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Re: I thought I understood how import worked...

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On 2012-08-08 06:14, Ben Finney wrote:
> Cameron Simpson <cs@zip.com.au> writes:
>
>> All of you are saying "two names for the same module", and variations
>> thereof. And that is why the doco confuses.
>>
>> I would expect less confusion if the above example were described as
>> _two_ modules, with the same source code.
> That's not true though, is it? It's the same module object with two
> different references, I thought.
They are not the same. Proof:

$ mkdir test
$ cd test
$ touch __init__.py
$ touch m.py
$ cd ..
$ python
Python 2.7.3 (default, Apr 20 2012, 22:39:59)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import sys
 >>> sys.path.append('test')
 >>> import m
 >>> from test import m
 >>> import m
 >>> from test import m as m2
 >>> m is m2
False
 >>> m.a = 3
 >>> m2.a
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'a'

So it is still true that top level code gets executed only once, when 
the module is first imported. The trick is that a module is not a file. 
It is a module object that is created from a file, with a name. If you 
change the name, then you create ("import") a new module.

You can also use the reload() function to execute module level code 
again, but it won't create a new module object. It will just update the 
contents of the very same module object:

What is more interesting is how the reload() function works:

Python 2.7.3 (default, Apr 20 2012, 22:39:59)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import test.m
 >>> a = test.m
 >>> import os
 >>> test.m is a
True
 >>> os.system("echo \"import sys\" >> test/m.py")
0
 >>> reload(test.m) # Updates the module object
<module 'test.m' from 'test/m.py'>
 >>> test.m is a # They are still the same
True
 >>> a.sys # So a.sys is a exist
<module 'sys' (built-in)>
 >>>

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I thought I understood how import worked... Roy Smith <roy@panix.com> - 2012-08-07 09:18 -0400
  Re: I thought I understood how import worked... Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-07 13:52 +0000
    Re: I thought I understood how import worked... Roy Smith <roy@panix.com> - 2012-08-07 08:25 -0700
      Re: I thought I understood how import worked... Paul Rubin <no.email@nospam.invalid> - 2012-08-07 08:53 -0700
      Re: I thought I understood how import worked... Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-07 17:54 +0000
    Re: I thought I understood how import worked... Cameron Simpson <cs@zip.com.au> - 2012-08-08 08:47 +1000
      Re: I thought I understood how import worked... Roy Smith <roy@panix.com> - 2012-08-07 19:05 -0400
      Re: I thought I understood how import worked... Ben Finney <ben+python@benfinney.id.au> - 2012-08-08 14:14 +1000
        Re: I thought I understood how import worked... Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-08 08:40 +0200
        Re: I thought I understood how import worked... Roy Smith <roy@panix.com> - 2012-08-08 09:12 -0400
        Re: I thought I understood how import worked... Cameron Simpson <cs@zip.com.au> - 2012-08-09 15:52 +1000
  Re: I thought I understood how import worked... Ben Finney <ben+python@benfinney.id.au> - 2012-08-07 23:55 +1000
    Re: I thought I understood how import worked... Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-07 16:14 +0200
    Re: I thought I understood how import worked... Roy Smith <roy@panix.com> - 2012-08-07 08:32 -0700
      Re: I thought I understood how import worked... Terry Reedy <tjreedy@udel.edu> - 2012-08-07 13:15 -0400
  Re: I thought I understood how import worked... Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-08-07 15:10 +0100
  Re: I thought I understood how import worked... Terry Reedy <tjreedy@udel.edu> - 2012-08-07 12:49 -0400
  Re: I thought I understood how import worked... Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-08-07 18:44 +0100
  Re: I thought I understood how import worked... Jean-Michel Pichavant <jeanmichel@sequans.com> - 2012-08-08 10:47 +0200

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