python metaprogramming

Question

I'm trying to archive a task which turns out to be a bit complicated since I'm not very good at Python metaprogramming.

I want to have a module locations with function get_location(name), which returns a class defined in a folder locations/ in the file with the name passed to function. Name of a class is something like NameLocation.

So, my folder structure:

program.py
locations/
    __init__.py
    first.py
    second.py

program.py will be smth with with:

from locations import get_location
location = get_location('first')

and the location is a class defined in first.py smth like this:

from locations import Location # base class for all locations, defined in __init__ (?)
class FirstLocation(Location):
    pass

etc.

Okay, I've tried a lot of import and getattribute statements but now I'm bored and surrender. How to archive such behaviour?

I don't know why, but this code

def get_location(name):
   module = __import__(__name__ + '.' + name)
   #return getattr(module, titlecase(name) + 'Location')
   return module

returns

>>> locations.get_location( 'first')
<module 'locations' from 'locations/__init__.py'>

the locations module! why?!

This idea is painful just to read. Are you really sure you want to go this route? — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Mar 20, 2010 at 5:29
I need to be able to do something like in my program.py example. If there is another route, just let me know! :) Sorry for painful reading, my English is not very good. — Valentin Golev
– Valentin Golev, Commented Mar 20, 2010 at 5:31
It's not so much the English. It's the idea. PLease explain why you need this. Also what does "smth" mean? Please use backticks ` around Python variables with _ in them. — S.Lott
– S.Lott, Commented Mar 20, 2010 at 12:30

Alex Martelli · Accepted Answer · 2010-03-20 06:05:08Z

6

You do need to __import__ the module; after that, getting an attr from it is not hard.

import sys

def get_location(name):
    fullpath = 'locations.' + name
    package = __import__(fullpath)
    module = sys.modules[fullpath]
    return getattr(module, name.title() + 'Location')

Edit: __import__ returns the package, so you need one more getattr, see the docs (and read all the section carefully -- "do as I say not as I do";-).

edited Mar 20, 2010 at 6:05

answered Mar 20, 2010 at 5:32

Alex Martelli

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4 Comments

Valentin Golev Over a year ago

(where was no titlecase method so I've used someone's snippet) I got AttributeError: 'module' object has no attribute 'FirstLocation'

Valentin Golev Over a year ago

please look at my question, I've added additional (painful) info

Valentin Golev Over a year ago

brilliant!!! thanks. (again, str has no titlecase method, or i don't know about it)

Alex Martelli Over a year ago

It's just .title(), not .titlecase() -- editing to fix, sorry.

MikeyB · Accepted Answer · 2010-03-20 05:30:32Z

1

I think you're looking for:

location = __import__('first')

answered Mar 20, 2010 at 5:30

MikeyB

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2 Comments

Valentin Golev Over a year ago

ImportError: No module named first, when I'm returning the location from get_location with your code

MikeyB Over a year ago

Well, adapt it to whatever the heck you're trying to do, but that's how you import a module using a string as the name.

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