Scrapy源码分析（二）：Setting相关类定义

Scrapy中的BaseSetting的行为是一个包含优先级的字典，当多次set一个key时，value中只保存优先级最大的一个。

首先看几个辅助的元素，优先级字典：

SETTINGS_PRIORITIES = {
    'default': 0,
    'command': 10,
    'project': 20,
    'spider': 30,
    'cmdline': 40,
}

对于优先级字典的包装函数，如果是字符串型参数，则返回dict中对应的值，否则返回参数本身（当做是int型了）：

def get_settings_priority(priority):
    """
    Small helper function that looks up a given string priority in the
    :attr:`~scrapytest.settings.SETTINGS_PRIORITIES` dictionary and returns its
    numerical value, or directly returns a given numerical priority.
    """
    if isinstance(priority, six.string_types):
        return SETTINGS_PRIORITIES[priority]
    else:
        return priority

BaseSetting中的属性类，考虑到了value本身是BaseSettings类实例的情况，需要取较高的优先级：

class SettingsAttribute(object):

    """Class for storing data related to settings attributes.

    This class is intended for internal usage, you should try Settings class
    for settings configuration, not this one.
    """

    def __init__(self, value, priority):
        self.value = value
        if isinstance(self.value, BaseSettings):
            self.priority = max(self.value.maxpriority(), priority)
        else:
            self.priority = priority

    def set(self, value, priority):
        """Sets value if priority is higher or equal than current priority."""
        if priority >= self.priority:
            if isinstance(self.value, BaseSettings):
                value = BaseSettings(value, priority=priority)
            self.value = value
            self.priority = priority

    def __str__(self):
        return "<SettingsAttribute value={self.value!r} " \
               "priority={self.priority}>".format(self=self)

    __repr__ = __str__

接下来是重头戏，BaseSetting类，这个类的特点类似于内置dict型，但是可以为属性设置优先级，并且冻结全部属性，使其无法被修改。

class BaseSettings(MutableMapping):
    """
    Instances of this class behave like dictionaries, but store priorities
    along with their ``(key, value)`` pairs, and can be frozen (i.e. marked
    immutable).

    Key-value entries can be passed on initialization with the ``values``
    argument, and they would take the ``priority`` level (unless ``values`` is
    already an instance of :class:`~scrapytest.settings.BaseSettings`, in which
    case the existing priority levels will be kept).  If the ``priority``
    argument is a string, the priority name will be looked up in
    :attr:`~scrapytest.settings.SETTINGS_PRIORITIES`. Otherwise, a specific integer
    should be provided.

    Once the object is created, new settings can be loaded or updated with the
    :meth:`~scrapytest.settings.BaseSettings.set` method, and can be accessed with
    the square bracket notation of dictionaries, or with the
    :meth:`~scrapytest.settings.BaseSettings.get` method of the instance and its
    value conversion variants. When requesting a stored key, the value with the
    highest priority will be retrieved.
    """

    def __init__(self, values=None, priority='project'):
        self.frozen = False
        self.attributes = {}
        self.update(values, priority)

    def __getitem__(self, opt_name):
        if opt_name not in self:
            return None
        return self.attributes[opt_name].value

    def __contains__(self, name):
        return name in self.attributes

    def get(self, name, default=None):
        """
        Get a setting value without affecting its original type.

        :param name: the setting name
        :type name: string

        :param default: the value to return if no setting is found
        :type default: any
        """
        return self[name] if self[name] is not None else default

    def getbool(self, name, default=False):
        """
        Get a setting value as a boolean.

        ``1``, ``'1'``, and ``True`` return ``True``, while ``0``, ``'0'``,
        ``False`` and ``None`` return ``False``.

        For example, settings populated through environment variables set to
        ``'0'`` will return ``False`` when using this method.

        :param name: the setting name
        :type name: string

        :param default: the value to return if no setting is found
        :type default: any
        """
        return bool(int(self.get(name, default)))

    def getint(self, name, default=0):
        """
        Get a setting value as an int.

        :param name: the setting name
        :type name: string

        :param default: the value to return if no setting is found
        :type default: any
        """
        return int(self.get(name, default))

    def getfloat(self, name, default=0.0):
        """
        Get a setting value as a float.

        :param name: the setting name
        :type name: string

        :param default: the value to return if no setting is found
        :type default: any
        """
        return float(self.get(name, default))

    def getlist(self, name, default=None):
        """
        Get a setting value as a list. If the setting original type is a list, a
        copy of it will be returned. If it's a string it will be split by ",".

        For example, settings populated through environment variables set to
        ``'one,two'`` will return a list ['one', 'two'] when using this method.

        :param name: the setting name
        :type name: string

        :param default: the value to return if no setting is found
        :type default: any
        """
        value = self.get(name, default or [])
        if isinstance(value, six.string_types):
            value = value.split(',')
        return list(value)

    def getdict(self, name, default=None):
        """
        Get a setting value as a dictionary. If the setting original type is a
        dictionary, a copy of it will be returned. If it is a string it will be
        evaluated as a JSON dictionary. In the case that it is a
        :class:`~scrapytest.settings.BaseSettings` instance itself, it will be
        converted to a dictionary, containing all its current settings values
        as they would be returned by :meth:`~scrapytest.settings.BaseSettings.get`,
        and losing all information about priority and mutability.

        :param name: the setting nam