Flask请求处理流程（request）[待续]

WSGI简介

WSGI(全称Web Server Gateway Interface)，是为 Python 语言定义的Web服务器和Web应用程序之间的一种简单而通用的接口，它封装了接受HTTP请求、解析HTTP请求、发送HTTP，响应等等的这些底层的代码和操作，使开发者可以高效的编写Web应用。

Flask的WSGI工具箱采用Werkzeug。工作原理如下：

from werkzeug.wrappers import Request, Response
from werkzeug.serving import run_simple

@Request.application
def hello(request):
    return Response('Hello World!')

if __name__ == '__main__':
    run_simple('localhost', 4000, hello)

一个最简单的Flask的Hello World示例：

from flask import Flask, request

app = Flask(__name__)

@app.route('/')
def index():
    if request.method == 'POST':
        return 'Hello World!'

if __name__ == '__main__':
    app.run()

启动先执行： app.run()

class Flask(_PackageBoundObject):

def run(self, host=None, port=None, debug=None, **options):
    　　from werkzeug.serving import run_simple
    　　try:
        　　# run_simple 是werkzeug 提供的方法，会执行第三个参数 self()
        　　run_simple(host, port, self, **options)

执行app()，对象()表示调用对象的__call__方法

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

又调用了app.wsgi_app方法

def wsgi_app(self, environ, start_response):
    # 1.
    ctx = self.request_context(environ)
    # 2
    ctx.push()
    error = None
    try:
        # 3
        try:
            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:
        # 4.
        if self.should_ignore_error(error):
            error = None
        ctx.auto_pop(error)

第1步：执行app.request_context方法，把请求的相关信息传进去了

class Flask(_PackageBoundObject):
　　 def request_context(self, environ):
        return RequestContext(self, environ)

返回了一个RequestContext类的实例对象

class RequestContext(object):
    def __init__(self, app, environ, request=None):
        self.app = app
        if request is None:
            request = app.request_class(environ)
        self.request = request

request_class = Request ，class Request(BaseRequest...)

class BaseRequest(object):
    def __init__(self, environ, populate_request=True, shallow=False):
        self.environ = environ

在RequestContext的__init__()方法中,app又调用了request_class方法，也就是Request 实例一个对象,Request又继承了RequestBase而，也就是在这里封装了请求进来的所有数据environ等。

第2步：执行ctx.push()方法

因为ctx是RequestContext类的对象，那我们就要去RequestContext类中找push方法

class RequestContext(object):
    def push(self):
        # 2.1
        top = _request_ctx_stack.top
        if top is not None and top.preserved:
            top.pop(top._preserved_exc)
        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()
        # 2.2
        _request_ctx_stack.push(self)
        self.session = self.app.open_session(self.request)
        if self.session is None:
            self.session = self.app.make_null_session()

2.1步，其中：_request_ctx_stack = LocalStack() ，_request_ctx_stack是LocalStack类的实例化对象，是一个全局对象。这一步，到_app_ctx_stack这个栈中取最后一个数据，没有就返回None

class LocalStack(object):
    def __init__(self):
        self._local = Local()
    @property
    def top(self):
        try:
            return self._local.stack[-1]
        except (AttributeError, IndexError):
            return None

2.2步，_request_ctx_stack.push(self)将封装了请求数据的LocalStack的对象封装到列表中。

class LocalStack(object):
    def __init__(self):
        self._local = Local()
    def push(self, obj):
        """Pushes a new item to the stack"""
        rv = getattr(self._local, 'stack', None)
        if rv is None:
            self._local.stack = rv = []
        rv.append(obj)
        return rv

第3步：app.full_dispatch_request() 执行视图函数时执行。此处后面再谈论，先进行下一步

第4步，执行了RequestContext 的 pop 方法

class RequestContext(object):
    def auto_pop(self, exc):
        else:
            self.pop(exc)

pop

class RequestContext(object):
    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 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()

            if clear_request:
                rv.request.environ['werkzeug.request'] = None

            if app_ctx is not None:
                app_ctx.pop(exc)

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

实则执行的是LocalStack对象的pop方法移除索引为-1的元素：

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

第3步，我们在试图函数中使用request的时候，发先request是一个全局变量。

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

其中LocalProxy的参数是一个偏函数，request作为参数，传入_lookup_req_object函数中。通过上下文管理器的原理，为每个线程创建独有的空间，进行request的封装。根据视图函数中的请求方式或者请求内容执行对应的方法，如在函数中print(request)将出发LocalProxy类对象的__str__()方法：

__str__ = lambda x: str(x._get_current_object())

其中name就是request

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

    def __init__(self, local, name=None):
        object.__setattr__(self, '_LocalProxy__local', local)
        object.__setattr__(self, '__name__', name)
        if callable(local) and not hasattr(local, '__release_local__'):
            object.__setattr__(self, '__wrapped__', local)

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

关于上下文，线程协程的相关文章之前整理的如下：

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Flask请求处理流程（request）[待续]

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