David Janes' Code Weblog

The normal way to load Python code is through the import statement:

import pprint
pprint.pprint('Hello, world.')

But what do you do if you want to dynamically load a module? A classic example of where you’d like to do this is adding ‘extensions’ to your application. Your application has no way of knowing the exact name of the module that it’s going to use; it only knows the filename(s). The way to do this is the imp module:

import md5
import os.path
import imp
import traceback

def load_module(code_path):
    try:
        try:
            code_dir = os.path.dirname(code_path)
            code_file = os.path.basename(code_path)

            fin = open(code_path, 'rb')

            return  imp.load_source(md5.new(code_path).hexdigest(), code_path, fin)
        finally:
            try: fin.close()
            except: pass
    except ImportError, x:
        traceback.print_exc(file = sys.stderr)
        raise
    except:
        traceback.print_exc(file = sys.stderr)
        raise

A few notes:

• call load_module with the path to a .py file that you want to load
• the md5.new generates a unique module identifier. If you don’t do this it’s difficult to import two modules in different directories with the same name!
• the different excepts are to give you a flavor of the issues you may see, ImportError is expected, the others are not

The return value is a module, which is a Python object that you can address in all the normal ways that you’d use a module. For example, if you have the following file extension.py:

def hello(x): print "Hello, %s" % x

You can use it as follows to get Hello, world.

m = load_module('extension.py')
m.hello("World")