alex23 <wuwe...@gmail.com> wrote:
> I must confess I've rarely had a need to use __import__ and don't
> think I've ever used the fromlist arg. I'm confused, though, because
> the docstring states:
>
> The fromlist should be a list of names to emulate ``from name
> import ...''
>
> But it also states that __import__ always returns a module, so I'm
> utterly confused as to the purpose of fromlist, or how to inject the
> specified entries into the calling namespace. If anyone could explain
> this for me, I'd really appreciate it.
Compare these:
>>> dom = __import__('xml').dom
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'dom'
>>> dom = __import__('xml', fromlist=['dom']).dom
>>> dom
<module 'xml.dom' from 'C:\Python26\lib\xml\dom\__init__.pyc'>
Then in a new session:
>>> import xml
>>> xml.dom
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'dom'
>>> from xml import dom
>>> dom
<module 'xml.dom' from 'C:\Python26\lib\xml\dom\__init__.pyc'>
>>>
When you import a package such as xml it only imports the top level.
Modules and subpackages within the imported package aren't available until
they are explicitly imported. The fromlist argument to __import__ allows
you to force the lower modules to also import.
>>> xml = __import__('xml', fromlist=['dom'])
is effectively the same as doing:
>>> import xml.dom
After either of these you have a name 'xml' which has an attribute 'dom':
>>> xml.dom
<module 'xml.dom' from 'C:\Python26\lib\xml\dom\__init__.pyc'>
To access the actual sub-objects using __import__ use getattr on the
returned module. So far as injecting names into the calling namespace is
concerned just assign to variables; you don't want to be injecting unknown
names into your namespace.
For the same effect if you only have one name and you know it is a module
you could do:
>>> xml = __import__('xml.dom')
but you need the fromlist parameter if you don't know which of the names
are modules.
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