Why doesn't `from pkg import mod' work after `del pkg.mod'?

[Piet van Oostrum]

>>>>> ryles <rylesny at gmail.com> (r) wrote:

>r> According to http://www.python.org/doc/essays/packages.html:
>r> "The import statement first tests whether the item is defined in the
>r> package; if not, it assumes it is a module and attempts to load it."

>r> However, I've noticed that once a module is imported using the
>r> `from pkg import mod' syntax, if its name is deleted from the
>r> package namespace then subsequent imports with `from' will fail.

This is incorrectly stated. Also on the initial import it will fail, not
just on subsequent imports.

piet$ python
Python 2.6.2 (r262:71600, Apr 16 2009, 09:17:39) 
[GCC 4.0.1 (Apple Computer, Inc. build 5250)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from pkg import _impl
_impl # this is from a print statement in _impl to show that it did import
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: cannot import name _impl
>>> 

According to the documentation
<http://docs.python.org/library/functions.html#__import__> the statement
from pkg import _impl

_temp = __import__('pkg', globals(), locals(), ['_impl'], -1)
_impl = _temp._impl

This fails because _temp (the imported module) doesn't have a binding
for _impl because you deleted it. By the way, if you have a `normal'
package that doesn't do anything with the name _impl, after the import
the pkg module namespace will have got a binding for _impl although it is not
in your code. It will have been put there by the import code.
In the following example pkg2/__init__.py just contains the line 
A = 2

piet$ python
Python 2.6.2 (r262:71600, Apr 16 2009, 09:17:39) 
[GCC 4.0.1 (Apple Computer, Inc. build 5250)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from pkg2 import _impl
_impl
>>> import sys
>>> dir(sys.modules['pkg2'])
['A', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__', '_impl']
>>> 

It is not clear to me, however what the order is of the statements in
__init__.py and the insertion of _impl in the module namespace.
-- 
Piet van Oostrum <piet at cs.uu.nl>
URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4]
Private email: piet at vanoostrum.org