Why does this str strict comparison fail in Python? [duplicate]

Question

This question already has answers here:

Why does comparing strings using either '==' or 'is' sometimes produce a different result? (15 answers)

Closed 9 years ago.

I have this code in a decorator:

is_local_valid = ['system_id', 'server_id', 'sensor_id']
local_params = [x for x in is_local_valid if x in kwargs.keys() and kwargs[x].lower() is 'local'] + [y for y in args if y.lower() is 'local']
if local_params == []:
    raise AssertionError("Tried to access the database from a non connected profile")

I noticed that the is infix operator for comparing two strings returns False in this case, even when kwargs[x].lower() equals to local. The == operator does return True, though. Both are str, of course.

Any clue on what's going on?

Answer 1

The operators is and is not test for object identity:
x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.

>>> id('local')
42745112
>>> a = {1: 'locAl'}
>>> id(a[1].lower())
53363408

They are not the same object

Answer 2

a is b determines whether two names a and b reference the same object (i.e. id(a) == id(b)). a == b determines whether they have equal values. It is rarely a good idea to compare strings with is; use == instead:

>>> "".join("hello")
'hello'
>>> "hello" is "".join("hello")
False
>>> "hello" == "".join("hello")
True

The only general case for comparison with is would be for None, e.g.:

if a is None: