Why do empty string and None evaluate differently?

Question

Why do those two evaluate differently?

In [34]: a = ''

In [35]: if a or a >=0:
    print 'y'
    ....:     
y

In [36]: a = None

In [37]: if a or a >=0:
    print 'y'
    ....:

I thought an empty string also evaluates to None (I know they are not the same)? Or is it just falsy and thus evaluates to 0 (which then evaluates to None if tested like if a: do something)?

I missed where you get confused. In the first example? It print `y` because of the `>= 0` part. — keyser, May 07 '14 at 07:38
It's because of how string vs int comparison works. The `if a` part doesn't kick in because, as you say, an empty string and `None` is not the same thing. — keyser, May 07 '14 at 07:40
Related: [How does Python compare string and int?](http://stackoverflow.com/q/3270680/142637) — sloth, May 07 '14 at 07:41
[Even more related](http://stackoverflow.com/q/2384078/645270). The short answer is: why would you ever want to do such a comparison? It's even fixed in python 3 — keyser, May 07 '14 at 07:42
Thanks for the link, indeed very related. I test for empty strings often like `if not x:` instead of `if x !=''`. So i was just adding a condition and then got confused... — LarsVegas, May 07 '14 at 07:47
@LarsVegas That makes sense. The condition you added kind of doesn't :) — keyser, May 07 '14 at 07:48
You should use `if a and a >= 0` if you expected the test to behave the same for both values. Or switch to Python 3, where strings and integers are not orderable relative to one another. — Martijn Pieters, May 07 '14 at 07:49
On Python 3 you'll get `TypeError: unorderable types: str() > int()`. — Matthias, May 07 '14 at 07:50
@LarsVegas : using `if not x` instead of `if x != ''` is the pythonic idiom (the empty string being False in a boolean context). Your problem comes from comparing oranges and bananas - in this case, either a string or `None` with an integer. — bruno desthuilliers, May 07 '14 at 08:44

score 0 · Answer 1 · answered May 07 '14 at 09:25

It's because in CPython int comes before string

from docs:

CPython implementation detail: Objects of different types except numbers are ordered by their type names; objects of the same types that don’t support proper comparison are ordered by their address.

so:

>>> '1' > 1
True

Why do empty string and None evaluate differently?

1 Answers1