0, in recent versions of numpy that expression will generate a deprecation warning: DeprecationWarning: The truth value of an empty array is ambiguous. Returning False, but in future this will result in an error. Use array.size > 0 to check that an array is not empty. And that messages suggests that checking the size attribute is the recommended method. — Warren Weckesser
– Warren Weckesser, Commented Jan 7, 2023 at 12:09
Also note that if a is a list such as a = [[], [], []], then not a will be False (because len(a) is 3), but np.array(a).size is 0 (because the array that is created has shape (3, 0)). — Warren Weckesser
– Warren Weckesser, Commented Jan 7, 2023 at 12:17

Daweo · Accepted Answer · 2023-01-07 12:10:54Z

2

If you want to check if size is zero, you might use numpy.size function to get more concise code

import numpy
a = []
b = [1,2]
c = [[1,2],[3,4]]
print(numpy.size(a) == 0)  # True
print(numpy.size(b) == 0)  # False
print(numpy.size(c) == 0)  # False

answered Jan 7, 2023 at 12:10

Daweo

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