Why can't I "from module import *" except at module level?

[Steven D'Aprano]

On Fri, 13 Jan 2006 10:17:32 -0800, Mudcat wrote:

> I have a directory structure that contains different modules that run
> depending on what the user selects. They are identical in name and
> structure, but what varies is the content of the functions. They will
> only need to be run once per execution.
...
> So if the user selects Sys1 during execution, I want to import the
> modules using "from *" and the run the content of those files.
> 
> Now the way my program is set up, it is very important that I be able
> to "from Sys1 import *" and not "import Sys1". 

Then change the way your program is set up :-)

Seriously, "from module import *" is generally a bad idea. Not always, but
generally, and in this specific instance I'm not sure that your use case
is one of those exceptions. However, be that as it may be, on to solving
your problem:

When you "import foo" or "from module import foo", the name foo is bound
in the local scope, not global. In other words, the name foo will then
only exist inside the function. So this does not work:

>>> def local_import():
...     import string
...
>>> local_import()
>>> string.whitespace
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'string' is not defined


However this does work as you would expect:

>>> def func_import():
...     global math
...     import math
...
>>> func_import()
>>> math.sin
<built-in function sin>


See http://docs.python.org/ref/import.html for more detail on what happens
when you import. Keep in mind that it states that "from module import *"
in function blocks is explicitly stated to be an error, and in Python 2.3
a warning is raised:


>>> def wild_import():
...     from math import *
...
<stdin>:1: SyntaxWarning: import * only allowed at module level




> The names of the
> functions inside are passed in from somewhere else and the program
> requires those function names to be globally scoped.
> 
> So I have a function that looks like this:
> 
> def importModules( type ):
>     cwd = os.getcwd()
>     path = cwd + "\\" + type
>     sys.path.append(path)
> 
>     from security import *
> 
> Obviously this is not working and I get a syntax error at runtime.

Should security be defined somewhere?

> So
> without this functionality, how do I target modules to import in other
> directories after program execution has begun?

I'd consider putting your import logic into a module of its own, at
the module-level and not inside a function. Then just call "from importer
import *" in the top level of your code and I think that should meet your
needs.



-- 
Steven.