Why is Python string the same if there's no space, but different if there is?

Asked Jan 08 '20 at 12:27

Active Jan 08 '20 at 12:32

Viewed 57 times

I happen to realize the following:

a = 'hello'
b = 'hello'
a is b
>> True

whereas

a = 'hello world'
b = 'hello world'
a is b
>> False

If there's no space, the two variables are the same, but if there is they are different.

Why is that?

(I don't need people telling me to use == instead of is. I know that.)

edited Jan 08 '20 at 12:32

asked Jan 08 '20 at 12:27

danqing

1

Never rely on literals producing the same immutable object. Also, never rely on literals producing *different* immutable objects. It's entirely up to the implementation how to handle literals for immutable types. – chepner Jan 08 '20 at 12:29
Because the rules for string interning are arcane. It doesn't matter, you should **never rely on this**. The fact that strings like this *are the same* is a fact that should be *surprising*, it is an optimization of the interpreter – juanpa.arrivillaga Jan 08 '20 at 12:29
@chepner of course. I'm simply curious as why that's the case. – danqing Jan 08 '20 at 12:30
Use `==`. `is` is essentially comparing references. – Mateen Ulhaq Jan 08 '20 at 12:30
`a is b` should only be used to see if either `a = b` or `b = a` took place. – chepner Jan 08 '20 at 12:30
Also related: https://stackoverflow.com/questions/132988/is-there-a-difference-between-and-is – Mateen Ulhaq Jan 08 '20 at 12:30
@juanpa.arrivillaga Thanks; I snuck "immutable" into my original comment :) – chepner Jan 08 '20 at 12:31
@MateenUlhaq Interning is expensive, so you want to do it as little as possible, or at least know that the time you spend on the various lookups involved will be worthwhile. – chepner Jan 08 '20 at 12:33
@chepner Did you mean to reply to someone else? :) – Mateen Ulhaq Jan 08 '20 at 12:36
Sigh. Yup. Sorry about that. – chepner Jan 08 '20 at 12:40

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