How can I discover classes in a specific package in python?

Question

I have a package of plug-in style modules. It looks like this:

/Plugins 
/Plugins/__init__.py
/Plugins/Plugin1.py
/Plugins/Plugin2.py 
etc...

Each .py file contains a class that derives from PluginBaseClass. So I need to list every module in the Plugins package and then search for any classes that implement PluginBaseClass. Ideally I want to be able to do something like this:

for klass in iter_plugins(project.Plugins):
    action = klass()
    action.run()

I have seen some other answers out there, but my situation is different. I have an actual import to the base package (ie: import project.Plugins) and I need to find the classes after discovering the modules.

Answer 1

If you don't know what's going to be in Plugins ahead of time, you can get a list of python files in the package's directory, and import them like so:

# compute a list of modules in the Plugins package

import os
import Plugins
plugin_modules = [f[:-3] for f in os.listdir(os.path.dirname(Plugins.__file__)) 
                  if f.endswith('.py') and f != '__init__.py']

Sorry, that comprehension might be a mouthful for someone relatively new to python. Here's a more verbose version (might be easier to follow):

plugin_modules = []

package_path = Plugins.__file__
file_list = os.listdir(os.path.dirname(package_path))
for file_name in file_list:
    if file_name.endswith('.py') and file_name != '__init__.py':
        plugin_modules.append(file_name)

Then you can use __import__ to get the module:

# get the first one
plugin = __import__('Plugins.' + plugin_modules[0])

Answer 2

Edit: here's a revised solution. I realised I was making a mistake while testing my previous one, and it doesn't really work the way you would expect. So here is a more complete solution:

import os
from imp import find_module
from types import ModuleType, ClassType

def iter_plugins(package):
    """Receives package (as a string) and, for all of its contained modules,
    generates all classes that are subclasses of PluginBaseClass."""

    # Despite the function name, "find_module" will find the package
    # (the "filename" part of the return value will be None, in this case)
    filename, path, description = find_module(package)

    # dir(some_package) will not list the modules within the package,
    # so we explicitly look for files. If you need to recursively descend
    # a directory tree, you can adapt this to use os.walk instead of os.listdir
    modules =  sorted(set(i.partition('.')[0]
                          for i in os.listdir(path)
                          if i.endswith(('.py', '.pyc', '.pyo'))
                          and not i.startswith('__init__.py')))
    pkg = __import__(package, fromlist=modules)
    for m in modules:
        module = getattr(pkg, m)
        if type(module) == ModuleType:  
            for c in dir(module):
                klass = getattr(module, c)
                if (type(klass) == ClassType and
                    klass is not PluginBaseClass and
                    issubclass(klass, PluginBaseClass)):
                    yield klass

My previous solution was:

You could try something like:

from types import ModuleType
import Plugins

classes = []
for item in dir(Plugins):
    module = getattr(Plugins, item)
    # Get all (and only) modules in Plugins
    if type(module) == ModuleType:
        for c in dir(module):
            klass = getattr(module, c)
            if isinstance(klass, PluginBaseClass):
                classes.append(klass)

Actually, even better, if you want some modularity:

from types import ModuleType

def iter_plugins(package):
    # This assumes "package" is a package name.
    # If it's the package itself, you can remove this __import__
    pkg = __import__(package)
    for item in dir(pkg):
        module = getattr(pkg, item)
        if type(module) == ModuleType:  
            for c in dir(module):
                klass = getattr(module, c)
                if issubclass(klass, PluginBaseClass):
                    yield klass

Answer 3

You may (and probably should) define __all__ in __init__.py as a list of the submodules in your package; this is so that you support people doing from Plugins import *. If you have done so, you can iterate over the modules with

import Plugins
import sys
modules = { }
for module in Plugins.__all__:
    __import__( module )
    modules[ module ] = sys.modules[ module ]
    # iterate over dir( module ) as above

The reason another answer posted here fails is that __import__ imports the lowest-level module, but returns the top-level one (see the docs). I don't know why.

Answer 4

Scanning modules isn't good idea. If you need class registry you should look at metaclasses or use existing solutions like zope.interface. Simple solution through metaclasses may look like that:

from functools import reduce
class DerivationRegistry(type):
    def __init__(cls,name,bases,cls_dict):
        type.__init__(cls,name,bases,cls_dict)
        cls._subclasses = set()
        for base in bases:
            if isinstance(base,DerivationRegistry):
                base._subclasses.add(cls)

    def getSubclasses(cls):
        return reduce( set.union,
                       ( succ.getSubclasses() for succ  in cls._subclasses if isinstance(succ,DerivationRegistry)),
                       cls._subclasses)

class Base(object):
    __metaclass__ = DerivationRegistry

class Cls1(object):
    pass

class Cls2(Base):
    pass

class Cls3(Cls2,Cls1):
    pass

class Cls4(Cls3):
    pass

print(Base.getSubclasses())