Dynamic module import in Python

I'm writing a Python application that takes as a command as an argument, for example:

$ python myapp.py command1

I want the application to be extensible, that is, to be able to add new modules that implement new commands without having to change the main application source. The tree looks something like:

myapp/
    __init__.py
    commands/
        __init__.py
        command1.py
        command2.py
    foo.py
    bar.py

So I want the application to find the available command modules at runtime and execute the appropriate one.

Currently this is implemented something like:

command = sys.argv[1]
try:
    command_module = __import__("myapp.commands.%s" % command, fromlist=["myapp.commands"])
except ImportError:
    # Display error message

command_module.run()

This works just fine, I'm just wondering if there is possibly a more idiomatic way to accomplish what we are doing with this code.

Note that I specifically don't want to get in to using eggs or extension points. This is not an open-source project and I don't expect there to be "plugins". The point is to simplify the main application code and remove the need to modify it each time a new command module is added.

---

With Python older than 2.7/3.1, that's pretty much how you do it. For newer versions, see importlib.import_module for 2.7+ and for 3.1+.

You can use exec if you want to as well.

Note you can import a list of modules by doing this:

>>> moduleNames = ['sys', 'os', 're', 'unittest'] 
>>> moduleNames
['sys', 'os', 're', 'unittest']
>>> modules = map(__import__, moduleNames)

Ripped straight from Dive Into Python.

---

Python 2.7及更高版本的推荐方法是使用importlib模块：

my_module = importlib.import_module('os.path')

---

As mentioned the imp module provides you loading functions:

imp.load_source(name, path)
imp.load_compiled(name, path)

I've used these before to perform something similar.

In my case I defined a specific class with defined methods that were required. Once I loaded the module I would check if the class was in the module, and then create an instance of that class, something like this:

import imp
import os

def load_from_file(filepath):
    class_inst = None
    expected_class = 'MyClass'

    mod_name,file_ext = os.path.splitext(os.path.split(filepath)[-1])

    if file_ext.lower() == '.py':
        py_mod = imp.load_source(mod_name, filepath)

    elif file_ext.lower() == '.pyc':
        py_mod = imp.load_compiled(mod_name, filepath)

    if hasattr(py_mod, expected_class):
        class_inst = getattr(py_mod, expected_class)()

    return class_inst

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