Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Ian Kelly <ian.g.kelly@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: Explaining names vs variables in Python
Date: Wed, 2 Mar 2016 08:13:16 -0700
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Xref: csiph.com comp.lang.python:103875

On Wed, Mar 2, 2016 at 2:35 AM, Jussi Piitulainen
<jussi.piitulainen@helsinki.fi> wrote:
> The following are too delicate for me. I suppose the answers could have
> been different, but I can't guess what mechanism actually leads to these
> results. Just idle curiosity on my part.
>
>>>> 890 is 890
> True
>>>> id(890) == id(890)
> True

This has to do with the way code blocks are compiled. In the
interactive interpreter, a single line like '890 is 890' is compiled
to a single code object. The constant 890 appears twice in the same
code block, so the optimizer uses the same constant for both. Note in
the following that the same index appears for both, so they're
actually the same object reference.

>>> import dis
>>> dis.dis('890 is 890')
  1           0 LOAD_CONST               0 (890)
              3 LOAD_CONST               0 (890)
              6 COMPARE_OP               8 (is)
              9 RETURN_VALUE
>>> compile('890 is 890', '', 'exec').co_consts
(890, None)


As for the earlier example of:

>>>> 1 + 1 is 2
> True
>>>> 800 + 90 + 0 is 890
> False

This one actually surprises me a little, because the optimizer is also
smart enough to evaluate '800 + 90 + 0' and just store a constant of
890:

>>> dis.dis('800 + 90 + 0 is 890')
  1           0 LOAD_CONST               5 (890)
              3 LOAD_CONST               3 (890)
              6 COMPARE_OP               8 (is)
              9 RETURN_VALUE
>>> compile('800 + 90 + 0 is 890', '', 'exec').co_consts
(800, 90, 0, 890, None, 890, 890)

Not smart enough to reuse the existing reference in this case
apparently, or even to prune out the original constants that are no
longer used in the code.