Is it better to use the “is” operator or the “==” operator to compare two numbers in Python?
>>> a = 1 >>> a is 1 True >>> a == 1 True >>> a is 0 False >>> a == 0 False
Sometimes, on some python implementations, by coincidence, integers from -5 to 256 will work with
is (in CPython implementations for instance). But don’t rely on this or use it in real programs.
Others have answered your question, but I’ll go into a little bit more detail:
is compares identity – it asks the question “is this one thing actually the same object as this other thing” (similar to
== in Java). So, there are some times when using
is makes sense – the most common one being checking for
foo is None. But, in general, it isn’t what you want.
==, on the other hand, asks the question “is this one thing logically equivalent to this other thing”. For example:
>>> [1, 2, 3] == [1, 2, 3] True >>> [1, 2, 3] is [1, 2, 3] False
And this is true because classes can define the method they use to test for equality:
>>> class AlwaysEqual(object): ... def __eq__(self, other): ... return True ... >>> always_equal = AlwaysEqual() >>> always_equal == 42 True >>> always_equal == None True
But they cannot define the method used for testing identity (ie, they can’t override
>>> a = 255556 >>> a == 255556 True >>> a is 255556 False
I think that should answer it 😉
The reason is that some often-used objects, such as the booleans True and False, all 1-letter strings and short numbers are allocated once by the interpreter, and each variable containing that object refers to it. Other numbers and larger strings are allocated on demand. The 255556 for instance is allocated three times, every time a different object is created. And therefore, according to
is, they are not the same.
That will only work for small numbers and I’m guessing it’s also implementation-dependent. Python uses the same object instance for small numbers (iirc <256), but this changes for bigger numbers.
>>> a = 2104214124 >>> b = 2104214124 >>> a == b True >>> a is b False
So you should always use
== to compare numbers.
== is what you want, “is” just happens to work on your examples.
>>> 2 == 2.0 True >>> 2 is 2.0 False