如何从模块导入类列表？

Question

Is there a way to import only a specific, predefined list of classes from a Python module? For example, say that I had modules a.py and b.py in the same directory with the following code:

#a.py
class Foo(object):
    '''definition of class Foo goes here'''

class Bar(object):
    '''definition of class Bar goes here'''

aTypes = [Foo, Bar]

_

#b.py
from a import aTypes

print Foo

Running b.py causes, of course, the print Foo line to raise a NameError . I didn't really think that this would work (the import statement in b.py gives me an a.Foo type instead of a Foo type), but I can't figure out the right syntax. Is there an alternative to from a import aTypes that gives the desired behavior?

Answer 1

You could import the names directly:

from a import Foo, Bar

Alternately, you could define __all__ in a.py :

__all__ = ['Foo', 'Bar']

and then do a wildcard import in b.py :

from a import *

For just two names though, it would be easier to use the first solution.

Answer 2

Supposing you really do need to do this (deal with list of types) and can't use tricks like __all__ that only work once (one such special list per module), then:

def gettypes(types):
    thismodule = sys.modules[__name__]
    for t in types:
        setattr(thismodule, t.__name__, t)

Use it like this:

import a
gettypes(a.aTypes)

I'm finding it difficult to imagine why you'd need this, but that's not my problem ;-)

Note the use of __name__ means this doesn't work for symbols in general, only for classes, modules, functions and anything else I've forgotten (or at a pinch for objects of your own design that have a __name__ ). It also won't work for things you've aliased in the module, for example by writing Baz = Bar in the module and then using Baz in the list. If the list was ['Foo', 'Bar'] instead of [Foo, Bar] , and you also passed the source module into gettypes , then you could avoid those restrictions.

Answer 3

You can define __all__ at the module level of a.py :

__all__ = ['Foo', 'Bar']

This will allow you to import it in the following way:

from a import *

Of course, this behaviour isn't always encouraged, and the alternative is preferred:

from a import Foo, Bar