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Perplexed by the behavior of import

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  Perplexed by the behavior of import Rod Kosloski <rkosloski@visi.com> - 2012-01-10 10:01 -0600
    Re: Perplexed by the behavior of import Peter Otten <__peter__@web.de> - 2012-01-10 17:40 +0100

#18769 — Perplexed by the behavior of import

I'm perlexed by an apparent inconsistency in the behavior of the import
statement.

First, the files. There is a simple package, pkg, containing two files: mod.py
and util.py, and a stand-alone module also named util.py:

   *** ./pkg/__init__.py ***
   from mod import *
   *** ./pkg/mod.py ***
   M = 8
   *** ./pkg/util.py ***
   V = 0
   *** ./util.py ***
   from pkg import *
   from pkg.util import *
   U = 0

Next, the Python session:

   Python 2.6.4 (r264:75706, Dec 13 2009, 19:46:11)
   [GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import util
   >>> globals()
   {'__builtins__': <module '__builtin__' (built-in)>, '__name__': '__main__',
    '__doc__': None 'util': <module 'util' from 'util.pyc'>, '__package__':
    None}
   >>> from pkg import *
   >>> globals()
   {'__builtins__': <module '__builtin__' (built-in)>, 'M': 8, '__package__':
    None, 'util': <module 'pkg.util' from 'pkg/util.pyc'>, '__name__':
    '__main__', '__doc__': None,
    'mod': <module 'pkg.mod' from 'pkg/mod.pyc'>}

Compare the output of the two globals() statements:
Variable util's value has changed from <module 'util' from 'util.py'....> to
<module 'pkg.util'...>

What's happening to util?

OK, maybe pkg.util replaces the original util because of the pkg import
statement. But then, what about the following:

   Python 2.6.4 (r264:75706, Dec 13 2009, 19:46:11)
   [GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
   Type "help", "copyright", "credits" or "license" for more information.
   >>> util = 0
   >>> globals()
   {'__builtins__': <module '__builtin__' (built-in)>, '__name__': '__main__',
    '__doc__': None, 'util': 0, '__package__': None}
   >>> from pkg import *
   >>> globals()
   {'__builtins__': <module '__builtin__' (built-in)>, 'M': 8, '__package__':
    None, 'util': 0, '__name__': '__main__', '__doc__': None, 'mod': <module
    'pkg.mod' from 'pkg/mod.pyc'>}
   >>>

Now the value of util is unchanged across the pkg import statement.

Why the difference?

Thanks.

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#18772

Rod Kosloski wrote:

> I'm perlexed by an apparent inconsistency in the behavior of the import
> statement.
> 
> First, the files. There is a simple package, pkg, containing two files:
> mod.py and util.py, and a stand-alone module also named util.py:
> 
>    *** ./pkg/__init__.py ***
>    from mod import *
>    *** ./pkg/mod.py ***
>    M = 8
>    *** ./pkg/util.py ***
>    V = 0
>    *** ./util.py ***
>    from pkg import *
>    from pkg.util import *
>    U = 0
> 
> Next, the Python session:
> 
>    Python 2.6.4 (r264:75706, Dec 13 2009, 19:46:11)
>    [GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
>    Type "help", "copyright", "credits" or "license" for more information.
>    >>> import util
>    >>> globals()
>    {'__builtins__': <module '__builtin__' (built-in)>, '__name__':
>    {'__main__',
>     '__doc__': None 'util': <module 'util' from 'util.pyc'>,
>     '__package__': None}
>    >>> from pkg import *
>    >>> globals()
>    {'__builtins__': <module '__builtin__' (built-in)>, 'M': 8,
>    {'__package__':
>     None, 'util': <module 'pkg.util' from 'pkg/util.pyc'>, '__name__':
>     '__main__', '__doc__': None,
>     'mod': <module 'pkg.mod' from 'pkg/mod.pyc'>}
> 
> Compare the output of the two globals() statements:
> Variable util's value has changed from <module 'util' from 'util.py'....>
> to <module 'pkg.util'...>
> 
> What's happening to util?
> 
> OK, maybe pkg.util replaces the original util because of the pkg import
> statement. 

That's indeed what happens.

> But then, what about the following:
> 
>    Python 2.6.4 (r264:75706, Dec 13 2009, 19:46:11)
>    [GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
>    Type "help", "copyright", "credits" or "license" for more information.
>    >>> util = 0
>    >>> globals()
>    {'__builtins__': <module '__builtin__' (built-in)>, '__name__':
>    {'__main__',
>     '__doc__': None, 'util': 0, '__package__': None}
>    >>> from pkg import *
>    >>> globals()
>    {'__builtins__': <module '__builtin__' (built-in)>, 'M': 8,
>    {'__package__':
>     None, 'util': 0, '__name__': '__main__', '__doc__': None, 'mod':
>     <module 'pkg.mod' from 'pkg/mod.pyc'>}
>    >>>
> 
> Now the value of util is unchanged across the pkg import statement.
> 
> Why the difference?

As long as you haven't imported the pkg.util module there isn't an "util" 
variable in the pkg namespace:

$ python -c 'import pkg; print "util" in vars(pkg); import pkg.util; print 
"util" in vars(pkg)'
False
True

Or, sticking closer to your example:

$ python -c 'util = 0; from pkg import *; print util'
0
$ python -c 'util = 0; import pkg.util; from pkg import *; print util'
<module 'pkg.util' from 'pkg/util.pyc'>

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