Question

Is it preferred to do:

if x is y:
    return True

or

if x == y
    return True

Same thing for “is not”

Answer 1

x is y is different than x == y.

x is y is true if and only if id(x) == id(y) — that is, x and y have to be one and the same object (with the same ids).

For all built-in Python objects (like strings, lists, dicts, functions, etc.), if x is y, then x == y is also True. However, this is not guaranteed in general. Strictly speaking, x == y is true if and only if x.__eq__(y) returns True.

It is possible to define an object x with a __eq__ method which always returns False, for example, and this would cause x == y to return False, even if x is y.

So the bottom line is, x is y and x == y are completely different tests.

Consider this for example:

In [1]: 0 is False
Out[1]: False

In [2]: 0 == False
Out[2]: True

PS. Instead of

if x is y:
    return True
else:
    return False

it is more Pythonic to write

return x is y

And similarly,

if x == y:
    return True
else:
    return False

can be replaced with

return x == y