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.. role:: bash(code)
:language: bash
.. role:: js(code)
:language: javascript
.. role:: python(code)
:language: python
=====================================
Migration Recipes and Their Lifecycle
=====================================
The actual migrations are performed running Python modules called **migration
recipes**, which contain directives on how to migrate strings, which files are
involved, transformations to apply, etc. These recipes are stored in
`mozilla-central`__.
When part of Firefox’s UI is migrated to Fluent, a migration recipe should be
attached to the same patch that adds new strings to `.ftl` files.
Migration recipes can quickly become obsolete, because the referenced strings
and files are removed from repositories as part of ongoing development.
For these reasons, l10n-drivers periodically clean up the `fluent_migrations`
folder in mozilla-central, keeping only recipes for 2
shipping versions (Nightly and Beta).
.. hint::
As a developer you don’t need to bother about updating migration recipes
already in `mozilla-central`: if a new patch removes a string or file that is
used in a migration recipe, simply ignore it, since the entire recipe will be
removed within a couple of cycles.
How to Write Migration Recipes
==============================
The migration recipe’s filename should start with a reference to the associated
bug number, and include a brief description of the bug, e.g.
:bash:`bug_1451992_preferences_applicationManager.py` is the migration recipe
used to migrate the Application Manager window in preferences. It’s also
possible to look at existing recipes in `mozilla-central`__ for inspiration.
General Recipe Structure
========================
A migration recipe is a Python module, implementing the :py:func:`migrate`
function, which takes a :py:class:`MigrationContext` as input. The API provided
by the context is
.. code-block:: python
class MigrationContext:
def add_transforms(self, target, reference, transforms):
"""Define transforms for target using reference as template.
`target` is a path of the destination FTL file relative to the
localization directory. `reference` is a path to the template FTL
file relative to the reference directory.
Each transform is an extended FTL node with `Transform` nodes as some
values.
For transforms that merely copy legacy messages or Fluent patterns,
using `fluent.migrate.helpers.transforms_from` is recommended.
"""
The skeleton of a migration recipe just implements the :py:func:`migrate`
function calling into :py:func:`ctx.add_transforms`, and looks like
.. code-block:: python
# coding=utf8
# Any copyright is dedicated to the Public Domain.
from __future__ import absolute_import
def migrate(ctx):
target = 'browser/browser/feature.ftl'
reference = 'browser/browser/feature.ftl'
ctx.add_transforms(
target,
reference,
[], # Actual transforms go here.
)
One can call into :py:func:`ctx.add_transforms` multiple times. In particular, one
can create migrated content in multiple files as part of a single migration
recipe by calling :py:func:`ctx.add_transforms` with different target-reference
pairs.
The *docstring* for this function will be used
as a commit message in VCS, that’s why it’s important to make sure the bug
reference is correct, and to keep the `part {index}` section: multiple strings
could have multiple authors, and would be migrated in distinct commits (part 1,
part 2, etc.).
Transforms
==========
The work of the migrations is done by the transforms that are passed as
last argument to :py:func:`ctx.add_transforms`. They're instances of either Fluent
:py:class:`fluent.syntax.ast.Message` or :py:class:`Term`, and their content
can depend on existing translation sources. The skeleton of a Message looks like
.. code-block:: python
FTL.Message(
id=FTL.Identifier(
name="msg",
),
value=FTL.Pattern(
elements=[
FTL.TextElement(
value="A string",
),
],
),
)
When migrating existing legacy translations, you'll replace an
``FTL.TextElement`` with a ``COPY(legacy_path, "old_id")``, or one of its
variations we detail :doc:`next <legacy>`. When migrating existing Fluent
translations, an ``FTL.Pattern`` is replaced with a
``COPY_PATTERN(old_path, "old-id")``.