Flask 中的 request

本文所使用的代码来源于
Flask version 0.12.2 commit sha 571334df8e26333f34873a3dcb84441946e6c64c
werkzeug version 0.14 commit sha 5b53d1539147c5db3210e0769d85397ab91f902d

Flask 使用全局变量 request 来获取请求数据,而不是像 Tornado 等那样将 request 作为参数传入 handler 中。不过这种全局变量实际上是一个 proxy,它会取上下文中的对应 request

Context 的生成

Flask 遵循 WSGI 规范,默认使用的 WSGI Server 为 werkzeug 提供的 threaded WSGI server

# app.py
from .ctx import RequestContext

class Flask(_PackageBoundObject):  
    def wsgi_app(self, environ, start_response):
        ctx = self.request_context(environ)
        ctx.push()
        error = None
        try:
            try:
                # 调用各种 hook、匹配路由、执行 handler、返回响应
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.handle_exception(e)
            except:
                error = sys.exc_info()[1]
                raise
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)  # 传入异常信息

    def __call__(self, environ, start_response):
        return self.wsgi_app(environ, start_response)

    def request_context(self, environ):
        return RequestContext(self, environ)

当一个请求到来时,我们会创建对应的 RequestContext,它保存了 HTTP Request 解析后的信息(request属性)

# ctx.py
class RequestContext(object):  
    def __init__(self, app, environ, request=None):
        self.app = app
        if request is None:
            request = app.request_class(environ)  # falsk.request 实际访问的对象
        self.request = request
        self.url_adapter = app.create_url_adapter(self.request)
        self.flashes = None
        self.session = None

        # Request contexts can be pushed multiple times and interleaved with
        # other request contexts.  Now only if the last level is popped we
        # get rid of them.  Additionally if an application context is missing
        # one is created implicitly so for each level we add this information
        self._implicit_app_ctx_stack = []

        # indicator if the context was preserved.  Next time another context
        # is pushed the preserved context is popped.
        self.preserved = False

        # remembers the exception for pop if there is one in case the context
        # preservation kicks in.
        self._preserved_exc = None

        # Functions that should be executed after the request on the response
        # object.  These will be called before the regular "after_request"
        # functions.
        self._after_request_functions = []

        self.match_request()

    def _get_g(self):
        return _app_ctx_stack.top.g
    def _set_g(self, value):
        _app_ctx_stack.top.g = value
    g = property(_get_g, _set_g)
    del _get_g, _set_g

    def match_request(self):
        """Can be overridden by a subclass to hook into the matching
        of the request.
        """
        try:
            url_rule, self.request.view_args = \
                self.url_adapter.match(return_rule=True)
            self.request.url_rule = url_rule
        except HTTPException as e:
            self.request.routing_exception = e

    def push(self):
        """Binds the request context to the current context."""
        # If an exception occurs in debug mode or if context preservation is
        # activated under exception situations exactly one context stays
        # on the stack.  The rationale is that you want to access that
        # information under debug situations.  However if someone forgets to
        # pop that context again we want to make sure that on the next push
        # it's invalidated, otherwise we run at risk that something leaks
        # memory.  This is usually only a problem in test suite since this
        # functionality is not active in production environments.
        top = _request_ctx_stack.top
        if top is not None and top.preserved:
            top.pop(top._preserved_exc)

        # Before we push the request context we have to ensure that there
        # is an application context.
        app_ctx = _app_ctx_stack.top
        if app_ctx is None or app_ctx.app != self.app:
            app_ctx = self.app.app_context()
            app_ctx.push()
            self._implicit_app_ctx_stack.append(app_ctx)
        else:
            self._implicit_app_ctx_stack.append(None)

        if hasattr(sys, 'exc_clear'):
            sys.exc_clear()

        _request_ctx_stack.push(self)

    def pop(self, exc=_sentinel):
        app_ctx = self._implicit_app_ctx_stack.pop()

        try:
            clear_request = False
            if not self._implicit_app_ctx_stack:
                self.preserved = False
                self._preserved_exc = None
                if exc is _sentinel:
                    exc = sys.exc_info()[1]
                self.app.do_teardown_request(exc)

                # If this interpreter supports clearing the exception information
                # we do that now.  This will only go into effect on Python 2.x,
                # on 3.x it disappears automatically at the end of the exception
                # stack.
                if hasattr(sys, 'exc_clear'):
                    sys.exc_clear()

                request_close = getattr(self.request, 'close', None)
                if request_close is not None:
                    request_close()
                clear_request = True
        finally:
            rv = _request_ctx_stack.pop()

            # get rid of circular dependencies at the end of the request
            # so that we don't require the GC to be active.
            if clear_request:
                rv.request.environ['werkzeug.request'] = None

            # Get rid of the app as well if necessary.
            if app_ctx is not None:
                app_ctx.pop(exc)

            assert rv is self, 'Popped wrong request context.  ' \
                '(%r instead of %r)' % (rv, self)

    def auto_pop(self, exc):
        # PRESERVE_CONTEXT_ON_EXCEPTION:
        # By default if the application is in debug mode the request context
        # is not popped on exceptions to enable debuggers to introspect the data.
        # This can be disabled by this key. You can also use this setting to
        # force-enable it for non debug execution which might be useful to
        # debug production applications (but also very risky).
        if self.request.environ.get('flask._preserve_context') or \
           (exc is not None and self.app.preserve_context_on_exception):
            self.preserved = True
            self._preserved_exc = exc
        else:
            self.pop(exc)

ctx.push() 中的 _request_ctx_stack.push(self) 将自身入栈

LocalProxy

Flask 中 request 的定义如下

# globals.py
from functools import partial  
from werkzeug.local import LocalStack, LocalProxy

_request_ctx_err_msg = '''\  
Working outside of request context.  
...
'''  
_app_ctx_err_msg = '''\  
Working outside of application context.  
...
'''

def _lookup_req_object(name):  
    top = _request_ctx_stack.top
    if top is None:
        raise RuntimeError(_request_ctx_err_msg)
    return getattr(top, name)

_request_ctx_stack = LocalStack()  
request = LocalProxy(partial(_lookup_req_object, 'request'))  

这部分代码依赖于 werkzeug,先来看一下 LocalProxy 的实现

@implements_bool
class LocalProxy(object):  
    __slots__ = ('__local', '__dict__', '__name__', '__wrapped__')

    def __init__(self, local, name=None):
        object.__setattr__(self, '_LocalProxy__local', local)  # name mangling
        object.__setattr__(self, '__name__', name)
        # werkzeug.Local 类的实例会拥有 `__release_local__` 属性
        if callable(local) and not hasattr(local, '__release_local__'):
            # "local" is a callable that is not an instance of Local or
            # LocalManager: mark it as a wrapped function.
            object.__setattr__(self, '__wrapped__', local)

    def _get_current_object(self):
        if not hasattr(self.__local, '__release_local__'):
            return self.__local()  # invoke callable
        try:
            return getattr(self.__local, self.__name__)
        except AttributeError:
            raise RuntimeError('no object bound to %s' % self.__name__)

    def __getattr__(self, name):
        if name == '__members__':
            return dir(self._get_current_object())
        return getattr(self._get_current_object(), name)

    __len__ = lambda x: len(x._get_current_object())
    __getitem__ = lambda x, i: x._get_current_object()[i]
    __iter__ = lambda x: iter(x._get_current_object())
    // 省略

它是一个对象代理,比如我们访问 request.args 时,其实是在访问 partial(_lookup_req_object, 'request')().args

_lookup_req_object 函数中 _request_ctx_stack.top 获取了请求上下文栈中栈顶的元素 即我们当时入栈的 RequestContext

_request_ctx_stack 是一个 LocalStack 对象,其实现如下

class LocalStack(object):  
    def __init__(self):
        self._local = Local()

    def _get__ident_func__(self):
        return self._local.__ident_func__

    def _set__ident_func__(self, value):
        object.__setattr__(self._local, '__ident_func__', value)

    __ident_func__ = property(_get__ident_func__, _set__ident_func__)
    del _get__ident_func__, _set__ident_func__

    def push(self, obj):
        rv = getattr(self._local, 'stack', None)
        if rv is None:
            self._local.stack = rv = []
        rv.append(obj)
        return rv

    def pop(self):
        stack = getattr(self._local, 'stack', None)
        if stack is None:
            return None
        elif len(stack) == 1:
            release_local(self._local)
            return stack[-1]
        else:
            return stack.pop()

    @property
    def top(self):
        try:
            return self._local.stack[-1]
        except (AttributeError, IndexError):
            return None

LocalStack 内部存储了一个 Local 对象,所有的操作都是对其的 stack 属性进行的

try:  
    from greenlet import getcurrent as get_ident
except ImportError:  
    try:
        from thread import get_ident
    except ImportError:
from _thread import get_ident

class Local(object):  
    __slots__ = ('__storage__', '__ident_func__')

    def __init__(self):
        object.__setattr__(self, '__storage__', {})
        object.__setattr__(self, '__ident_func__', get_ident)

    // ...

    def __getattr__(self, name):
        try:
            return self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

    def __setattr__(self, name, value):
        ident = self.__ident_func__()
        storage = self.__storage__
        try:
            storage[ident][name] = value
        except KeyError:
            storage[ident] = {name: value}

    def __delattr__(self, name):
        try:
            del self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

Local 内部维护了 __storage__,其将线程标识符与实际存储属性的字典进行映射,实现了 TLS

也就是说其实代码中简单的地访问 request.args,背后绕了一个大圈子

那么问题就来了,为什么不直接使用 threading.local 做 TLS,为什么要设计成一个栈?

这些可以参考 Flask 的 Context 机制

Reference

Flask source code
werkzeug source code