实例检查良好做法之前没有检查？

Question

Is it good practice to test for not None preceding an isinstance check in Python? For example:

if x is not None and isinstance(x, (int, float)):
    # Some code over here 
    pass

Or can I do without and is this sufficient?

if isinstance(x, (int, float)):
    # Some code over there 
    pass

Answer 1

To answer your question, doing a is not None check is unnecessary, because -

>>> isinstance(None, (int, float))
False

Meaning, if x is None , then the isinstance condition would return False anyway, making the x is not None check redundant. So, in summary,

if isinstance(x, (int, float)):
    ...

Works just as well.

---

Furthermore, if you want to test whether an object is numeric in nature, a simpler way to do so would be using the numbers module -

import numbers
if isinstance(x, numbers.Number):
    ...

Note that numbers.Number tests all numeric objects from the very root of the hierarchy. This also includes complex numbers, so if that isn't what you want, you can test with numbers.Real to numbers.Rational instead.

Answer 2

Python 2.7.14 (default, Sep 20 2017, 01:25:59) 
[GCC 7.2.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> isinstance(None, (int, float))
False
>>> 

It raises no error, so I would actually prefer the version without the 'is None' check, as it is just less code.