Writing a python plugin API/Architecture

The first time I attempted to write a plugin API was for a C++ project. Long story short, the project didn’t have one when it was released (and never did). Since then I was always scared of writing one in any language, but since my python project (relinux) required one, I decided to try. It was actually quite a lot easier than I expected, since all that was required to do was to get a list of plugins and to load them each into a variable.

We’ll start by creating the “header” of the plugin loader

import imp
import os

PluginFolder = "./plugins"
MainModule = "__init__"

The imp module is needed for finding and importing the plugins, and the rest should be self-explanatory.
Next we’ll create a function for finding plugins

def getPlugins():
    plugins = []
    possibleplugins = os.listdir(PluginFolder)
    for i in possibleplugins:
        location = os.path.join(PluginFolder, i)
        if not os.path.isdir(location) or not MainModule + ".py" in os.listdir(location):
            continue
        info = imp.find_module(MainModule, [location])
        plugins.append({"name": i, "info": info})
    return plugins

Most of this code should be self-explanatory. It simply looks through the directories in the plugin folder, and tries to find MainModule.py in each of them. If it finds the MainModule module, it will load information about it using imp.find_module (this information is used by imp’s module loader).
All that is left now is to add the plugin loading function

def loadPlugin(plugin):
    return imp.load_module(MainModule, *plugin["info"])

This function is simple enough (the asterisk simply extends the “info” list into arguments). The module returned can be used like any module object as it’s simply a namespace.

Here is the full code of the plugin loader:

import imp
import os

PluginFolder = "./plugins"
MainModule = "__init__"

def getPlugins():
    plugins = []
    possibleplugins = os.listdir(PluginFolder)
    for i in possibleplugins:
        location = os.path.join(PluginFolder, i)
        if not os.path.isdir(location) or not MainModule + ".py" in os.listdir(location):
            continue
        info = imp.find_module(MainModule, [location])
        plugins.append({"name": i, "info": info})
    return plugins

def loadPlugin(plugin):
    return imp.load_module(MainModule, *plugin["info"])

Here is an example of using this module (assuming you named it “pluginloader”)

for i in pluginloader.getPlugins():
    print("Loading plugin " + i["name"])
    plugin = pluginloader.loadPlugin(i)
    plugin.run()

Here is a sample plugin (in ./plugins/hello/__init__.py):

def run():
    print("Hello from a plugin!")

Now, of course, this plugin API is very simple, and can easily (and should) be extended for your program’s needs.