How to contribute to Flask

Thank you for considering contributing to Flask!

Support questions

Please, don’t use the issue tracker for this. Use one of the following resources for questions about your own code:

  • The IRC channel #pocoo on FreeNode.
  • The IRC channel #python on FreeNode for more general questions.
  • The mailing list for long term discussion or larger issues.
  • Ask on Stack Overflow. Search with Google first using: flask {search term, exception message, etc.}

Reporting issues

  • Describe what you expected to happen.
  • If possible, include a minimal, complete, and verifiable example to help us identify the issue. This also helps check that the issue is not with your own code.
  • Describe what actually happened. Include the full traceback if there was an exception.
  • List your Python, Flask, and Werkzeug versions. If possible, check if this issue is already fixed in the repository.

Submitting patches

  • Include tests if your patch is supposed to solve a bug, and explain clearly under which circumstances the bug happens. Make sure the test fails without your patch.
  • Try to follow PEP8, but you may ignore the line length limit if following it would make the code uglier.

First time setup

  • Download and install the latest version of git.

  • Configure git with your username and email:

    git config --global 'your name'
    git config --global 'your email'
  • Make sure you have a GitHub account.

  • Fork Flask to your GitHub account by clicking the Fork button.

  • Clone your GitHub fork locally:

    git clone{username}/flask
    cd flask
  • Add the main repository as a remote to update later:

    git remote add pallets
    git fetch pallets
  • Create a virtualenv:

    python3 -m venv env
    . env/bin/activate
    # or "env\Scripts\activate" on Windows
  • Install Flask in editable mode with development dependencies:

    pip install -e ".[dev]"

Start coding

  • Create a branch to identify the issue you would like to work on (e.g. 2287-dry-test-suite)
  • Using your favorite editor, make your changes, committing as you go.
  • Try to follow PEP8, but you may ignore the line length limit if following it would make the code uglier.
  • Include tests that cover any code changes you make. Make sure the test fails without your patch. Run the tests..
  • Push your commits to GitHub and create a pull request.
  • Celebrate 🎉

Running the tests

Run the basic test suite with:


This only runs the tests for the current environment. Whether this is relevant depends on which part of Flask you’re working on. Travis-CI will run the full suite when you submit your pull request.

The full test suite takes a long time to run because it tests multiple combinations of Python and dependencies. You need to have Python 2.6, 2.7, 3.3, 3.4, 3.5 3.6, and PyPy 2.7 installed to run all of the environments. Then run:


Running test coverage

Generating a report of lines that do not have test coverage can indicate where to start contributing. Run pytest using coverage and generate a report on the terminal and as an interactive HTML document:

coverage run -m pytest
coverage report
coverage html
# then open htmlcov/index.html

Read more about coverage.

Running the full test suite with tox will combine the coverage reports from all runs.

make targets

Flask provides a Makefile with various shortcuts. They will ensure that all dependencies are installed.

  • make test runs the basic test suite with pytest
  • make cov runs the basic test suite with coverage
  • make test-all runs the full test suite with tox
  • make docs builds the HTML documentation

Caution: zero-padded file modes

This repository contains several zero-padded file modes that may cause issues when pushing this repository to git hosts other than GitHub. Fixing this is destructive to the commit history, so we suggest ignoring these warnings. If it fails to push and you’re using a self-hosted git service like GitLab, you can turn off repository checks in the admin panel.

These files can also cause issues while cloning. If you have

fsckobjects = true


fsckObjects = true

set in your git configuration file, cloning this repository will fail. The only solution is to set both of the above settings to false while cloning, and then setting them back to true after the cloning is finished.