Flask 启动流程
请结合
Flask上下文一起阅读:https://wenyan.online/2020/10/19/flask-shang-xia-wen/
Flask依赖于werkzeug和jinja两个核心库,werkzeug是HTTP与WSGI相关的包集合.jinja主要用于渲染前端模板文件.
werkzeug实现了WSGI协议,一个典型的WSGI类似于:
wsgi
WSGI:全称Web Server Gateway Interface,他不是服务器,Python模块,框架,API,只是一种规范,用来描述web server如何与web application通信的规范.根据它,可以实现很多的相关框架,比如Flask,DjangoWSGI server主要负责从客户端接受request,并转发给application,application处理完数据后,WSGI server在把Response返回给客户端.WSGI application主要是来处理request,它可以包含多个中间件.environ指的是一个包含请求信息的对象.
1.Flask 简单应用
一个简单的
Flask服务器如下.
from flask import Flask
app = Flask(__name__)
@app.route('/')
def index():
return 'This is {}'.format(username)
if __name__ == '__main__':
app.run(debug=True)
from flask import Flask
app = Flask(__name__)
@app.route('/')
def index():
return 'This is {}'.format(username)
if __name__ == '__main__':
app.run(debug=True)这里从app.run() 来一步一步的看,它都执行了什么.
app.run()
# 定义在 flask/app.py中
class Flask(_PackageBoundObject):
# .....
def run(self, host=None, port=None, debug=None, load_dotenv=True, **options):
# ....
from werkzeug.serving import run_simple
try:
run_simple(host, port, self, **options)
finally:
self._got_first_request = False
# 可以看到 run() 方法最重要的是使用了 werkzeug.serving.run_simgle()方法
# 查看 run_simple()
# werkzeug.serving.run_simgle()
def run_sinple():
# ...
srv = make_server(
hostname,
port,
application,
threaded,
processes,
request_handler,
passthrough_errors,
ssl_context,
fd=fd,
)
#...
# 可以看到这里主要使用了一个 make_server的方法
# 此方法也定义在 werkzeug.serving.make_server()
def make_server():
# ''''''
if threaded and processes > 1:
raise ValueError("cannot have a multithreaded and multi process server.")
elif threaded:
return ThreadedWSGIServer(
host, port, app, request_handler, passthrough_errors, ssl_context, fd=fd
)
elif processes > 1:
return ForkingWSGIServer(
host,
port,
app,
processes,
request_handler,
passthrough_errors,
ssl_context,
fd=fd,
)
else:
return BaseWSGIServer(
host, port, app, request_handler, passthrough_errors, ssl_context, fd=fd
)
# 查看 ThreadedWSGIServer,ForkingWSGIServer,BaseWSGIServer
# 这三个方法同样定义在 werkzeug.serving.BaseWSGIServer
class ThreadedWSGIServer(ThreadingMixIn, BaseWSGIServer):
pass
class ForkingWSGIServer(ForkingMixIn, BaseWSGIServer):
pass
class BaseWSGIServer(HTTPServer, object):
pass
# 可以看到最主要的类是 BaseWSGIServer
class BaseWSGIServer(HTTPServer, object):
"""Simple single-threaded, single-process WSGI server."""
multithread = False
multiprocess = False
request_queue_size = LISTEN_QUEUE
def __init__(
self,
host,
port,
app,
handler=None,
passthrough_errors=False,
ssl_context=None,
fd=None,
):
if handler is None:
handler = WSGIRequestHandler
HTTPServer.__init__(self, server_address, handler)
# ....
# 这里使用了最重要的一个类 WSGIRequestHandler
# 这个类也定义在 werkzeug.serving.WSGIRequestHandler
class WSGIRequestHandler(BaseHTTPRequestHandler, object):
# ...
def execute(app):
application_iter = app(environ, start_response)
# ...
try:
execute(self.server.app)
# ...
# 可以看到这个类最主要就是执行了 execute(app) 这个方法,而根据传递 这里的
# app 就是最开始生成的app=Flask(__name__)
# 执行到最后相当于执行 app() 这个函数
# 找到 flask/app.py 下的 Flask.__call__
class Flask:
#...
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: # noqa: B001
error = sys.exc_info()[1]
raise
return response(environ, start_response)
finally:
if self.should_ignore_error(error):
error = None
ctx.auto_pop(error)
def __call__(self, environ, start_response):
return self.wsgi_app(environ, start_response)
# 最终的逻辑定义在 Flask.wsgi_app()中.
app.run()
# 定义在 flask/app.py中
class Flask(_PackageBoundObject):
# .....
def run(self, host=None, port=None, debug=None, load_dotenv=True, **options):
# ....
from werkzeug.serving import run_simple
try:
run_simple(host, port, self, **options)
finally:
self._got_first_request = False
# 可以看到 run() 方法最重要的是使用了 werkzeug.serving.run_simgle()方法
# 查看 run_simple()
# werkzeug.serving.run_simgle()
def run_sinple():
# ...
srv = make_server(
hostname,
port,
application,
threaded,
processes,
request_handler,
passthrough_errors,
ssl_context,
fd=fd,
)
#...
# 可以看到这里主要使用了一个 make_server的方法
# 此方法也定义在 werkzeug.serving.make_server()
def make_server():
# ''''''
if threaded and processes > 1:
raise ValueError("cannot have a multithreaded and multi process server.")
elif threaded:
return ThreadedWSGIServer(
host, port, app, request_handler, passthrough_errors, ssl_context, fd=fd
)
elif processes > 1:
return ForkingWSGIServer(
host,
port,
app,
processes,
request_handler,
passthrough_errors,
ssl_context,
fd=fd,
)
else:
return BaseWSGIServer(
host, port, app, request_handler, passthrough_errors, ssl_context, fd=fd
)
# 查看 ThreadedWSGIServer,ForkingWSGIServer,BaseWSGIServer
# 这三个方法同样定义在 werkzeug.serving.BaseWSGIServer
class ThreadedWSGIServer(ThreadingMixIn, BaseWSGIServer):
pass
class ForkingWSGIServer(ForkingMixIn, BaseWSGIServer):
pass
class BaseWSGIServer(HTTPServer, object):
pass
# 可以看到最主要的类是 BaseWSGIServer
class BaseWSGIServer(HTTPServer, object):
"""Simple single-threaded, single-process WSGI server."""
multithread = False
multiprocess = False
request_queue_size = LISTEN_QUEUE
def __init__(
self,
host,
port,
app,
handler=None,
passthrough_errors=False,
ssl_context=None,
fd=None,
):
if handler is None:
handler = WSGIRequestHandler
HTTPServer.__init__(self, server_address, handler)
# ....
# 这里使用了最重要的一个类 WSGIRequestHandler
# 这个类也定义在 werkzeug.serving.WSGIRequestHandler
class WSGIRequestHandler(BaseHTTPRequestHandler, object):
# ...
def execute(app):
application_iter = app(environ, start_response)
# ...
try:
execute(self.server.app)
# ...
# 可以看到这个类最主要就是执行了 execute(app) 这个方法,而根据传递 这里的
# app 就是最开始生成的app=Flask(__name__)
# 执行到最后相当于执行 app() 这个函数
# 找到 flask/app.py 下的 Flask.__call__
class Flask:
#...
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: # noqa: B001
error = sys.exc_info()[1]
raise
return response(environ, start_response)
finally:
if self.should_ignore_error(error):
error = None
ctx.auto_pop(error)
def __call__(self, environ, start_response):
return self.wsgi_app(environ, start_response)
# 最终的逻辑定义在 Flask.wsgi_app()中.2.wsgi_app
Flask 虽然运行,但是并没有真正的去处理请求,当真正有请求时,是调用Flask.wsgi_app() 去处理
class Flask:
def request_context(self, environ):
return RequestContext(self, environ)
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: # noqa: B001
error = sys.exc_info()[1]
raise
return response(environ, start_response)
finally:
if self.should_ignore_error(error):
error = None
ctx.auto_pop(error)
class Flask:
def request_context(self, environ):
return RequestContext(self, environ)
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: # noqa: B001
error = sys.exc_info()[1]
raise
return response(environ, start_response)
finally:
if self.should_ignore_error(error):
error = None
ctx.auto_pop(error)ctx = self.request_context(environ)
# 开始就进行了 请求上下文
# 这个方法同样定义在Flask中,引用的是 RequestContext
# 此方法定义在 flask/ctx.py 中
class RequestContext(object):
def __init__(self, app, environ, request=None, session=None):
self.app = app
# 最开始 request 为None
if request is None:
# 定义在 flask/app.py中
# request_class =Request
# 这个Request,就是 flask/wrappers.py 下的Request类
# 这个类主要是用来实现路由的.
request = app.request_class(environ)
self.request = request
self.url_adapter = None
try:
# 上下文对象的 url_adapter,主要是将请求中的URL和Map实例中的URL
# 进行匹配
self.url_adapter = app.create_url_adapter(self.request)
except HTTPException as e:
self.request.routing_exception = e
self.flashes = None
self.session = session
self._implicit_app_ctx_stack = []
self.preserved = False
self._preserved_exc = None
self._after_request_functions = []
#全局变量g
@property
def g(self):
return _app_ctx_stack.top.g
@g.setter
def g(self, value):
_app_ctx_stack.top.g = value
def copy(self):
return self.__class__(
self.app,
environ=self.request.environ,
request=self.request,
session=self.session,
)
def match_request(self):
try:
result = self.url_adapter.match(return_rule=True)
self.request.url_rule, self.request.view_args = result
except HTTPException as e:
self.request.routing_exception = e
# 最主要的是实现了 push 和 pop方法
# 将请求上下文绑定到当前上下文
def push(self):
# 获取请求上下文,开始是None
top = _request_ctx_stack.top
if top is not None and top.preserved:
top.pop(top._preserved_exc)
# 获取应用上下文,开始为None
app_ctx = _app_ctx_stack.top
# 先创建 application context,并push到堆栈结构
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()
# self是RequestContext对象,其中包含了请求的所有数据
# push RequestContext 到堆栈结构
# push当前上下文到堆栈结构中
_request_ctx_stack.push(self)
# 这是最重要的方法,保证了_request_ctx_stack中永远指向了当前的上下文
if self.session is None:
session_interface = self.app.session_interface
self.session = session_interface.open_session(self.app, self.request)
if self.session is None:
self.session = session_interface.make_null_session(self.app)
if self.url_adapter is not None:
self.match_request()
ctx = self.request_context(environ)
# 开始就进行了 请求上下文
# 这个方法同样定义在Flask中,引用的是 RequestContext
# 此方法定义在 flask/ctx.py 中
class RequestContext(object):
def __init__(self, app, environ, request=None, session=None):
self.app = app
# 最开始 request 为None
if request is None:
# 定义在 flask/app.py中
# request_class =Request
# 这个Request,就是 flask/wrappers.py 下的Request类
# 这个类主要是用来实现路由的.
request = app.request_class(environ)
self.request = request
self.url_adapter = None
try:
# 上下文对象的 url_adapter,主要是将请求中的URL和Map实例中的URL
# 进行匹配
self.url_adapter = app.create_url_adapter(self.request)
except HTTPException as e:
self.request.routing_exception = e
self.flashes = None
self.session = session
self._implicit_app_ctx_stack = []
self.preserved = False
self._preserved_exc = None
self._after_request_functions = []
#全局变量g
@property
def g(self):
return _app_ctx_stack.top.g
@g.setter
def g(self, value):
_app_ctx_stack.top.g = value
def copy(self):
return self.__class__(
self.app,
environ=self.request.environ,
request=self.request,
session=self.session,
)
def match_request(self):
try:
result = self.url_adapter.match(return_rule=True)
self.request.url_rule, self.request.view_args = result
except HTTPException as e:
self.request.routing_exception = e
# 最主要的是实现了 push 和 pop方法
# 将请求上下文绑定到当前上下文
def push(self):
# 获取请求上下文,开始是None
top = _request_ctx_stack.top
if top is not None and top.preserved:
top.pop(top._preserved_exc)
# 获取应用上下文,开始为None
app_ctx = _app_ctx_stack.top
# 先创建 application context,并push到堆栈结构
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()
# self是RequestContext对象,其中包含了请求的所有数据
# push RequestContext 到堆栈结构
# push当前上下文到堆栈结构中
_request_ctx_stack.push(self)
# 这是最重要的方法,保证了_request_ctx_stack中永远指向了当前的上下文
if self.session is None:
session_interface = self.app.session_interface
self.session = session_interface.open_session(self.app, self.request)
if self.session is None:
self.session = session_interface.make_null_session(self.app)
if self.url_adapter is not None:
self.match_request()刚启动app,应该还没有生成上下文,也就是上下文为{} 的时候,因为我们并没有去访问网页.
from flask import Flask, request
from flask.globals import _app_ctx_stack, _request_ctx_stack
app = Flask(__name__)
# 堆栈结构
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
# {}
# {}
@app.route('/')
def index():
username = request.args.get('username')
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
return 'This is {}'.format(username)
if __name__ == '__main__':
app.run(debug=True)
from flask import Flask, request
from flask.globals import _app_ctx_stack, _request_ctx_stack
app = Flask(__name__)
# 堆栈结构
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
# {}
# {}
@app.route('/')
def index():
username = request.args.get('username')
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
return 'This is {}'.format(username)
if __name__ == '__main__':
app.run(debug=True)3.路由
现在来看一下路由的形成.
@app.route('/')
def index():
username = request.args.get('username')
return 'This is {}'.format(username)
@app.route('/')
def index():
username = request.args.get('username')
return 'This is {}'.format(username)这里route 方法是一个装饰器,具体的实现逻辑在add_url_rule()
# flask/app.py
class Flask():
# ...
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
# add_url_rule
@setupmethod
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)
# ....
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
# flask/app.py
class Flask():
# ...
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
# add_url_rule
@setupmethod
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)
# ....
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它主要的作用是将路由添加到self.url_map 中.通过rule 和endpoint 的对应,可以很方便的访问到网页.
以上,当执行完ctx.push() 后,Local 对象中已经有数据了,接着执行self.full_dispatch_request,也就是执行视图函数,它的作用是根据请求的url找到对应的蓝本里面的视图函数,并生成一个response对象.
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 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
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 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最后,会清空堆栈结构中的数据,为下一次的上下文做准备.
ctx.auto_pop(error)
# auto_pop中实现了 ResponseContext.pop()方法.
ctx.auto_pop(error)
# auto_pop中实现了 ResponseContext.pop()方法.4.发起访问
from flask import Flask, request
from flask.globals import _app_ctx_stack, _request_ctx_stack
app = Flask(__name__)
# 堆栈结构
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
# {}
# {}
@app.route('/')
def index():
username = request.args.get('username')
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
return 'This is {}'.format(username)
if __name__ == '__main__':
app.run(debug=True)
from flask import Flask, request
from flask.globals import _app_ctx_stack, _request_ctx_stack
app = Flask(__name__)
# 堆栈结构
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
# {}
# {}
@app.route('/')
def index():
username = request.args.get('username')
print(_app_ctx_stack._local.__storage__)
print(_request_ctx_stack._local.__storage__)
return 'This is {}'.format(username)
if __name__ == '__main__':
app.run(debug=True)在浏览器中发起访问http://127.0.0.1:5000/?username=python
{: {'stack': []}}
{: {'stack': ['http://127.0.0.1:5000/?username=python' [GET] of app>]}}
{: {'stack': []}}
{: {'stack': ['http://127.0.0.1:5000/?username=python' [GET] of app>]}}可以看到其中有一个RequestContext 对象.当真正发起访问http://127.0.0.1:5000/?username=python ,就会调用Flask.wsgi_app() ,此时调用了RequestContext.push(self) ,也就是当前上下文被push 到了__request_ctx_stack 堆栈结构中.这就是上下文结构.
当访问结束后,ctx.auto_pop(error)会清空堆栈结构.