Welcome to Flask

Welcome to Flask’s documentation. Get started with Installation and then get an overview with the Quickstart. There is also a more detailed Tutorial that shows how to create a small but complete application with Flask. Common patterns are described in the Patterns for Flask section. The rest of the docs describe each component of Flask in detail, with a full reference in the API section.

Flask depends on the Jinja template engine and the Werkzeug WSGI toolkit. The documentation for these libraries can be found at:

• Jinja documentation
• Werkzeug documentation

User’s Guide

This part of the documentation, which is mostly prose, begins with some background information about Flask, then focuses on step-by-step instructions for web development with Flask.

• Foreword
• What does “micro” mean?
• Configuration and Conventions
• Growing with Flask
• Foreword for Experienced Programmers
• Thread-Locals in Flask
• Develop for the Web with Caution
• Installation
• Python Version
• Dependencies
• Virtual environments
• Install Flask
• Quickstart
• A Minimal Application
• What to do if the Server does not Start
• Debug Mode
• HTML Escaping
• Routing
• Static Files
• Rendering Templates
• Accessing Request Data
• Redirects and Errors
• About Responses
• Sessions
• Message Flashing
• Logging
• Hooking in WSGI Middleware
• Using Flask Extensions
• Deploying to a Web Server
• Tutorial
• Project Layout
• Application Setup
• Define and Access the Database
• Blueprints and Views
• Templates
• Static Files
• Blog Blueprint
• Make the Project Installable
• Test Coverage
• Deploy to Production
• Keep Developing!
• Templates
• Jinja Setup
• Standard Context
• Controlling Autoescaping
• Registering Filters
• Context Processors
• Testing Flask Applications
• Identifying Tests
• Fixtures
• Sending Requests with the Test Client
• Following Redirects
• Accessing and Modifying the Session
• Running Commands with the CLI Runner
• Tests that depend on an Active Context
• Handling Application Errors
• Error Logging Tools
• Error Handlers
• Custom Error Pages
• Blueprint Error Handlers
• Returning API Errors as JSON
• Logging
• Debugging
• Debugging Application Errors
• In Production
• The Built-In Debugger
• External Debuggers
• Logging
• Basic Configuration
• Email Errors to Admins
• Injecting Request Information
• Other Libraries
• Configuration Handling
• Configuration Basics
• Environment and Debug Features
• Builtin Configuration Values
• Configuring from Python Files
• Configuring from Data Files
• Configuring from Environment Variables
• Configuration Best Practices
• Development / Production
• Instance Folders
• Signals
• Subscribing to Signals
• Creating Signals
• Sending Signals
• Signals and Flask’s Request Context
• Decorator Based Signal Subscriptions
• Core Signals
• Pluggable Views
• Basic Principle
• Method Hints
• Method Based Dispatching
• Decorating Views
• Method Views for APIs
• The Application Context
• Purpose of the Context
• Lifetime of the Context
• Manually Push a Context
• Storing Data
• Events and Signals
• The Request Context
• Purpose of the Context
• Lifetime of the Context
• Manually Push a Context
• How the Context Works
• Callbacks and Errors
• Context Preservation on Error
• Notes On Proxies
• Modular Applications with Blueprints
• Why Blueprints?
• The Concept of Blueprints
• My First Blueprint
• Registering Blueprints
• Nesting Blueprints
• Blueprint Resources
• Building URLs
• Blueprint Error Handlers
• Extensions
• Finding Extensions
• Using Extensions
• Building Extensions
• Command Line Interface
• Application Discovery
• Run the Development Server
• Open a Shell
• Environments
• Debug Mode
• Environment Variables From dotenv
• Environment Variables From virtualenv
• Custom Commands
• Plugins
• Custom Scripts
• PyCharm Integration
• Development Server
• Command Line
• In Code
• Working with the Shell
• Command Line Interface
• Creating a Request Context
• Firing Before/After Request
• Further Improving the Shell Experience
• Patterns for Flask
• Large Applications as Packages
• Application Factories
• Application Dispatching
• Using URL Processors
• Deploying with Setuptools
• Deploying with Fabric
• Using SQLite 3 with Flask
• SQLAlchemy in Flask
• Uploading Files
• Caching
• View Decorators
• Form Validation with WTForms
• Template Inheritance
• Message Flashing
• JavaScript, fetch, and JSON
• Lazily Loading Views
• MongoDB with MongoEngine
• Adding a favicon
• Streaming Contents
• Deferred Request Callbacks
• Adding HTTP Method Overrides
• Request Content Checksums
• Celery Background Tasks
• Subclassing Flask
• Single-Page Applications
• Deploying to Production
• Self-Hosted Options
• Hosting Platforms
• Using async and await
• Performance
• Background tasks
• When to use Quart instead
• Extensions
• Other event loops

API Reference

If you are looking for information on a specific function, class or method, this part of the documentation is for you.

• API
• Application Object
• Blueprint Objects
• Incoming Request Data
• Response Objects
• Sessions
• Session Interface
• Test Client
• Test CLI Runner
• Application Globals
• Useful Functions and Classes
• Message Flashing
• JSON Support
• Template Rendering
• Configuration
• Stream Helpers
• Useful Internals
• Signals
• Class-Based Views
• URL Route Registrations
• View Function Options
• Command Line Interface

Additional Notes

Design notes, legal information and changelog are here for the interested.

• Design Decisions in Flask
• The Explicit Application Object
• The Routing System
• One Template Engine
• Micro with Dependencies
• Thread Locals
• Async/await and ASGI support
• What Flask is, What Flask is Not
• HTML/XHTML FAQ
• History of XHTML
• History of HTML5
• HTML versus XHTML
• What does “strict” mean?
• New technologies in HTML5
• What should be used?
• Security Considerations
• Cross-Site Scripting (XSS)
• Cross-Site Request Forgery (CSRF)
• JSON Security
• Security Headers
• Copy/Paste to Terminal
• Flask Extension Development
• Naming
• The Extension Class and Initialization
• Adding Behavior
• Configuration Techniques
• Data During a Request
• Views and Models
• Recommended Extension Guidelines
• How to contribute to Flask
• Support questions
• Reporting issues
• Submitting patches
• License
• BSD-3-Clause Source License
• Artwork License
• Changes
• Version 2.1.3
• Version 2.1.2
• Version 2.1.1
• Version 2.1.0
• Version 2.0.3
• Version 2.0.2
• Version 2.0.1
• Version 2.0.0
• Version 1.1.4
• Version 1.1.3
• Version 1.1.2
• Version 1.1.1
• Version 1.1.0
• Version 1.0.4
• Version 1.0.3
• Version 1.0.2
• Version 1.0.1
• Version 1.0
• Version 0.12.5
• Version 0.12.4
• Version 0.12.3
• Version 0.12.2
• Version 0.12.1
• Version 0.12
• Version 0.11.1
• Version 0.11
• Version 0.10.1
• Version 0.10
• Version 0.9
• Version 0.8.1
• Version 0.8
• Version 0.7.2
• Version 0.7.1
• Version 0.7
• Version 0.6.1
• Version 0.6
• Version 0.5.2
• Version 0.5.1
• Version 0.5
• Version 0.4
• Version 0.3.1
• Version 0.3
• Version 0.2
• Version 0.1