Check if return value is instance of Py_None

Question

I have to use a c function (myAPI.readArr) that returns a scalar,numpy.ndarray or Py_None on failure.

This works for a failure:

data = myAPI.readArr(arg1, arg2)
if not data:
   raise Exception("Problem!")

but for valid arguments I get:

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

How can I check that the call succeeded or not?

PEP 8 explicitly recommends testing `is None` when you mean `is None`, in part because many objects besides `None` are falsey—like, say, `False`. The fact that some objects are illegal to test for falsiness, like NumPy arrays, just means that you get an early error message for your mistake instead of code that seems to work until you deploy it and it silently screws up your data. :) — abarnert, Apr 20 '15 at 08:34
I think [this](http://stackoverflow.com/questions/14247373/python-none-comparison-should-i-use-is-or) is the canonical answer, but the linked one seems to more directly answer the OP's question. — abarnert, Apr 20 '15 at 08:36

score 3 · Accepted Answer · edited May 23 '17 at 12:21

3

data = myAPI.readArr(arg1, arg2)
if data is None:
   raise Exception("Problem!")

This answer contains more details about comparison to None

edited May 23 '17 at 12:21

Community

answered Apr 20 '15 at 08:14

Konstantin

Jérôme · Answer 2 · 2015-04-20T08:27:56.443

3

if data is None:
    raise Exception("Problem!")

Testing is None is better because it returns True if and only if the value is None, not if it evaluates to zero.

edited Apr 20 '15 at 08:27

answered Apr 20 '15 at 08:14

Jérôme

2 Answers2