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This function allows you to do "from module import name", where both "module" and "name" are dynamic values (i.e. expressions or variables). For example, this can be used to implement a "plugin" mechanism to extend an application by external modules that adhere to some common interface.

This pattern is used in MoinMoin (http://moin.sourceforge.net/) to load extensions implementing variations of a common interface, like "action", "macro", and "formatter".

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def importName(modulename, name):
    """ Import a named object from a module in the context of this function,
        which means you should use fully qualified module paths.

        Return None on failure.
    """
    try:
        module = __import__(modulename, globals(), locals(), [name])
    except ImportError:
        return None
        
    return vars(module)[name]

### MyApp/extensions/spam.py
class Handler:
    def handleSomething(self):
        print "spam!"

### MyApp/extensions/eggs.py
class Handler:
    def handleSomething(self):
        print "eggs!"

### Example
extension_name = "spam" # could be "eggs", too!
Handler = importName("MyApp.extensions." + extension_name, "Handler")
handler = Handler()
handler.handleSomething()

3 comments

Chris Green 17 years, 8 months ago  # | flag

Don't forget to have __init__.py. Since this module uses a __import__ call, the extensions folder will have to have an __init__.py in it so that python treats the directory as module path.

Walter Brunswick 16 years, 1 month ago  # | flag

Improve it. Could you perhaps expand this recipe to allow either all, or multiple/selected names to be imported, for example,

importModule('module',('*'))

to import all names, and

importModule('module',('func','func2','func3','class'))

to import selected names.

Also, it would be nice if the names did not have to be assigned manually, i.e., instead of

func, func2, func3, class = importModule('module',('func','func2','func3','class'))

one could do, for instance, simply

importModule('module',('func','func2','func3','class'))

and 'func', 'func2', 'func3', and 'class' would all be imported as attributes of 'module'.

Thanks.

Walter Brunswick 16 years, 1 month ago  # | flag

hmm, no, wait...

If the second argument is NOT given it should just import module with all the attributes: 'module.func', 'module.func2', 'module.func3', 'module.class', and if the second argument IS given, it should import all the names into the global scope: 'func', 'func2', 'func3', 'class'.

Created by Jürgen Hermann on Thu, 15 Mar 2001 (PSF)
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