Comparing strings using '==' and 'is' [duplicate]

Question

Possible Duplicates:
Types for which “is” keyword may be equivalent to equality operator in Python
Python “is” operator behaves unexpectedly with integers

Hi.

I have a question which perhaps might enlighten me on more than what I am asking.

Consider this:

>>> x = 'Hello'
>>> y = 'Hello'
>>> x == y
True
>>> x is y
True

I have always used the comparison operator. Also I read that is compares the memory address and hence in this case, returns True

So my question is, is this another way to compare variables in Python? If yes, then why is this not used?

Also I noticed that in C++, if the variables have the same value, their memory addresses are different.

{ int x = 40; int y = 40; cout << &x, &y; }
0xbfe89638, 0xbfe89634

What is the reason for Python having the same memory addresses?

Answer 1

This is an implementation detail and absolutely not to be relied upon. is compares identities, not values. Short strings are interned, so they map to the same memory address, but this doesn't mean you should compare them with is. Stick to ==.

Answer 2

There are two ways to check for equality in Python: == and is. == will check the value, while is will check the identity. In almost every case, if is is true, then == must be true.

Sometimes, Python (specifically, CPython) will optimize values together so that they have the same identity. This is especially true for short strings. Python realizes that 'Hello' is the same as 'Hello' and since strings are immutable, they become the same through string interning / string pooling.

See a related question: Python: Why does ("hello" is "hello") evaluate as True?