The Request Context

This is similar to The Application Context, which keeps track of the application-level data independent of a request. A corresponding application context is pushed when a request context is pushed.

When the application handles a request, it creates a Request object based on the environment it received from the WSGI server. Because a worker (thread, process, or coroutine depending on the server) handles only one request at a time, the request data can be considered global to that worker during that request. Flask uses the term context local for this.

Flask automatically pushes a request context when handling a request. View functions, error handlers, and other functions that run during a request will have access to the proxy, which points to the request object for the current request.

Lifetime of the Context

When a Flask application begins handling a request, it pushes a request context, which also pushes an . When the request ends it pops the request context then the application context.

The context is unique to each thread (or other worker type). request cannot be passed to another thread, the other thread will have a different context stack and will not know about the request the parent thread was pointing to.

Context locals are implemented in Werkzeug. See for more information on how this works internally.

If you try to access request, or anything that uses it, outside a request context, you’ll get this error message:

This should typically only happen when testing code that expects an active request. One option is to use the to simulate a full request. Or you can use test_request_context() in a block, and everything that runs in the block will have access to , populated with your test data.

If you see that error somewhere else in your code not related to testing, it most likely indicates that you should move that code into a view function.

For information on how to use the request context from the interactive Python shell, see Working with the Shell.

How the Context Works

The Flask.wsgi_app() method is called to handle each request. It manages the contexts during the request. Internally, the request and application contexts work as stacks, and _app_ctx_stack. When contexts are pushed onto the stack, the proxies that depend on them are available and point at information from the top context on the stack.

When the request starts, a is created and pushed, which creates and pushes an AppContext first if a context for that application is not already the top context. While these contexts are pushed, the , g, , and session proxies are available to the original thread handling the request.

After the request is dispatched and a response is generated and sent, the request context is popped, which then pops the application context. Immediately before they are popped, the and teardown_appcontext() functions are executed. These execute even if an unhandled exception occurred during dispatch.

Flask dispatches a request in multiple stages which can affect the request, response, and how errors are handled. The contexts are active during all of these stages.

A can add handlers for these events that are specific to the blueprint. The handlers for a blueprint will run if the blueprint owns the route that matches the request.

1. Before each request, before_request() functions are called. If one of these functions return a value, the other functions are skipped. The return value is treated as the response and the view function is not called.

2. If the functions did not return a response, the view function for the matched route is called and returns a response.

3. After the response is returned, the contexts are popped, which calls the teardown_request() and functions. These functions are called even if an unhandled exception was raised at any point above.

If an exception is raised before the teardown functions, Flask tries to match it with an errorhandler() function to handle the exception and return a response. If no error handler is found, or the handler itself raises an exception, Flask returns a generic 500 Internal Server Error response. The teardown functions are still called, and are passed the exception object.

If debug mode is enabled, unhandled exceptions are not converted to a 500 response and instead are propagated to the WSGI server. This allows the development server to present the interactive debugger with the traceback.

The teardown callbacks are independent of the request dispatch, and are instead called by the contexts when they are popped. The functions are called even if there is an unhandled exception during dispatch, and for manually pushed contexts. This means there is no guarantee that any other parts of the request dispatch have run first. Be sure to write these functions in a way that does not depend on other callbacks and will not fail.

During testing, it can be useful to defer popping the contexts after the request ends, so that their data can be accessed in the test function. Use the as a block to preserve the contexts until the with block exits.

Signals

If is true, the following signals are sent:

2. request_finished is sent after the functions are called.

3. got_request_exception is sent when an exception begins to be handled, but before an is looked up or called.

4. request_tearing_down is sent after the functions are called.

Context Preservation on Error

At the end of a request, the request context is popped and all data associated with it is destroyed. If an error occurs during development, it is useful to delay destroying the data for debugging purposes.

When the development server is running in development mode (the FLASK_ENV environment variable is set to ), the error and data will be preserved and shown in the interactive debugger.

This behavior can be controlled with the config. As described above, it defaults to True in the development environment.

Do not enable PRESERVE_CONTEXT_ON_EXCEPTION in production, as it will cause your application to leak memory on exceptions.

Some of the objects provided by Flask are proxies to other objects. The proxies are accessed in the same way for each worker thread, but point to the unique object bound to each worker behind the scenes as described on this page.

Most of the time you don’t have to care about that, but there are some exceptions where it is good to know that this object is actually a proxy:

• The reference to the proxied object is needed in some situations, such as sending or passing data to a background thread.

If you need to access the underlying object that is proxied, use the _get_current_object() method: