python之importlib模块

1、import_module方法

　　解析字符串，并导入改路径的模块

# 根据字符串导入模块
# 通畅用来导入包下面的模块
o = importlib.import_module("aa.bb")
s2 = "Person"

# 由字符串找函数、方法、类  利用 反射
the_class = getattr(o, "Person")
p2 = the_class("小黑")
p2.dream()

　　从"aa.bb"路径导入bb模块。

2、import_module在Django中的使用

2.1、Django中使用import_module实现import_string方法

　　在Django源码目录"django/utils/module_loading.py",使用importlib中的import_module函数实现了Django中自定义的import_string(dotted_path)方法

　　源码：

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 as err:
        raise ImportError("%s doesn't look like a module path" % dotted_path) from err

    module = import_module(module_path)

    try:
        return getattr(module, class_name)
    except AttributeError as err:
        raise ImportError('Module "%s" does not define a "%s" attribute/class' % (
            module_path, class_name)
        ) from err

　　解析：

　　　　例如Django项目settings.py中的'django.middleware.clickjacking.XFrameOptionsMiddleware'，在dotted_path.rsplit('.', 1)后生成元组('django.middleware.clickjacking','XFrameOptionsMiddleware'),再使用import_module导入'django.middleware.clickjacking'模块，最后使用映射获取类XFrameOptionsMiddleware。

 2.2、Django中使用import_string导入模块中的类

　　在"djangocorehandlersase.py"中class BaseHandler中有load_middleware()方法用来导入settings中的中间件，其中调用了上面的import_string()方法，源码如下：

def load_middleware(self):
    """
    Populate middleware lists from settings.MIDDLEWARE.

    Must be called after the environment is fixed (see __call__ in subclasses).
    """
    self._request_middleware = []
    self._view_middleware = []
    self._template_response_middleware = []
    self._response_middleware = []
    self._exception_middleware = []

    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 str(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)

    # We only assign to this when initialization is complete as it is used
    # as a flag for initialization being complete.