Python string multiplication with "is" comparison is inconsistent

Asked Apr 24 '19 at 17:30

Active Apr 24 '19 at 17:35

Viewed 156 times

I know that python's "is" comparison uses the ID of the object to compare, but it seems to behave inconsistently specifically when comparing a string concatenated with itself. Up to 20, for a single character str, it returns True, then False:

>>>'a'*20 is 'a'*20
True
>>>'a'*21 is 'a'*21
False

With strings of different lengths it seems to happen wherever the total string surpasses 21:

>>>'abcdefghijk'*2 is 'abcdefghijk'*2
False
>>>'abcdefghijk'*1 is 'abcdefghijk'*1
True

But this doesn't apply to strings which are already longer than 21 characters:

>>>'abcdefghijklmnopqrstuvwxyz'*1 is 'abcdefghijklmnopqrstuvwxyz'*1
True

What is python doing that causes this change of behavior?

EDIT: I'm using Python 3.6.5

edited Apr 24 '19 at 17:33

asked Apr 24 '19 at 17:30

Aya Maguire

1

In Python 2.7, and I think up to around Python 3.6, identifier-like strings of up to length 20 are interened; those bigger are not. – Brad Solomon Apr 24 '19 at 17:32
See also: http://guilload.com/python-string-interning/ – Brad Solomon Apr 24 '19 at 17:34
1

see https://github.com/satwikkansal/wtfpython#-strings-can-be-tricky-sometimes- – Michael H. Apr 24 '19 at 17:35
Unless at least one of the arguments to `is` is a simple variable name or function call, you are probably misusing `is`. – chepner Apr 24 '19 at 17:36
I'm not actually using `is` in this way, I just wanted to understand what was happening. I didn't know about interning! Thanks for the links, great reads. – Aya Maguire Apr 24 '19 at 17:48

Python string multiplication with "is" comparison is inconsistent

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