What determines which strings are interned and when?

Question

>>> 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.

Are you asking how long a string has to be before python starts making separate copies of it? — Matt Fenwick, May 16 '12 at 16:12
It's completely implementation dependent and could change in the next version of Python without warning. Why do you want to know? — Mark Ransom, May 16 '12 at 16:13
Yes, I am asking how long a string has to be before python starts making separate copies of it. — krzyhub, May 16 '12 at 16:13
@Cris: What are you trying to accomplish? The behavior is implementation-defined and cannot be relied on. Why not just check for equality? — Daenyth, May 16 '12 at 16:14
I just want know for knowing. Don't know that this is no important and very flexible. — krzyhub, May 16 '12 at 16:15
if you just want to know, why don't you test it on the implementation *you*'re using? — Karoly Horvath, May 16 '12 at 16:16
if i will only testing i will never get knowledge that this is implementation dependent — krzyhub, May 16 '12 at 16:18
Note that you may get a different result if you put your example statements in a script, instead of trying them interactively. — Janne Karila, May 16 '12 at 17:45

score 14 · Accepted Answer · answered May 16 '12 at 16:15

14

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

answered May 16 '12 at 16:15

MattH

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...implementation specific. This is also answer. Thank You. – krzyhub May 16 '12 at 16:16
@Cris As a concrete example, [ideone's python version seems to intern your "long" strings](http://ideone.com/ZO3ni) as well as 1000-character-eval-constructed strings. – trutheality May 16 '12 at 16:32
It gives me little more knowlednge and many more questions, byt it is ok. – krzyhub May 16 '12 at 17:32
So is there never a legit use case for users to test `s1 is s2` ? – smci Sep 13 '18 at 23:36

score 12 · Answer 2 · answered May 16 '12 at 16:19

12

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

answered May 16 '12 at 16:19

Spaceghost

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For some bizarre reason, good to know. – krzyhub May 16 '12 at 16:21
2

Additionally, intern is moved into module sys in python 3000 along with id. Also good to know. – Spaceghost May 16 '12 at 16:23
For some reasons, the most. Could You tell is `intern()` only python 3000 toy? – krzyhub May 16 '12 at 16:27
2

@Cris `intern()` has been in Python at least since version 1.5 – trutheality May 16 '12 at 17:24
intern is in the root namespace. – Spaceghost May 16 '12 at 17:44
It is built-in, you don't need to import anything. – Karl Knechtel May 16 '12 at 17:48

score 6 · Answer 3 · edited Jan 07 '16 at 15:16

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.

What determines which strings are interned and when?

3 Answers3

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