Tornado源码分析1-Configurable

class Configurable(object):    """    从下面的注释可以看出Configurable为工厂函数的实现提供了便利，用户可以通过调用configure或者其他设置获取所需要的类实例。    """    """Base class for configurable interfaces.    A configurable interface is an (abstract) class whose constructor    acts as a factory function for one of its implementation subclasses.    The implementation subclass as well as optional keyword arguments to    its initializer can be set globally at runtime with `configure`.    By using the constructor as the factory method, the interface    looks like a normal class, `isinstance` works as usual, etc.  This    pattern is most useful when the choice of implementation is likely    to be a global decision (e.g. when `~select.epoll` is available,    always use it instead of `~select.select`), or when a    previously-monolithic class has been split into specialized    subclasses.    Configurable subclasses must define the class methods    `configurable_base` and `configurable_default`, and use the instance    method `initialize` instead of ``__init__``.    """    __impl_class = None   # 具化且已实现的类    __impl_kwargs = None    """这是一个用户可以调用的函数，cls需要是当前类A或其子类，在A.configure(cls)后，以后的A()会返回cls的实例"""    @classmethod    def configure(cls, impl, **kwargs):        """Sets the class to use when the base class is instantiated.        Keyword arguments will be saved and added to the arguments passed        to the constructor.  This can be used to set global defaults for        some parameters.        """        base = cls.configurable_base()        if isinstance(impl, (unicode_type, bytes)):            impl = import_object(impl)        if impl is not None and not issubclass(impl, cls):            raise ValueError("Invalid subclass of %s" % cls)        # 指定__impl_class        base.__impl_class = impl        base.__impl_kwargs = kwargs    """返回当前指定的具化cls"""    @classmethod    def configured_class(cls):        """Returns the currently configured class."""        base = cls.configurable_base()        if cls.__impl_class is None:            base.__impl_class = cls.configurable_default()        return base.__impl_class    # 可以通过重载此方法来配置具化类的生成，参考IOLoop的函数    @classmethod    def configurable_default(cls):        pass    def __new__(cls, *args, **kwargs):        base = cls.configurable_base()        init_kwargs = {}        if cls is base:            impl = cls.configured_class()            if base.__impl_kwargs:                init_kwargs.update(base.__impl_kwargs)        else:            impl = cls        init_kwargs.update(kwargs)        instance = super(Configurable, cls).__new__(impl)        # initialize vs __init__ chosen for compatibility with AsyncHTTPClient        # singleton magic.  If we get rid of that we can switch to __init__        # here too.        instance.initialize(*args, **init_kwargs)        return instance

IOLoop的configurable_default实现

    @classmethod    def configurable_default(cls):        #从当前的操作系统环境给出具体的IOLoop类        if hasattr(select, "epoll"):            from tornado.platform.epoll import EPollIOLoop            return EPollIOLoop        if hasattr(select, "kqueue"):            # Python 2.6+ on BSD or Mac            from tornado.platform.kqueue import KQueueIOLoop            return KQueueIOLoop        from tornado.platform.select import SelectIOLoop        return SelectIOLoop