When does python choose to intern a string

14
votes
>>> s1 = "spam"
>>> s2 = "spam"
>>> s1 is s2
True
>>> q = 'asdalksdjfla;ksdjf;laksdjfals;kdfjasl;fjasdf'
>>> r = 'asdalksdjfla;ksdjf;laksdjfals;kdfjasl;fjasdf'
>>> q is r
False

How many characters should have to s1 is s2 give False? Where is limit? I.e., I am asking how long a string has to be before python starts making separate copies of it.

3
pythonstring-interning
Are you asking how long a string has to be before python starts making separate copies of it? - Matt Fenwick
It's completely implementation dependent and could change in the next version of Python without warning. Why do you want to know? - Mark Ransom
Yes, I am asking how long a string has to be before python starts making separate copies of it. - krzyhub
I just want know for knowing. Don't know that this is no important and very flexible. - krzyhub
if i will only testing i will never get knowledge that this is implementation dependent - krzyhub

3 Answers

13
votes

String interning is implementation specific and shouldn't be relied upon, use equality testing if you want to check two strings are identical.

12
votes

If you want, for some bizarre reason, to force the comparison to be true then use the intern function:

>>> a = intern('12345678012345678901234567890qazwsxedcrfvtgbyhnujmikolp')
>>> b = intern('12345678012345678901234567890qazwsxedcrfvtgbyhnujmikolp')
>>> a is b
True
5
votes

Here is a piece of comment about interned string from CPython 2.5.0 source file (stringobject.h)

/* ... ... This is generally restricted to strings that **"look like" Python identifiers**, although the intern() builtin can be used to force interning of any string ... ... */

Accordingly, strings contain only underscores, digits or alphabets will be interned. In your example, q and ``r contain ;, so they will not be interned.