[Python-Dev] Caching float(0.0)

[Jean-Paul Calderone]

On Sun, 1 Oct 2006 13:54:31 -0400, Terry Reedy <tjreedy at udel.edu> wrote:
>
>"Nick Craig-Wood" <nick at craig-wood.com> wrote in message
>news:20061001093846.GA20938 at craig-wood.com...
>> On Fri, Sep 29, 2006 at 12:03:03PM -0700, Guido van Rossum wrote:
>>> I see some confusion in this thread.
>>>
>>> If a *LITERAL* 0.0 (or any other float literal) is used, you only get
>>> one object, no matter how many times it is used.
>>
>> For some reason that doesn't happen in the interpreter which has been
>> confusing the issue slightly...
>>
>> $ python2.5
>>>>> a=0.0
>>>>> b=0.0
>>>>> id(a), id(b)
>> (134737756, 134737772)
>
>Guido said *a* literal (emphasis shifted), reused as in a loop or function
>recalled, while you used *a* literal, then *another* literal, without
>reuse.  Try a=b=0.0 instead.

Actually this just has to do with, um, "compilation units", for lack of a
better term:

  exarkun at kunai:~$ python
  Python 2.4.3 (#2, Apr 27 2006, 14:43:58) 
  [GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> a = 0.0
  >>> b = 0.0
  >>> print a is b
  False
  >>> ^D
  exarkun at kunai:~$ cat > test.py
  a = 0.0
  b = 0.0
  print a is b
  ^D
  exarkun at kunai:~$ python test.py
  True
  exarkun at kunai:~$ cat > test_a.py
  a = 0.0
  ^D
  exarkun at kunai:~$ cat > test_b.py
  b = 0.0
  ^D
  exarkun at kunai:~$ cat > test.py
  from test_a import a
  from test_b import b
  print a is b
  ^D
  exarkun at kunai:~$ python test.py
  False
  exarkun at kunai:~$ python
  Python 2.4.3 (#2, Apr 27 2006, 14:43:58) 
  [GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> a = 0.0; b = 0.0
  >>> print a is b
  True
  >>> 
  exarkun at kunai:~$

Each line in an interactive session is compiled separately, like modules
are compiled separately.  With the current implementation, literals in a
single compilation unit have a chance to be "cached" like this.  Literals
in different compilation units, even for the same value, don't.

Jean-Paul