Python: Why does ("hello" is "hello") evaluate as True? [duplicate]

Question

Why does "hello" is "hello" produce True in Python?

I read the following here:

If two string literals are equal, they have been put to same memory location. A string is an immutable entity. No harm can be done.

So there is one and only one place in memory for every Python string? Sounds pretty strange. What's going on here?

Answer 1

Python (like Java, C, C++, .NET) uses string pooling / interning. The interpreter realises that "hello" is the same as "hello", so it optimizes and uses the same location in memory.

Another goodie: "hell" + "o" is "hello" ==> True

Answer 2

So there is one and only one place in memory for every Python string?

No, only ones the interpreter has decided to optimise, which is a decision based on a policy that isn't part of the language specification and which may change in different CPython versions.

eg. on my install (2.6.2 Linux):

>>> 'X'*10 is 'X'*10 True >>> 'X'*30 is 'X'*30 False 

similarly for ints:

>>> 2**8 is 2**8 True >>> 2**9 is 2**9 False 

So don't rely on 'string' is 'string': even just looking at the C implementation it isn't safe.