DEV Community

Python 'is' vs '=='

Kelvin Wangonya on September 06, 2019

A lot of times when I'm doing ifs in Python, I find myself wondering whether to use is or == for the check. # do I do if a is b: ... # or i...

Read full post

Jason C. McDonald • Sep 7 '19 • Edited

Very nice, but a word of warning, identity (what is checks) is generally a language implementation detail, not something that should be relied upon.

Consider:

spam = 42
eggs = spam
print(spam is eggs)  # True
print(spam is 42)  # True

spam = 420
eggs = spam
print(spam is eggs)  # True
print(spam is 420)  # False

That same example might not work precisely that way in another implementation of Python.

Mutable and immutable values behave very differently in terms of assignment and identity.

Except in some extremely rare, hack-y scenarios, you should only see is in the context of is None.

Kelvin Wangonya • Sep 8 '19

Interesting. Thanks for the heads up.

Except in some extremely rare, hack-y scenarios, you should only see is in the context of is None.

You know, I wanted to mention something similar to this but didn't because we wouldn't usually do if a is None... a better way would be if not a - so I didn't know exactly how to frame it.

Jason C. McDonald • Sep 8 '19 • Edited

Well, no, you wouldn't want to do that. if not a is the accepted shorthand for if a == False, but False and None are distinct values. You should always explicitly test for None, although you can implicitly test for "not None":

if foo:
    # foo has value (not None) OR foo is True

if not foo:
    # foo is False

if foo is None:
    # foo is None

Kelvin Wangonya • Sep 8 '19

Awesome.

Dylan Anthony • Oct 23 '19

There's actually a bit more to it than that. if not checks "truthyness", not just equality to False. So not foo will evaluate to True if foo is any of these and more:

False
None
0
[]
{}
""

The same thing applies in the other direction, if [1] will succeed because the list is not empty, empty list are "falsey" whereas lists containing elements are "truthy".

The lesson here is pretty much don't use if without an explicit comparison (== or is) unless you're sure you know how truthy the value will be in all cases.

Mikhail Merzlyutin • Sep 9 '19

There are some corner cases when you would like to use the full comparison form. For example if None is used as an "no argument value was passed" flag:

def foo(a=None):
 if not a:
  print("No value provided")

it will print message for calls foo(), foo(0), foo(""), foo([]).

Jason C. McDonald • Sep 9 '19

Yes, it's always best to test for the presence of None explicitly, rather than implicitly.

hidden_dude • Sep 8 '19

wouldn't 'is' be used in s scenario where we're comparing pointers like in a linked list?

while p is not q :
p = p.next

I think that is how 'is' should be used.

Kelvin Wangonya • Sep 8 '19 • Edited

I've not used linked lists much so I wouldn't know but yeah, I guess that would work.

Jason C. McDonald • Sep 8 '19

Well, Python doesn't have pointers, per se, nor would there ordinarily be a cause to implement your own linked list. It's virtually always best to use Python's own data structures, or at least build upon them.

hidden_dude • Sep 9 '19

yes.. but if you needed something fancy like a left leaning red black tree or a concurrent skip list you'd need 'is' to work for you for completeness.

(ps. I know they don't really have "pointers" but in these scenarios you use it like a pointer)

Ali Sherief • Jan 29 '20

If you try to compare a variable with a literal using is or is not you will get a syntax warning like this:

>>> x=1
>>> x is 0
<stdin>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
False
>>> x is not 0
<stdin>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?
True