Strange python 'is' operator in arithmetic

Question

I tried this operation in python interactive mode :

>>> (1*1) is 1
True
>>> (377*35) is 13195
False
>>> 377*35
13195
>>> 377*35 is 377*35
False
>>> 1*1 is 1
True

Could anybody explain why ' (377*35) is 13195 ' is false?

Thanks in advance!

I was doing mental math, just tried to type the answer directly instead of hitting enter. But '==' gets no problem at all. Thanks. — hugle, Sep 09 '14 at 13:35

score 3 · Accepted Answer · answered Sep 09 '14 at 13:26

A is B checks that A and B refer to the same object. It does not check whether A equals B numerically.

The reason for the different behaviour in your examples is that ints with small values (typically between -1 and 99 inclusive) are "interned" by the interpreter -- whenever a result has such a value, an existing short int with the same value is returned.

This explains why is returns True for your examples involving small numbers but not for those involving large numbers.

The interned range is -5 to +256 inclusive. – khelwood Sep 09 '14 at 13:37 — khelwood, Sep 09 '14 at 13:37

Strange python 'is' operator in arithmetic

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