int - Python: False vs 0

10
votes

In PHP you use the === notation to test for TRUE or FALSE distinct from 1 or 0.

For example if FALSE == 0 returns TRUE, if FALSE === 0 returns FALSE. So when doing string searches in base 0 if the position of the substring in question is right at the beginning you get 0 which PHP can distinguish from FALSE.

Is there a means of doing this in Python?

3
pythonintboolean
you can distinguish, but you usually shouldn't have to. False looks like 0 in almost every way (and True like 1). e.g. True + True + True + False == 3 - mgilson
a very good point ... although there are cases where you do want to distinguish - Joran Beasley
@mgilson I don't usually have to distinguish. However, in this case I am looking to validate a hex color string. So I am first checking for the # symbol at the beginning of the string and then checking the length. I want to be able to tell the user at the time they pass in the color value that it is invalid, not wait until later when the class attempts to use it. - Gabe Spradlin

3 Answers

26
votes

In Python,

  • The is operator tests for identity (False is False, 0 is not False).

  • The == operator which tests for logical equality (and thus 0 == False).

Technically neither of these is exactly equivalent to PHP's ===, which compares logical equality and type - in Python, that'd be a == b and type(a) is type(b).

Some other differences between is and ==:

Mutable type literals

  • {} == {}, but {} is not {} (and the same holds true for lists and other mutable types)
  • However, if a = {}, then a is a (because in this case it's a reference to the same instance)

Strings

  • "a"*255 is not "a"*255", but "a"*20 is "a"*20 in most implementations, due to how Python handles string interning. This behavior isn't guaranteed, though, and you probably shouldn't be using is in this case. "a"*255 == "a"*255 and is almost always the right comparison to use.

Numbers

  • 12345 is 12345 but 12345 is not 12345 + 1 - 1 in most implementations, similarly. You pretty much always want to use equality for these cases.
5
votes
if something is False:

is what you should do

if something is None:

also works

the moral is use is ... (although you should never do something is 123457, or simillar)

for why you should never do this with ints and things see http://ideone.com/iKmWCn

0
votes

The strict equivalent of x === y in Python is type(x) is type(y) and x == y. You don't really want to do this as Python is duck typed. If an object has the appropriate method or attribute then you shouldn't be too worried about its actual type.

If you are checking for a specific unique object such as (True, False, None, or a class) then you should use is and is not. For example: x is True.