[Python-Dev] distutils.sys: None in sys.modules

[M.-A. Lemburg]

Ka-Ping Yee wrote:
> 
> On Tue, 10 Apr 2001, M.-A. Lemburg wrote:
> > > What are 'distutils.sys', 'distutils.os', 'distutils.string',
> > > 'distutils.re', 'distutils.distutils' doing in there?
> > > (The sys.modules dictionary maps all these keys to None.)
> >
> > This basically means that the corresponding modules have already
> > been loaded at top-level.
> 
> But there's no 'sys' module in the distutils package.

The None entry is used to cache the import miss. Please see
Python/import.c for details (function mark_miss).

> If there were one, it would be called 'distutils.sys'
> everywhere, even within the distutils package, since
> we decided that packages would always use absolute
> module paths, right?
> 
> This behaviour seems quite confusing to me:
> 
>     localhost[1]% ls -al foo
>     total 9
>     drwxr-xr-x   2 ping     users        1024 Apr 10 04:50 ./
>     drwxr-xr-x  12 ping     users        5120 Apr 10 04:49 ../
>     -rw-r--r--   1 ping     users           0 Apr 10 04:49 __init__.py
>     -rw-r--r--   1 ping     users         106 Apr 10 04:50 __init__.pyc
>     -rw-r--r--   1 ping     users          50 Apr 10 04:50 sys.py
>     -rw-r--r--   1 ping     users         216 Apr 10 04:50 sys.pyc
>     localhost[2]% cat foo/sys.py
>     import sys, os
> 
>     print 'here is foo.sys'
> 
>     blah = 1
>     localhost[3]% python -S
>     Python 2.1b2 (#28, Apr 10 2001, 02:49:05)
>     [GCC egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)] on linux2
>     Type "copyright", "credits" or "license" for more information.
>     >>> import sys, foo
>     >>> sys.modules.keys()
>     ['__main__', '__builtin__', 'sys', 'foo', 'signal', 'exceptions']
>     >>> import foo.sys
>     here is foo.sys
>     >>> sys.modules.keys()
>     ['os.path', 'os', 'foo', 'foo.sys', 'exceptions', '__main__', 'foo.os', 'posix', 'sys', '__builtin__', 'signal', 'UserDict', 'posixpath', 'stat']
>     >>> sys.modules['foo.os']
>     >>> sys.modules['foo.sys']
>     <module 'foo.sys' from '/home/ping/python/foo/sys.py'>
>     >>> import foo.os
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in ?
>     ImportError: no module named 'os' could be found
>     >>> import foo.sys
> 
> At this point sys.modules['foo.sys'] is a real module, as it should
> be, but sys.modules['foo.os'] is None.  I don't see why 'foo.os'
> should be present at all.

-- 
Marc-Andre Lemburg
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