python之以字符串形式导入模块

示例

调用方法

class CorsMiddleware:
    def process(self):
        print('from auth.cors.CorsMiddleware.process')

import importlib

path = 'auth.cors.CorsMiddleware'
module_path, class_name = path.rsplit('.', maxsplit=1)
module = importlib.import_module(module_path)
cls = getattr(module, class_name)
obj = cls()
obj.process()

# from auth.cors.CorsMiddleware.process

获取静态字段

class Dev:
    DEBUG = True

import importlib

path = 'settings.Dev'
module_path, class_name = path.rsplit('.', maxsplit=1)
module = importlib.import_module(module_path)
cls = getattr(module, class_name)
for key in dir(cls):
    if key.isupper():
        print(getattr(cls,key)) # True

源码

其实很多在配置文件中通过类路径配置类的操作都是这样实现的，例如 Django 中在执行 wsgi 模块时加载配置文件：

  def load_middleware(self):
        self._request_middleware = []
        self._view_middleware = []
        self._template_response_middleware = []
        self._response_middleware = []
        self._exception_middleware = []

        if settings.MIDDLEWARE is None:
            warnings.warn(
                "Old-style middleware using settings.MIDDLEWARE_CLASSES is "
                "deprecated. Update your middleware and use settings.MIDDLEWARE "
                "instead.", RemovedInDjango20Warning
            )
            handler = convert_exception_to_response(self._legacy_get_response)
            for middleware_path in settings.MIDDLEWARE_CLASSES:
                mw_class = import_string(middleware_path)
                try:
                    mw_instance = mw_class()
                except MiddlewareNotUsed as exc:
                    if settings.DEBUG:
                        if six.text_type(exc):
                            logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
                        else:
                            logger.debug('MiddlewareNotUsed: %r', middleware_path)
                    continue

                if hasattr(mw_instance, 'process_request'):
                    self._request_middleware.append(mw_instance.process_request)
                if hasattr(mw_instance, 'process_view'):
                    self._view_middleware.append(mw_instance.process_view)
                if hasattr(mw_instance, 'process_template_response'):
                    self._template_response_middleware.insert(0, mw_instance.process_template_response)
                if hasattr(mw_instance, 'process_response'):
                    self._response_middleware.insert(0, mw_instance.process_response)
                if hasattr(mw_instance, 'process_exception'):
                    self._exception_middleware.insert(0, mw_instance.process_exception)
        else:
            handler = convert_exception_to_response(self._get_response)
            for middleware_path in reversed(settings.MIDDLEWARE):
                middleware = import_string(middleware_path)
                try:
                    mw_instance = middleware(handler)
                except MiddlewareNotUsed as exc:
                    if settings.DEBUG:
                        if six.text_type(exc):
                            logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
                        else:
                            logger.debug('MiddlewareNotUsed: %r', middleware_path)
                    continue

                if mw_instance is None:
                    raise ImproperlyConfigured(
                        'Middleware factory %s returned None.' % middleware_path
                    )

                if hasattr(mw_instance, 'process_view'):
                    self._view_middleware.insert(0, mw_instance.process_view)
                if hasattr(mw_instance, 'process_template_response'):
                    self._template_response_middleware.append(mw_instance.process_template_response)
                if hasattr(mw_instance, 'process_exception'):
                    self._exception_middleware.append(mw_instance.process_exception)

                handler = convert_exception_to_response(mw_instance)
        self._middleware_chain = handler

def import_string(dotted_path):
    """
    Import a dotted module path and return the attribute/class designated by the
    last name in the path. Raise ImportError if the import failed.
    """
    try:
        module_path, class_name = dotted_path.rsplit('.', 1)
    except ValueError:
        msg = "%s doesn't look like a module path" % dotted_path
        six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])

    module = import_module(module_path)

    try:
        return getattr(module, class_name)
    except AttributeError:
        msg = 'Module "%s" does not define a "%s" attribute/class' % (
            module_path, class_name)
        six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])