Flask學習-Flask app接受第一個HTTP請求

1、__call__()

在Flask app啓動後，一旦uwsgi收到來自web server的請求，就會調用後端app，其實此時就是調用app的__call__（environ，start_response）.

flask.py:

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

　　

2、wsgi_app()

當http請求從server發送過來的時候，他會啓動__call__功能，這時候就啓動了最關鍵的wsgi_app（environ，start_response）函數。

fask.py:

def wsgi_app(self, environ, start_response):
        
        with self.request_context(environ):  #2.1,把環境變量入棧
            rv = self.preprocess_request()   #2.2，請求前的處理操做，主要執行一些函數，主要是準備工做。
            if rv is None:   #請求前若是沒有須要作的事情，就會進入到請求分發
                rv = self.dispatch_request()  #2.3 進行請求分發
            response = self.make_response(rv)  #2.4 返回一個response_class的實例對象，也就是能夠接受environ和start_reponse兩個參數的對象
            response = self.process_response(response)  #2.5 進行請求後的處理操做，只要是執行一些函數，和preprocesses_request()相似，主要是清理工做。
            return response(environ, start_response)   #2.6 對web server（uwsgi）進行正式迴應

　　

2.1 環境變量入棧

經過with上下文管理，生成request請求對象和請求上下文環境，併入棧。

執行以下代碼塊：

   def request_context(self, environ):
        
        return _RequestContext(self, environ) #調用_RequestContext（）類

　　

2.1.1 調用_RequestContext（）類

首先 會執行__enter__（）函數，執行_request_ctx_stack的push方法，把環境變量進行入棧操做。

注意：_request_ctx_stack = LocalStack()

#請求上下文初始化
class _RequestContext(object):
   
    def __init__(self, app, environ):
        self.app = app
        self.url_adapter = app.url_map.bind_to_environ(environ)
        self.request = app.request_class(environ)
        self.session = app.open_session(self.request)
        self.g = _RequestGlobals()
        self.flashes = None
'''
調用with的時候就會執行
'''
    def __enter__(self):
        _request_ctx_stack.push(self)

'''
with進行上下文管理的例子：
with離開後就會執行自定義上下文管理
class Diycontextor:
    def __init__(self,name,mode):
        self.name = name
        self.mode = mode

    def __enter__(self):
        print "Hi enter here!!"
        self.filehander = open(self.name,self.mode)
        return self.filehander

    def __exit__(self,*para):
        print "Hi exit here"
        self.filehander.close()

with Diycontextor('py_ana.py','r') as f:
    for i in f:
        print i
'''
    def __exit__(self, exc_type, exc_value, tb):
       
        if tb is None or not self.app.debug:
            _request_ctx_stack.pop()

　　

這一步最主要的是LocalStack類。

class LocalStack(object):

    """This class works similar to a :class:`Local` but keeps a stack
    of objects instead.  This is best explained with an example::
LocalStack相似於Local類，可是它保持了一個棧功能。它可以將對象入棧、出棧，以下：

        >>> ls = LocalStack()
        >>> ls.push(42)
        >>> ls.top
        42
        >>> ls.push(23)
        >>> ls.top
        23
        >>> ls.pop()
        23
        >>> ls.top
        42

    They can be force released by using a :class:`LocalManager` or with
    the :func:`release_local` function but the correct way is to pop the
    item from the stack after using.  When the stack is empty it will
    no longer be bound to the current context (and as such released).
入棧的對象能夠經過LocalManage類進行強制釋放，或者經過函數release_local函數。可是，正確的姿式是經過pop函數把他們出棧。
當棧爲空時，它再也不彈出上下文對象，這樣就徹底釋放了。

    By calling the stack without arguments it returns a proxy that resolves to
    the topmost item on the stack.
經過調用調用無參數的stack，返回一個proxy。

　　

從LocalStack的說明能夠得知這幾個事情：

一、LocalStack是一個相似Local的類，可是它具有棧的功能，可以讓對象入棧、出棧以及銷燬對象。

二、能夠經過LocalManager類進行強制釋放對象。

由引伸出兩個類：Local類和Localmanager類

Local類用於添加、存儲、或刪除上下文變量。

LocalManager類：因爲Local對象不能管理本身，因此經過LocalManager類用來管理Local對象。

 

咱們再回到wsgi_app函數中的2.2步驟：

2.2  preprocess_request

preprocess_request()方法，主要是進行flask的hook鉤子, before_request功能的實現，也就是在真正發生請求以前，有些準備工做須要提早作。好比，鏈接數據庫。

代碼以下：

   def preprocess_request(self):
       
        for func in self.before_request_funcs:
            rv = func()
            if rv is not None:
                return rv

　　

它會執行preprocess_request列表裏面的每一個函數。執行完成後，他會進入到dispatch_request方法。

2.3 dispatch_request

'''
分發請求
一、獲取endpoint和values，即請求url和參數
二、調用視圖函數view_functions[endpoint](**values)
三、處理異常錯誤
'''		
    def dispatch_request(self):
        
        try:
            endpoint, values = self.match_request()
            return self.view_functions[endpoint](**values)
        except HTTPException, e:
            handler = self.error_handlers.get(e.code)
            if handler is None:
                return e
            return handler(e)
        except Exception, e:
            handler = self.error_handlers.get(500)
            if self.debug or handler is None:
                raise
            return handler(e)

　　

進入dispatch_request()後，先執行match_request()，match_request()定義以下：

    def match_request(self):
        rv = _request_ctx_stack.top.url_adapter.match()
        request.endpoint, request.view_args = rv
        return rv

　　

注意：函數裏面的url_adapter。

self.url_adapter = app.url_map.bind_to_environ(environ),其實他使一個Map()對象，Map（）對象位於werkzeug.routing模塊，用於處理routeing。

從函數能夠看出，它會返回一個元組（endpoint,view_args）

endpoint:是一個url

view_args:是url的參數

例如：

>>> m = Map([
        ...     Rule('/', endpoint='index'),
        ...     Rule('/downloads/', endpoint='downloads/index'),
        ...     Rule('/downloads/<int:id>', endpoint='downloads/show')
        ... ])
        >>> urls = m.bind("example.com", "/")
        >>> urls.match("/", "GET")
        ('index', {})
        >>> urls.match("/downloads/42")
        ('downloads/show', {'id': 42})

　　

match_request（）執行完成後，此時已經獲取到了url和url裏面包含的參數的信息。接着進入視圖函數的執行：view_functions（）

 在Flask app啓動一節咱們知道，view_functions()是一個字典，相似view_functions = {'hello_world':hello_world}，當咱們執行

self.view_functions[endpoint](**values)

就至關於執行了hello_wold(**values),即自行了視圖函數。也就是咱們最開始app裏面的hello_world視圖函數：

@app.route('/hello')
def hello_world():
    return 'Hello World!'

　　

這時就會返回'Hello  World！‘若是有異常，則返回異常內容。

 至此，就完成了dispatch_request（），請求分發工做。這時，咱們再次回到wsgi_app中的2.4步：response = self.make_response(rv)

2.4 make_response

def make_response(self, rv):
    """Converts the return value from a view function to a real
    response object that is an instance of :attr:`response_class`.

    The following types are allowd for `rv`:

    ======================= ===========================================
    :attr:`response_class`  the object is returned unchanged
    :class:`str`            a response object is created with the
                            string as body
    :class:`unicode`        a response object is created with the
                            string encoded to utf-8 as body
    :class:`tuple`          the response object is created with the
                            contents of the tuple as arguments
    a WSGI function         the function is called as WSGI application
                            and buffered as response object
    ======================= ===========================================

    :param rv: the return value from the view function
    """
    if isinstance(rv, self.response_class):
        return rv
    if isinstance(rv, basestring):
        return self.response_class(rv)
    if isinstance(rv, tuple):
        return self.response_class(*rv)
    return self.response_class.force_type(rv, request.environ)

　　

經過make_response函數，將剛纔取得的 rv 生成響應，從新賦值response

2.5 process_response

再經過process_response功能主要是處理一個after_request的功能，好比你在請求後，要把數據庫鏈接關閉等動做，和上面提到的before_request對應和相似。

def after_request(self, f):
        self.after_request_funcs.append(f)
        return f

　　

以後再進行request_finished.send的處理，也是和socket處理有關，暫時不詳細深刻。以後返回新的response對象。

這裏特別須要注意的是，make_response函數是一個很是重要的函數，他的做用是返回一個response_class的實例對象，也就是能夠接受environ和start_reponse兩個參數的對象

 

當全部清理工做完成後，就會進入response(environ, start_response)函數，進行正式迴應。

2.6 response(environ, start_response)

　最後進入response()函數，說response函數以前，咱們先來看看response函數的由來：

response = self.make_response(rv)
response = self.process_response(response)

　　

2.6.1 response=make_response()

而make_response的返回值爲：return self.response_class.force_type(rv, request.environ)

其中的response_class = Response，而Response最終來自werkzeug.

werkzeug import Response as ResponseBase

@classmethod
    def force_type(cls, response, environ=None):
"""Enforce that the WSGI response is a response object of the current
        type.  Werkzeug will use the :class:`BaseResponse` internally in many
        situations like the exceptions.  If you call :meth:`get_response` on an
        exception you will get back a regular :class:`BaseResponse` object, even
        if you are using a custom subclass.

        This method can enforce a given response type, and it will also
        convert arbitrary WSGI callables into response objects if an environ
        is provided::

            # convert a Werkzeug response object into an instance of the
            # MyResponseClass subclass.
            response = MyResponseClass.force_type(response)

            # convert any WSGI application into a response object
            response = MyResponseClass.force_type(response, environ)

        This is especially useful if you want to post-process responses in
        the main dispatcher and use functionality provided by your subclass.

        Keep in mind that this will modify response objects in place if
        possible!

　　

從上面能夠知道，force_type（）是一個類方法，強制使當前對象成爲一個WSGI的reponse對象。

 

 2.6.2  response = self.process_response(response)

def process_response(self, response):
        
        session = _request_ctx_stack.top.session
        if session is not None:
            self.save_session(session, response)
        for handler in self.after_request_funcs:
            response = handler(response)
        return response

　　

process_response()執行了2個操做：

一、保存會話

二、執行請求後的函數列表中每個函數，並返回response對象

 

最後response函數會加上environ, start_response的參數並返回給uwsgi（web服務器），再由uwsgi返回給nginx，nignx返回給瀏覽器，最終咱們看到的內容顯示出來。

 至此，一個HTTP從請求到響應的流程就完畢了.

 

3、總結

總的來講，一個流程的關鍵步驟能夠簡單歸結以下：

 

 最後附上flask 0.1版本的註釋源碼：

# -*- coding: utf-8 -*-
from __future__ import with_statement
import os
import sys
from threading import local
from jinja2 import Environment, PackageLoader, FileSystemLoader
from werkzeug import Request as RequestBase, Response as ResponseBase, \
     LocalStack, LocalProxy, create_environ, cached_property, \
     SharedDataMiddleware
from werkzeug.routing import Map, Rule
from werkzeug.exceptions import HTTPException, InternalServerError
from werkzeug.contrib.securecookie import SecureCookie
from werkzeug import abort, redirect
from jinja2 import Markup, escape

try:
    import pkg_resources
    pkg_resources.resource_stream
except (ImportError, AttributeError):
    pkg_resources = None

#Request繼承werkzeug的Request類
class Request(RequestBase):
 
    def __init__(self, environ):
        RequestBase.__init__(self, environ)
        self.endpoint = None
        self.view_args = None
		
#Response繼承werkzeug的Response類
class Response(ResponseBase):
    default_mimetype = 'text/html'

#請求的全局變量類
class _RequestGlobals(object):
    pass

#請求上下文初始化
class _RequestContext(object):
   
    def __init__(self, app, environ):
        self.app = app
        self.url_adapter = app.url_map.bind_to_environ(environ)
        self.request = app.request_class(environ)
        self.session = app.open_session(self.request)
        self.g = _RequestGlobals()
        self.flashes = None
'''
調用with的時候就會執行
'''
    def __enter__(self):
        _request_ctx_stack.push(self)

'''
with離開後就會執行
自定義上下文管理
class Diycontextor:
    def __init__(self,name,mode):
        self.name = name
        self.mode = mode

    def __enter__(self):
        print "Hi enter here!!"
        self.filehander = open(self.name,self.mode)
        return self.filehander

    def __exit__(self,*para):
        print "Hi exit here"
        self.filehander.close()

with Diycontextor('py_ana.py','r') as f:
    for i in f:
        print i
'''
    def __exit__(self, exc_type, exc_value, tb):
       
        if tb is None or not self.app.debug:
            _request_ctx_stack.pop()

'''
 self.url_adapter = app.url_map.bind_to_environ(environ)
 >>> m = Map([
        ...     Rule('/', endpoint='index'),
        ...     Rule('/downloads/', endpoint='downloads/index'),
        ...     Rule('/downloads/<int:id>', endpoint='downloads/show')
        ... ])
        >>> urls = m.bind("example.com", "/")
        >>> urls.build("index", {})
        '/'
        >>> urls.build("downloads/show", {'id': 42})
        '/downloads/42'
        >>> urls.build("downloads/show", {'id': 42}, force_external=True)
        'http://example.com/downloads/42'
'''
def url_for(endpoint, **values):
  
    return _request_ctx_stack.top.url_adapter.build(endpoint, values)

'''
flash實現，經過get_flashed_messages獲取消息
'''
def flash(message):
   
    session['_flashes'] = (session.get('_flashes', [])) + [message]

def get_flashed_messages():
    
    flashes = _request_ctx_stack.top.flashes
    if flashes is None:
        _request_ctx_stack.top.flashes = flashes = \
            session.pop('_flashes', [])
    return flashes

'''
實現render_template功能，模板名和{content}
'''
def render_template(template_name, **context):
    
    current_app.update_template_context(context)
    return current_app.jinja_env.get_template(template_name).render(context)

'''
實現render_template_string功能，模板名和{content}
'''
def render_template_string(source, **context):
    
    current_app.update_template_context(context)
    return current_app.jinja_env.from_string(source).render(context)

def _default_template_ctx_processor():
   
    reqctx = _request_ctx_stack.top
    return dict(
        request=reqctx.request,
        session=reqctx.session,
        g=reqctx.g
    )

def _get_package_path(name):
    try:
        return os.path.abspath(os.path.dirname(sys.modules[name].__file__))
    except (KeyError, AttributeError):
        return os.getcwd()

'''
Flask類
'''
class Flask(object):

'''
全局變量
'''
    request_class = Request
    response_class = Response
    static_path = '/static'
    secret_key = None
    session_cookie_name = 'session'
    jinja_options = dict(
        autoescape=True,
        extensions=['jinja2.ext.autoescape', 'jinja2.ext.with_']
    )

'''
當前模塊名
app=Flask(__name__)

'''
    def __init__(self, package_name):
        self.debug = False
        self.package_name = package_name
        self.root_path = _get_package_path(self.package_name)
        self.view_functions = {}
        self.error_handlers = {}
        self.before_request_funcs = []
        self.after_request_funcs = []
        self.template_context_processors = [_default_template_ctx_processor]
		
        self.url_map = Map()
		
"""
A WSGI middleware that provides static content for development
    environments or simple server setups. Usage is quite simple::

        import os
        from werkzeug.wsgi import SharedDataMiddleware

        app = SharedDataMiddleware(app, {
            '/shared': os.path.join(os.path.dirname(__file__), 'shared')
        })
"""

        if self.static_path is not None:
            self.url_map.add(Rule(self.static_path + '/<filename>',
                                  build_only=True, endpoint='static'))
            if pkg_resources is not None:
                target = (self.package_name, 'static')
            else:
                target = os.path.join(self.root_path, 'static')
            self.wsgi_app = SharedDataMiddleware(self.wsgi_app, {
                self.static_path: target
            })       
        self.jinja_env = Environment(loader=self.create_jinja_loader(),
                                     **self.jinja_options)
        self.jinja_env.globals.update(
            url_for=url_for,
            get_flashed_messages=get_flashed_messages
        )

'''
FileSystemLoader(BaseLoader)
Loads templates from the file system.  This loader can find templates
    in folders on the file system and is the preferred way to load them.

    The loader takes the path to the templates as string, or if multiple
    locations are wanted a list of them which is then looked up in the
    given order::

    >>> loader = FileSystemLoader('/path/to/templates')
    >>> loader = FileSystemLoader(['/path/to/templates', '/other/path'])
	
	 A very basic example for a loader that looks up templates on the file
    system could look like this::

        from jinja2 import BaseLoader, TemplateNotFound
        from os.path import join, exists, getmtime

        class MyLoader(BaseLoader):

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

            def get_source(self, environment, template):
                path = join(self.path, template)
                if not exists(path):
                    raise TemplateNotFound(template)
                mtime = getmtime(path)
                with file(path) as f:
                    source = f.read().decode('utf-8')
                return source, path, lambda: mtime == getmtime(path)
				
PackageLoader(BaseLoader)
    If the package path is not given, ``'templates'`` is assumed.
	默認值爲templates
'''
    def create_jinja_loader(self):
        
        if pkg_resources is None:
            return FileSystemLoader(os.path.join(self.root_path, 'templates'))
        return PackageLoader(self.package_name)

'''
template_context_processors=[dict(
        request=reqctx.request,
        session=reqctx.session,
        g=reqctx.g]
'''
    def update_template_context(self, context):
       
        reqctx = _request_ctx_stack.top
        for func in self.template_context_processors:
            context.update(func())

'''
總體APP啓動入口，啓動後進行以下幾個步驟：
一、判斷是否有設置DEBUG
二、運行rum_simple()
app = Flask(__name__)
if __name__ == '__main__():
	app.run()
	
app.run() 
	run_simple(host, port, self, **options) 
	Start a WSGI application. Optional features include a reloader,
    multithreading and fork support.	

'''
    def run(self, host='localhost', port=5000, **options):
       
        from werkzeug import run_simple
        if 'debug' in options:
            self.debug = options.pop('debug')
        options.setdefault('use_reloader', self.debug)
        options.setdefault('use_debugger', self.debug)
        return run_simple(host, port, self, **options)

    def test_client(self):
        
        from werkzeug import Client
        return Client(self, self.response_class, use_cookies=True)

    def open_resource(self, resource):
       
        if pkg_resources is None:
            return open(os.path.join(self.root_path, resource), 'rb')
        return pkg_resources.resource_stream(self.package_name, resource)

    def open_session(self, request):
       
        key = self.secret_key
        if key is not None:
            return SecureCookie.load_cookie(request, self.session_cookie_name,
                                            secret_key=key)

    def save_session(self, session, response):
      
        if session is not None:
            session.save_cookie(response, self.session_cookie_name)

    def add_url_rule(self, rule, endpoint, **options):
       
        options['endpoint'] = endpoint
        options.setdefault('methods', ('GET',))
        self.url_map.add(Rule(rule, **options))


'''
@app.route('/test')
def test():
	pass
	
route是一個裝飾器，它幹了2個事情：
一、添加url到map裏
二、添加函數名到視圖函數{'test':test}
'''
    def route(self, rule, **options):
        
        def decorator(f):
            self.add_url_rule(rule, f.__name__, **options)
            self.view_functions[f.__name__] = f
            return f
        return decorator

'''
A decorator that is used to register a function give a given
        error code.  Example::

            @app.errorhandler(404)
            def page_not_found():
                return 'This page does not exist', 404

        You can also register a function as error handler without using
        the :meth:`errorhandler` decorator.  The following example is
        equivalent to the one above::

            def page_not_found():
                return 'This page does not exist', 404
            app.error_handlers[404] = page_not_found
			
			error_handlers = {page_not_found:404}

'''	
    def errorhandler(self, code):
       
        def decorator(f):
            self.error_handlers[code] = f
            return f
        return decorator
'''
請求開始前作準備工做，好比數據庫鏈接，用戶驗證
@app.before_request  
def before_request():  
    #可在此處檢查jwt等auth_key是否合法，  
    #abort(401)  
    #而後根據endpoint，檢查此api是否有權限，須要自行處理  
    #print(["endpoint",connexion.request.url_rule.endpoint])  
    #abort(401)  
    #也可作ip檢查，以阻擋受限制的ip等   
'''
    def before_request(self, f):
        
        self.before_request_funcs.append(f)
        return f

    def after_request(self, f):
        self.after_request_funcs.append(f)
        return f

    def context_processor(self, f):
        self.template_context_processors.append(f)
        return f
'''
 Here is a small example for matching:

        >>> m = Map([
        ...     Rule('/', endpoint='index'),
        ...     Rule('/downloads/', endpoint='downloads/index'),
        ...     Rule('/downloads/<int:id>', endpoint='downloads/show')
        ... ])
        >>> urls = m.bind("example.com", "/")
        >>> urls.match("/", "GET")
        ('index', {})
        >>> urls.match("/downloads/42")
        ('downloads/show', {'id': 42})
'''
    def match_request(self):
        rv = _request_ctx_stack.top.url_adapter.match()
        request.endpoint, request.view_args = rv
        return rv
		
'''
分發請求
一、獲取endpoint和values，即請求url和參數
二、調用視圖函數view_functions[endpoint](**values)
三、處理異常錯誤
'''		
    def dispatch_request(self):
        
        try:
            endpoint, values = self.match_request()
            return self.view_functions[endpoint](**values)
        except HTTPException, e:
            handler = self.error_handlers.get(e.code)
            if handler is None:
                return e
            return handler(e)
        except Exception, e:
            handler = self.error_handlers.get(500)
            if self.debug or handler is None:
                raise
            return handler(e)
'''
from werkzeug.wrappers import BaseResponse as Response

        def index():
            return Response('Index page')

        def application(environ, start_response):
            path = environ.get('PATH_INFO') or '/'
            if path == '/':
                response = index()
            else:
                response = Response('Not Found', status=404)
            return response(environ, start_response)
'''
    def make_response(self, rv):
      
        if isinstance(rv, self.response_class):
            return rv
        if isinstance(rv, basestring):
            return self.response_class(rv)
        if isinstance(rv, tuple):
            return self.response_class(*rv)
        return self.response_class.force_type(rv, request.environ)

    def preprocess_request(self):
       
        for func in self.before_request_funcs:
            rv = func()
            if rv is not None:
                return rv

    def process_response(self, response):
        
        session = _request_ctx_stack.top.session
        if session is not None:
            self.save_session(session, response)
        for handler in self.after_request_funcs:
            response = handler(response)
        return response

    def wsgi_app(self, environ, start_response):
        
        with self.request_context(environ):
            rv = self.preprocess_request()
            if rv is None:
                rv = self.dispatch_request()
            response = self.make_response(rv)
            response = self.process_response(response)
            return response(environ, start_response)

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

    def test_request_context(self, *args, **kwargs):
       
        return self.request_context(create_environ(*args, **kwargs))
'''
app=Flask(__name__)

call(self, environ, start_response) 
			wsgi_app(self, environ, start_response) 
			wsgi_app是flask核心:

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

# context locals
_request_ctx_stack = LocalStack()
current_app = LocalProxy(lambda: _request_ctx_stack.top.app)
request = LocalProxy(lambda: _request_ctx_stack.top.request)
session = LocalProxy(lambda: _request_ctx_stack.top.session)
g = LocalProxy(lambda: _request_ctx_stack.top.g)