Python equality testing¶

I ran into this great article on python equality.

I think the discussion on how Python handles interning especially useful.

From the article:

• Most of the time, you want ==.

• Use arg is None when you have a function with an argument defaulting to None. That’s okay, because there’s only one None.

• For testing whether two classes, functions, or modules are the same object, is is okay. Stylistic choice.

• Never use is with str, int, float, complex, or any other core immutable value type! Interning makes the response worthless!

• Other valid uses of is are fairly rare and obscure, for example:

  • If I have a large tree structure and want to find the location of a subtree, == will recursively compare values (potentially very slow) but is will tell me if I’ve found the exact same node.

  • A caching mechanism may want to treat all objects as distinct, without having to care about or rely on how they implement ==. is can be appropriate here.

  • Demonstrating to newbies that interning exists is only possible with is :)