Firefox/Python 3 Migration

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Why This is Important

In mozilla-central there are over 3500 Python files (excluding third party files), comprising roughly 230k lines of code. Most of it is Python 2.[1]

Python 2 will reach it's End Of Life on January 1st 2020. This page collects links and resources to assist the migration of the Firefox ecosystem off of Python 2 and onto Python 3.

How We Get There

As of today (May 2019) we require both the Python 2 and Python 3 interpreters to be installed to build and develop Firefox.

Next we:

  1. Make Python 3 porting and development safe by adding a Python 3 option to the developer toolchain. (mach lint,, parts of the test suite)
  2. Make key support libraries run in both Python 2 and Python 3. (mach-core)
  3. Build interpreter switching mechanisms into the toolchain so we can run sub-components in just 2 or just 3. (mach sub-commands, py_action)
  4. Port Python 2-only components to Python 3 piecemeal.
  5. Remove Python 2 compatibility. (When that might happen needs discussion. Possibly sometime in 2020.)

Getting Involved

Find the others

Most discussion happens in the #python channel on Slack (NDA'd Mozillians only).

What you'll need

You will need a working Firefox build environment.

Where to start:

Remove more of the "excluded" paths from the py2 and py3 linters

See this Etherpad for instructions.


  • Pick a directory from the list, and put your name beside it so we know it's ported
  • Edit py2.yml and/or py3.yml and remove that directory from the exclusion list
  • Run `./mach lint -l py2 -l py3 <your directory name>` to get a list of errors
  • To get a list of files that need fixing:
./mach lint -l py2 -l py3 <your directory name> --format unix > tofix.txt
  • To install `futurize`:
pip install --user future
  • To run through futurize
cat tofix.txt | cut -f1 -d: | sort -u | xargs futurize -1 -w
  • Visually inspect the patch, especially around `except:` clauses. Watch out for "xxx_todo_changeme" in the patch. These are places where futurize couldn't port the code and you need to fix it manually.

When your patch is ready file:

  1. Use moz-phab for submitting the patch -
  2. Use bug 1559975
  3. Set one of callek, ahal, or catlee as the code reviewer

Run all python source tests with Python 3 and see what breaks

Make sure each of the tasks listed in has Python 3 specified, and do a try push, enabling all the python source tests.

See what breaks, then fix it!

Help port mach-core to Python 3 (bug 1473498)

  1. Edit python/mach/mach/test/python.ini to remove the skip-if
  2. Run: ./mach python-test --python 3 python/mach
  3. Fix the errors until all tests pass

See the #References for links to other ideas and tracking bugs.

How to Port the Code

Before starting

If you haven't ported modules before (or need a refresher), read the Porting Guide on It has a good outline of the porting process.

Porting modules in the Firefox source tree

  • We've already standardized on Python 2.7 across the tree.
  • Make sure you can test the code you're working on with python3!
    • If using `tox`, then adding `py35`, and `py36` to the list of environments should be sufficient.
    • If using |mach python-test|, make sure the relevant manifest doesn't skip-if Python 3 and run |mach python-test --python 3 <path>|.
  • `futurize` Stage 1 fixes can be used to transform code into code that's compatible with both python2.7 and python3.
  • The `six` module is available in-tree, feel free to use it as a dependency.
  • Black can format Python 2 code and Python 3 code (but it requires Py3.6+ to run).

Porting examples

Prefer six when possible. Make use of six.PY2, six.PY3, six.moves, six.string_types, six.integer_types, etc. This make it easy to find the py2 code again so we can remove it.

import statements

Use the six.moves module:


import __builtin__

__builtin__.__import__ = myimporthook()


from six.moves import builtins

builtins.__import__ = myimporthook()

if statements

Use six.PY2 and six.PY3:

import six

if six.PY2:
    # py2 stuff
    # py3 stuff

If six isn't available or you are in the imports section of the module then this is fine too:

if sys.version_info < (3,):
   # py2 stuff
   # py3 stuff

String data

Use six.string_types, six.text_type, six.binary_type, etc. Also see the six docs for "Binary and text data":


# using unicode
def write(self, buf):
    if isinstance(buf, unicode):
        buf = buf.encode('utf-8')
    BytesIO.write(self, buf)

# using types.StringTypes
if isinstance(value, types.StringTypes):


import six

# fixed unicode
def write(self, buf):
    if isinstance(buf, six.text_type):
        buf = buf.encode('utf-8')
    BytesIO.write(self, buf)

# fixed types.StringTypes
if isinstance(value, six.string_types):

Porting Resources

Porting Guide on

Cheat Sheet: Writing Python 2-3 compatible code. Note: misses some advanced changes, like porting __eq__() to __hash__() and changes to the slice interface.


Bug 1496527 tracks the migration

Python 3 tree migration notes

mach Python 3 migration roadmap