from flask import Flask app = Flask(__name__) if __name__ == '__main__': app.run()
这样就能启动一个flask项目了，那么app.run()做了什么呢？
简单的来说就是这个方法调用了run_simple()方法，make_server()又反回了一个单线程单进程的WSGI server；客户端发送请求时WSGI server会调用Flask的__call__方法，__call__再调用wsgi_app
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)
ctx = self.request_context(environ)会执行 RequestContext(self, environ)，并将这个对象返回，这个self指的是app
ctx封装了request对象和session，session为None
再看看这个RequestContext类里有什么
class RequestContext(object): """The request context contains all request relevant information. It is created at the beginning of the request and pushed to the `_request_ctx_stack` and removed at the end of it. It will create the URL adapter and request object for the WSGI environment provided. Do not attempt to use this class directly, instead use :meth:`~flask.Flask.test_request_context` and :meth:`~flask.Flask.request_context` to create this object. When the request context is popped, it will evaluate all the functions registered on the application for teardown execution (:meth:`~flask.Flask.teardown_request`). The request context is automatically popped at the end of the request for you. In debug mode the request context is kept around if exceptions happen so that interactive debuggers have a chance to introspect the data. With 0.4 this can also be forced for requests that did not fail and outside of ``DEBUG`` mode. By setting ``'flask._preserve_context'`` to ``True`` on the WSGI environment the context will not pop itself at the end of the request. This is used by the :meth:`~flask.Flask.test_client` for example to implement the deferred cleanup functionality. You might find this helpful for unittests where you need the information from the context local around for a little longer. Make sure to properly :meth:`~werkzeug.LocalStack.pop` the stack yourself in that situation, otherwise your unittests will leak memory. """ def __init__(self, app, environ, request=None): self.app = app if request is None: request = app.request_class(environ) 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 copy(self): """Creates a copy of this request context with the same request object. This can be used to move a request context to a different greenlet. Because the actual request object is the same this cannot be used to move a request context to a different thread unless access to the request object is locked. .. versionadded:: 0.10 """ return self.__class__(self.app, environ=self.request.environ, request=self.request ) 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) # Open the session at the moment that the request context is available. # This allows a custom open_session method to use the request context. # Only open a new session if this is the first time the request was # pushed, otherwise stream_with_context loses the session. 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) def pop(self, exc=_sentinel): """Pops the request context and unbinds it by doing that. This will also trigger the execution of functions registered by the :meth:`~flask.Flask.teardown_request` decorator. .. versionchanged:: 0.9 Added the `exc` argument. """ 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): 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) def __enter__(self): self.push() return self def __exit__(self, exc_type, exc_value, tb): # do not pop the request stack if we are in debug mode and an # exception happened. This will allow the debugger to still # access the request object in the interactive shell. Furthermore # the context can be force kept alive for the test client. # See flask.testing for how this works. self.auto_pop(exc_value) if BROKEN_PYPY_CTXMGR_EXIT and exc_type is not None: reraise(exc_type, exc_value, tb) def __repr__(self): return '<%s \'%s\' [%s] of %s>' % ( self.__class__.__name__, self.request.url, self.request.method, self.app.name, )
回到wsgi_app，继续往下走执行 ctx.push()
ctx就是RequestContext对象
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) # Open the session at the moment that the request context is available. # This allows a custom open_session method to use the request context. # Only open a new session if this is the first time the request was # pushed, otherwise stream_with_context loses the session. 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)
_request_ctx_stack是一个flask全局变量，在全局变量中是_request_ctx_stack = LocalStack()
LocalStack的__init__中定义self._local = Local()
LocalStack()作用是将Local对象中stack键对应的列表作为栈来维护
Local对象负责存数据，键为协程ID，如果抛出异常则为线程ID
push就是将封装好的ctx放进了Local对象的栈里，
继续回到wsgi_app，往下执行response = self.full_dispatch_request()
def full_dispatch_request(self): """Dispatches the request and on top of that performs request pre and postprocessing as well as HTTP exception catching and error handling. .. versionadded:: 0.7 """ 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)
full_dispatch_request会执行视图函数
视图函数里使用的request和session其实都是从全局变量里导入过来的
request = LocalProxy(partial(_lookup_req_object, 'request'))
def _lookup_req_object(name):
top = _request_ctx_stack.top
if top is None:
raise RuntimeError(_request_ctx_err_msg)
return getattr(top, name)
我们知道，使用对象.属性会调用__getattr__方法
def __getattr__(self, name):
if name == '__members__':
return dir(self._get_current_object())
return getattr(self._get_current_object(), name)
再来看看_get_current_object
def _get_current_object(self):
"""Return the current object. This is useful if you want the real
object behind the proxy at a time for performance reasons or because
you want to pass the object into a different context.
"""
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__)
__local就是偏函数，进行反射，执行了top方法，得到了栈顶的ctx，ctx封装了request，这样就可以取得request对象了
现在再看看session = LocalProxy(partial(_lookup_req_object, 'session'))，我们知道，session是像字典一样写入的，那就来看看setitem
def __setitem__(self, key, value):
self._get_current_object()[key] = value
这样就可以在字典里写值了
full_dispatch_request()还会将session写入浏览器的cookie里
最后再来看ctx.auto_pop(error)
def auto_pop(self, exc):
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)
执行Response方法将Local对象内的数据清除
wsgi_app就执行完毕了……