如何在python中发现特定包中的类？

Question

我有一个插件式模块包.它看起来像这样:

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

每个.py文件都包含一个派生自的类PluginBaseClass.所以我需要列出Plugins包中的每个模块,然后搜索任何实现的类PluginBaseClass.理想情况下,我希望能够做到这样的事情:

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

我已经看到了其他一些答案,但我的情况有所不同.我有一个实际的导入基础包(即:) import project.Plugins,我需要在发现模块后找到类.

Answer 1

编辑：这是一个修改后的解决方案。我意识到我在测试前一个时犯了一个错误，而且它并没有真正按照您期望的方式工作。所以这里有一个更完整的解决方案：

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

我之前的解决方案是：

你可以尝试这样的事情：

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)

实际上，如果你想要一些模块化，那就更好了：

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 2

你可以(并且可能应该)定义__all__在__init__.py为你的包的子模块的列表; 这是为了让你支持人们这样做from Plugins import *.如果您已经这样做,您可以使用迭代模块

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

这里发布的另一个答案失败的原因是__import__导入最低级别的模块,但返回顶级模块(请参阅文档).我不知道为什么.

Answer 3

扫描模块不是一个好主意。如果您需要类注册表，您应该查看元类或使用现有的解决方案，例如zope.interface。通过元类的简单解决方案可能如下所示：

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())