Check if variable is defined with Numpy array?

Question

Sometimes I have a situation where I want to test whether a variable is 0 or None or not. In pure Python, this is simply

foo == True

but when foo is possibly a Numpy object (such as numpy.ndarray), this does not work anymore and I get the error:

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

and in this case I want a.any(), however this fails on non-iterable objects.

Of course I can explicitly check if foo is 0 or None, but I am wondering if there is a more elegant way to do this.

How can I check if an arbitrary object (both iterable and not) is True or not?

More Pythonically, it's if foo - and that'll work with most things. Unfortunately, you'll have to special case numpy.ndarray or any other type that's more complex than 0, None, False, len() == 0 equates to False. — Jon Clements
– Jon Clements, Commented Oct 19, 2012 at 12:08
I usually check numpy object to be None with array is None, but this doesn't check for zero though. See here for a similar question, stackoverflow.com/questions/5086178/… — gg349
– gg349, Commented Oct 19, 2012 at 14:05

the Tin Man · Accepted Answer · 2012-10-24 19:17:31Z

2

Just use np.all or np.any instead of the .all method. np.all will create an array if it is not one yet.

edited Oct 24, 2012 at 19:17

the Tin Man

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answered Oct 19, 2012 at 12:09

seberg

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2 Comments

Joe Kington Over a year ago

Just be aware that this doesn't work as you might think for all iterables: np.any(False for _ in range(10)) yields True. (Of course, it would work if this were a list comprehension instead of a generator.) Basically, it evaluates the logical truth of the iterable instance instead of making an array from the contents. You can work around this with np.fromiter, but it's an extra step. Still, using np.all, etc, is by far the cleanest solution if you just want to deal with most cases.

Tim Over a year ago

This seems like a suitable solution, thanks! The generator exception is not really an issue for me, but it's good to know there are potential pitfalls.

Ben Allison · Accepted Answer · 2012-10-19 12:07:24Z

1

The recipe I use is to surround the statement assuming the existence of a variable in a try except block:

try:
    do_something(foo)
except NameError:
    foo = something

answered Oct 19, 2012 at 12:07

Ben Allison

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