On 04Nov2017 01:47, Chris Angelico <ros...@gmail.com> wrote:
On Fri, Nov 3, 2017 at 8:24 PM, Ali Rıza KELEŞ <ali.r.ke...@gmail.com> wrote:
Yesterday, while working with redis, i encountered a strange case.
I want to ask why is the following `True`
```
"s" is b"s".decode()
```
while the followings are `False`?
```
"so" is b"so".decode()
"som" is b"som".decode()
"some" is b"some".decode()
```
Or vice versa?
I read that `is` compares same objects, not values. So my question is
why "s" and b"s".decode() are same objects, while the others aren't?
My python version is 3.6.3.
You shouldn't be comparing string objects with 'is'. Sometimes two
equal strings will be identical, and sometimes they won't. All you're
seeing is that the interpreter happened to notice and/or cache this
particular lookup.
To be more clear here, usually when humans say "identical" they mean having
exactly the same value or attributes.
Here, Chris means that the two strings are actually the same object rather than
two equivalent objects. "is" tests the former (the same object). "==" is for
testing the latter (the objects have the same value).
For speed and memory reasons, Python notices small values of strings and ints,
and allocates them only once. So When your write:
a = "s"
b = "s"
Python will reuse the same str object for both. Because of this, not only is "a
== b" (i.e. they have the same value) but also "a is b" (a and b refer to the
same object). But this is not guarrenteed, and certainly for larger values
Python doesn't bother. Eg:
a = "ghghghghghg"
b = "ghghghghghg"
Here they will have the same value but be different objects. So "==" will still
return True, but "is" would return False.
You should usually be using "==" to compare things. "is" has its place, but it
is usually not what you're after.
In your example code, b"s".decode() returns the string value "s", and Python is
internally deciding to reuse the existing "s" from the left half of your
comparison. It can do this because strings are immutable. (For example, "+=" on
a string makes a new string).
Hoping this is now more clear,
Cameron Simpson <c...@cskk.id.au> (formerly c...@zip.com.au)
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