The Flask class is defined in the app.py module. The class has a
defined on Line 2460 (Flask version 1.1.2). If you have an object
app that is an
instance of Flask class,
you can treat the object as a function (or callable). When you call the object
it will actually run the code in
__call__ method in Flask class is simply calling another method
I will discuss in this article. The code in the method is not long (list below)
because it just calls a few other methods.
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() raise return response(environ, start_response) finally: if self.should_ignore_error(error): error = None ctx.auto_pop(error)
If you ignore the exception handling code for a minute, the code is not too difficult to understand.
Let’s look the first line of the method in detail.
ctx = self.request_context(environ)
request_context method is defined on Line 2345, and the method is returning a
RequestContext object. Notice the first argument to the
RequestContext constructor is
self, which is an instance of the Flask class.
def request_context(self, environ): return RequestContext(self, environ)
RequestContext class is defined on Line 255 of the ctx.py module. The first
sentence of the class documentation states that “the request context contains all
request relevant information”. Let’s look at the instance variables defined in
The class defines a property
g for easy access to the variable on the app stack.
The main methods of the class are
pop defined on Lines 355 and 398.
To better understand those two methods, you can read
Patrick Kennedy’s article.
The interesting part of
push method is that session is part of the request
context object and is initialized here. The
_request_ctx_stack object is
a global variable defined in global.py module.
def push(self): 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) 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()
I will stop here for now and look at other code in
wsgi_app at a later time.