Bottle源码阅读（二）

在《Bottle源码阅读（一）》中，我们了解了bottle如何接收请求，处理请求以及如何检测模块变化重启server。在ServerHandler类中的run函数中，application接受了两个参数，一个envrion， 和一个start_response的方法，接下来我们就来了解我们写的应用函数是如何被封装成适配的application。

1.上层调用

先看一个简单的例子

from bottle import get, run
@get('/hello')
def hello():
    return "hello"

run(host='localhost', port=8080)

通过get装饰器，使requestline ’/hello‘ 路由到hello函数，并将函数返回的结果通过WSGIRequestHandler 中的wfile返回。

接下来我们看一下get源码是怎么实现的

route     = make_default_app_wrapper('route')
# 经过封装后，get最终获得的是具备有Bottle.get一些属性的装饰器
get       = make_default_app_wrapper('get')
post      = make_default_app_wrapper('post')
put       = make_default_app_wrapper('put')
delete    = make_default_app_wrapper('delete')
error     = make_default_app_wrapper('error')
mount     = make_default_app_wrapper('mount')
hook      = make_default_app_wrapper('hook')
install   = make_default_app_wrapper('install')
uninstall = make_default_app_wrapper('uninstall')
url       = make_default_app_wrapper('get_url')

def make_default_app_wrapper(name):
    ''' Return a callable that relays calls to the current default app. '''
    
    # 当name为'get'时，将Bottle.get的属性传递给wrapper，使其具备相同的属性。
    @functools.wraps(getattr(Bottle, name))
    def wrapper(*a, **ka):
        # 使用默认的app， 也就是说，应用函数的装饰器实际是Bottle.get(*a, **ka)
        return getattr(app(), name)(*a, **ka)
    return wrapper
    

app = default_app = AppStack()
app.push()
class AppStack(list):
    """ A stack-like list. Calling it returns the head of the stack. """

    def __call__(self):
        """ Return the current default application. """
        return self[-1]

    def push(self, value=None):
        """ Add a new :class:`Bottle` instance to the stack """
        if not isinstance(value, Bottle):
            value = Bottle()
        self.append(value)
        return value

我们可以顺便看一下wraps和update_wrapper的源码， wraps实际调用了partial函数（C写的),update_wrapper保留了被装饰函数原来的属性('__module__', '__name__', '__doc__')

def update_wrapper(wrapper,
                   wrapped,
                   assigned = WRAPPER_ASSIGNMENTS,
                   updated = WRAPPER_UPDATES):
    for attr in assigned:
        setattr(wrapper, attr, getattr(wrapped, attr))
    for attr in updated:
        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
    # Return the wrapper so this can be used as a decorator via partial()
    return wrapper

def wraps(wrapped,
          assigned = WRAPPER_ASSIGNMENTS,
          updated = WRAPPER_UPDATES):
    return partial(update_wrapper, wrapped=wrapped,
                   assigned=assigned, updated=updated)

2.Bottle类中的应用函数装饰器

由上述我们可以知道应用函数装饰器@get(’/hello‘)，实际为Bottle.get(a, *ka) 。get实际调用route，hello函数作为callback， ’/hello‘作为path， 并将Route实例后添加进self.route，当接收请求时再由self.match调用self.router.match(environ)返回对应的Route实例，并将callback执行结果响应返回

    def get(self, path=None, method='GET', **options):
        """ Equals :meth:`route`. """
        return self.route(path, method, **options)
        
    def route(self, path=None, method='GET', callback=None, name=None,
              apply=None, skip=None, **config):

        if callable(path): path, callback = None, path
        plugins = makelist(apply)
        skiplist = makelist(skip)
        def decorator(callback):
            # TODO: Documentation and tests
            if isinstance(callback, basestring): callback = load(callback)
            for rule in makelist(path) or yieldroutes(callback):
                for verb in makelist(method):
                    verb = verb.upper()
                    route = Route(self, rule, verb, callback, name=name,
                                  plugins=plugins, skiplist=skiplist, **config)
                    self.add_route(route)
            return callback
        return decorator(callback) if callback else decorator

3.断点查看请求响应过程

1. ServerHandler类中的run函数中, application 为Bottle一个实例
    self.result = application(self.environ, self.start_response)

2. bottle.Bottle类中
    def __init__(self, catchall=True, autojson=True):
        self.router = Router()
    
    # Router类主要建立url，method和应用函数的映射字典，同时实现match方法
    def add_route(self, route):
        ''' Add a route object, but do not change the :data:`Route.app`
            attribute.'''
        self.routes.append(route)
        self.router.add(route.rule, route.method, route, name=route.name)
        if DEBUG: route.prepare()

    
    def __call__(self, environ, start_response):
        ''' Each instance of :class:'Bottle' is a WSGI application. '''
        return self.wsgi(environ, start_response)
        
    
    def wsgi(self, environ, start_response):
            ...
            out = self._cast(self._handle(environ))
            ...
            return out
            
    def _handle(self, environ):
            ...
                # 上述中应用函数的装饰器，就将应用函数和路径作为参数实例化了一个Route实例，并将它添加到了router中，调用match时，通过正则表达式匹配调用相应的route实例
                route, args = self.router.match(environ)
            ...
                return route.call(**args)
            ...
            
3.bottle.Route类，主要是封装了应用函数及其对应的metadata和配置
    @cached_property
    def call(self):
        ''' The route callback with all plugins applied. This property is
            created on demand and then cached to speed up subsequent requests.'''
        return self._make_callback()
     
    def _make_callback(self):
        callback = self.callback
        for plugin in self.all_plugins():
            try:
                if hasattr(plugin, 'apply'):
                    api = getattr(plugin, 'api', 1)
                    context = self if api > 1 else self._context
                    callback = plugin.apply(callback, context)
            ...
        return callback
        
4. 在上述的调用过程的plugin在本例中使用JSONPlugin， 将应用函数的结果序列化之后返回
class JSONPlugin(object):
    name = 'json'
    api  = 2

    def __init__(self, json_dumps=json_dumps):
        self.json_dumps = json_dumps

    def apply(self, callback, route):
        dumps = self.json_dumps
        if not dumps: return callback
        def wrapper(*a, **ka):
            try:
                rv = callback(*a, **ka)
            except HTTPError:
                rv = _e()

            if isinstance(rv, dict):
                #Attempt to serialize, raises exception on failure
                json_response = dumps(rv)
                #Set content type only if serialization succesful
                response.content_type = 'application/json'
                return json_response
            elif isinstance(rv, HTTPResponse) and isinstance(rv.body, dict):
                rv.body = dumps(rv.body)
                rv.content_type = 'application/json'
            return rv

        return wrapper
        

4.缓存响应结果

上述代码中，我们注意到在调用call函数时使用了cached_property的装饰器，当第一次发起请求时，应用函数会被执行，而结果也将被保存。当再次请求，只需要从__dict__中获取相应的属性值即可

class cached_property(object):
    ''' A property that is only computed once per instance and then replaces
        itself with an ordinary attribute. Deleting the attribute resets the
        property. '''

    def __init__(self, func):
        self.__doc__ = getattr(func, '__doc__')
        self.func = func

    def __get__(self, obj, cls):
        if obj is None: return self
        # 在调用时，会先从__dict__属性中查找对应的属性值，如果找不到则再使用func函数的执行结果
        value = obj.__dict__[self.func.__name__] = self.func(obj)
        return value

看到这里，我们大概就清楚了从发起请求到解析请求，路由分发，返回应用函数结果的大概流程。url与callback的映射关系建立和match需要重点关注一下Router类，而callback的结果解析与相关的metadata则需要继续关注Route类