Flask Contexts: A Comprehensive Guide
As a beginner in the world of Flask, you may have heard of Flask Contexts but do not quite understand what they are and their relevance. Flask Contexts are integral parts of the Flask framework that handle certain processes involved in building web applications.
Understanding Flask Contexts is crucial as it provides you with a more in-depth understanding of the Flask framework and how it works. In this article, we will define Flask Contexts, the types, and their roles in Flask.
1) What is a Flask Context?
At its simplest definition, a Flask Context is a runtime environment in which the Flask application runs.
It is an environment that Flask provides you with to run your application, and it contains the context required to handle client requests. Flask Contexts provide a way to share data across modules without having to pass variables around.
When handling a client request, a Flask application must be able to associate the client request with the right Flask application instance. Flask Contexts serve this purpose by holding information about the application, and every request is handled within a context.
2) Flask Views and Request Object
Flask Views are the route functions or endpoint functions that listen to client requests and processes it accordingly. Views are where client requests are handled, and they are executed within a Flask Context.
Flask Views are usually decorated with @app.route specifying the route of the request, and they accept a client request. The Request Object represents a client’s HTTP request in Flask, and it provides access to the clients headers, form data and submitted files, among other characteristics.
3) Global Variables in Flask
In Flask applications, global variables refer to variables that are accessible throughout the application. Flask global variables are associated with the context of the application and are accessible during the entire lifetime of a request.
However, it is crucial to note that global variables are not the same across different requests. Two contexts of the same application running parallel to serve a different client request will maintain their unique global variable values.
Types of Flask Contexts
There are two primary Flask contexts, namely Application Context and Request Context. Each of these contexts plays a different role in the application.
4) Application Context in Flask
The Application Context is created when the Flask application starts and is used to store data that is permanent-things such as database connections and configurations. It provides a way to establish a connection between the Flask application and third-party tools such as databases and message queues.
This context is usually activated on start-up of the Flask application automatically.
Definition and Purpose of Application Context
Every Flask application usually has one Application Context, and it is created once during the lifetime of the application. The Purpose of the Application Context is to provide access to an application-level configuration that is managed throughout the lifetime of the application.
It is a place to store data that is needed throughout the lifetime of the application. The Application Context is also used to establish database connections and obtain the applications current running configuration.
Exposed Objects in Application Context: current_app
When an Application Context is created, Flask provides us with an object that gives us access to the current application instance, the current_app object. current_app is a reference to the Flask application that is handling the request.
It can be used throughout the REST API to retrieve information such as the authentication and database connections.
Exposed Objects in Application Context: g variable
The g variable is another object that is exposed in the Application Context.
The g variable represents a space that is available for storing data throughout the lifetime of a request, but its values are not carried over to the next request. Flask provides us with a before_request function that allows us to store data live across the current request’s context.
Activation and Deactivation of Application Context
There’s no need to activate or deactivate Application Context by a developer as Flask activates and deactivates the context automatically during the application’s lifetime. When the application starts, Flask initializes the Application Context and sets up the configurations and connections.
When the application is terminated, Flask deactivated the Application Context and frees the resources and connections associated with it.
5) Request Context in Flask
In the Flask framework, the Request Context is a context environment that contains data for a single client request to the Flask server. The Request Context provides information about the current HTTP request being made to the server.
Each Request Context is unique and created for every HTTP request received by Flasks application instance.
Definition and Purpose of Request Context
The purpose of the Request Context is to provide a safe container for the request data. It is essential in handling and processing client requests as it creates a safe, isolated environment for request data, ensuring that the data in one request does not mix with the data of another request.
The Request Context also provides a way to access and manipulate global data created during a request, ensuring that they are not lost after the request is completed. Exposed Objects in Request Context: requests
Exposed Objects in Request Context: requests
Within the Request Context, Flask provides tools to access the incoming data and provides different objects to work with the request.
One of the notable objects available in the Request Context is the ‘request’ object. This object provides data about the current HTTP request, including HTTP headers, POSTed data, and query parameters.
Developers can access the request object by calling it from within the Flask view function. Exposed Objects in Request Context: sessions
Exposed Objects in Request Context: sessions
Another object available in the Request Context is the session object.
The session object allows developers to store data that is unique to a particular user and is shared between requests made by the same user. Once stored, the ‘session’ object remains available in all upcoming requests made by the same user.
Flask sessions are signed with a secret key that ensures that the data stored cannot be modified by the user.
Activation and Deactivation of Request Context
The Request Context is automatically activated by Flask when a request is made to the application. Flask creates a new instance of the Request Context for each incoming request making sure that information from previous requests does not get mixed with new requests, ensuring that the responses are correct and relevant to the client.
Once the request is processed, Flask shuts down the Request Context, cleaning up memory space that the request occupied.
6) Manually Pushing Flask Context in the Shell
While Flask automatically creates and manages Flask Contexts, developers can manually create and manage them using specific functions in the Flask class.
Automatic Creation of Flask Contexts
When a flask application runs, Flask automatically creates an Application Context and a Request Context to handle requests from clients. The Request Context is unique and created for each distinct HTTP request that the Flask application receives.
The Flask framework provides tools to create these contexts directly in the Python interactive shell, allowing developers to test their application in the shell environment.
Accessing Objects Outside the View Function
To access request context variables outside of the current view function, developers can access a special object called current_app. Current_app gives access to the current application context, which has the request context.
This object can be imported and used to access request context variables in a view function.
Creating Application Context using app_context() method
To create a new instance of the Application Context explicitly instead of when the Flask application starts, Flask provides an app_context() method. The app_context() method returns an Application Context and installs it as the active application context, creating a container for the application-level data.
Developers can use this method when they need to access application-level data without the Request Context, such as during background tasks and processing tasks.
Creating Request Context using test_request_context() method
Similarly, Flask provides developers with the test_request_context() method to allow them to manually push a Request Context into the stack. This method can be used in testing, to test request context objects such as the request and session objects outside of the view function.
The test_request_context() method creates a Request Context stack and sets up context locals such as the request, session and g variables, which can be accessed from within the test case.
Conclusion
Flask Contexts are an essential part of Flask development as they provide data isolation and ensure that the application is stable and safe for handling client requests. We have discussed the two main Flask contexts, the Application Context and the Request Context, and the objects that are exposed in each context.
Flask provides us with tools to manage these contexts, and we have looked at functions such as app_context() and test_request_context() that allow us to create and manage Flask Contexts manually. Understanding Flask Contexts and their applications in Flask development gives us insight into how Flask processes user requests and provides insights into the inner workings of the Flask framework.