Check if variable is None or numpy.array

Question

I look up in a table if keys have associated arrays, or not. By design, my table.__getitem__() somtimes returns None rather than KeyError-s. I would like this value to be either None, or the numpy array associated with w.

value = table[w] or table[w.lower()]
# value should be a numpy array, or None
if value is not None:
    stack = np.vstack((stack, value))

Only if I go with the above code, and the first lookup is a match, I get :

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

and if I go with value = table[w].any() or table[w.lower()].any(), then if it's a mismatch, I expectedly bump into :

AttributeError: 'NoneType' object has no attribute 'any'

I must be missing the correct way to do this, how to do ?

Answer 1

if type(value) is numpy.ndarray:
    #do numpy things
else
    # Handle None

Though the above would work, I would suggest to keep signatures simple and consistent, ie table[w] should always return numpy array. In case of None, return empty array.

Answer 2

The question is answered, but other folks who encounter this error may want a general solution. With the idea of being explicit in mind we can use the function isinstance. Here is a working example.

import numpy as np

a = np.array([1,2,3])
b = None
for itm in [a,b]:
    isinstance(itm,np.ndarray)

So in the context of the question

value = table[w]
if not isinstance(value,np.ndarray):
    value = table[w.lower()]