文章转载自https://blog.csdn.net/weixin_37923128/article/details/80992645 , 感谢原作者
当客户端向服务器发送一个请求时，服务器会将请求转发给web应用程序，应用程序处理完这个请求后将会返回一个response。在这篇文章我们分析一下flask怎样处理request，又是怎样生成response的，同时我们应该思考，在这个过程中，flask是怎样让url、endpoint、视图函数一一对应的。
一旦web应用接收到request，flask就会调用Flask类的call函数。在wsgi_app()函数中，我们看到，在调用了full_dispatch_request()函数后，就生成了response（Response类实例）。
class Flask:
def __call__(self, environ, start_response):
return self.wsgi_app(environ, start_response) def wsgi_app(self, environ, start_response):
ctx = self.request_context(environ)
error = None
try:
try:
ctx.push()
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)
在函数full_dispatch_request中，我们看到了dispatch_request函数，一路寻找下去，在这个函数里找到了view_functions这个字典。这个字典被多个函数在多个函数中出现。
def full_dispatch_request(self):
self.try_trigger_before_first_request_functions()
try:
request_started.send(self)
rv = self.preprocess_request()
if rv is None:
rv = self.dispatch_request()
except Exception as e:
rv = self.handle_user_exception(e)
return self.finalize_request(rv) def dispatch_request(self):
req = _request_ctx_stack.top.request
if req.routing_exception is not None:
self.raise_routing_exception(req)
rule = req.url_rule if getattr(rule, 'provide_automatic_options', False) \
and req.method == 'OPTIONS':
return self.make_default_options_response()
return self.view_functions[rule.endpoint](**req.view_args) self.view_functions = {}
那这个add_url_rule函数又是在哪里被调用的呢？往下看。
def add_url_rule(self, rule, endpoint=None, view_func=None,provide_automatic_options=None, **options):
if endpoint is None:
endpoint = _endpoint_from_view_func(view_func)
options['endpoint'] = endpoint
methods = options.pop('methods', None) # if the methods are not given and the view_func object knows its
# methods we can use that instead. If neither exists, we go with
# a tuple of only ``GET`` as default.
if methods is None:
methods = getattr(view_func, 'methods', None) or ('GET',)
if isinstance(methods, string_types):
raise TypeError('Allowed methods have to be iterables of strings, ''for example: @app.route(..., methods=["POST"])')
methods = set(item.upper() for item in methods) # Methods that should always be added
required_methods = set(getattr(view_func, 'required_methods', ())) # starting with Flask 0.8 the view_func object can disable and
# force-enable the automatic options handling.
if provide_automatic_options is None:
provide_automatic_options = getattr(view_func,'provide_automatic_options', None) if provide_automatic_options is None:
if 'OPTIONS' not in methods:
provide_automatic_options = True
required_methods.add('OPTIONS')
else:
provide_automatic_options = False # Add the required methods now.
methods |= required_methods rule = self.url_rule_class(rule, methods=methods, **options)
rule.provide_automatic_options = provide_automatic_options self.url_map.add(rule)
if view_func is not None:
old_func = self.view_functions.get(endpoint)
if old_func is not None and old_func != view_func:
raise AssertionError('View function mapping is overwriting an ''existing endpoint function: %s' % endpoint)
self.view_functions[endpoint] = view_func
看到这里就应该明白了。在我们写的web应用程序中，往往需要对视图函数添加路由修饰，如：app.route(‘/’)。正是在路由函数中调用了add_url_rule函数，而这个函数负责绑定相应的视图函数、url、以及endpoint。由此，这三个参数建立联系。而后，在程序运行中，当web程序收到请求后，根据请求中的url找到endpoint，再根据endpoint找到视图函数，然后调用视图函数，在视图函数中，会处理那些需要处理的值（return self.view_functionsrule.endpoint）。接下来这个函数的返回值会交给finalize_request（）函数处理。
def route(self, rule, **options):
def decorator(f):
endpoint = options.pop('endpoint', None)
self.add_url_rule(rule, endpoint, f, **options)
return f
return decorator
正是在这个函数里，生成了response。（response是Response类的实例），最终，这个response被返回给客户端，然后显示为页面。
def finalize_request(self, rv, from_error_handler=False):
response = self.make_response(rv)
try:
response = self.process_response(response)
request_finished.send(self, response=response)
except Exception:
if not from_error_handler:
raise
self.logger.exception('Request finalizing failed with an ''error while handling an error')
return response