.. role:: bash(code)
:language: bash
.. role:: js(code)
:language: javascript
.. role:: python(code)
:language: python
===========================
Fluent to Fluent Migrations
===========================
When migrating existing Fluent messages,
it's possible to copy a source directly with :python:`COPY_PATTERN`,
or to apply string replacements and other changes
by extending the :python:`TransformPattern` visitor class.
These transforms work with individual Fluent patterns,
i.e. the body of a Fluent message or one of its attributes.
Copying Fluent Patterns
-----------------------
Consider for example a patch modifying an existing message to move the original
value to a :js:`alt` attribute.
Original message:
.. code-block:: fluent
about-logins-icon = Warning icon
.title = Breached website
New message:
.. code-block:: fluent
about-logins-breach-icon =
.alt = Warning icon
.title = Breached website
This type of changes requires a new message identifier, which in turn causes
existing translations to be lost. It’s possible to migrate the existing
translated content with:
.. code-block:: python
from fluent.migrate import COPY_PATTERN
ctx.add_transforms(
"browser/browser/aboutLogins.ftl",
"browser/browser/aboutLogins.ftl",
transforms_from(
"""
about-logins-breach-icon =
.alt = {COPY_PATTERN(from_path, "about-logins-icon")}
.title = {COPY_PATTERN(from_path, "about-logins-icon.title")}
""",from_path="browser/browser/aboutLogins.ftl"),
)
In this specific case, the destination and source files are the same. The dot
notation is used to access attributes: :js:`about-logins-icon.title` matches
the :js:`title` attribute of the message with identifier
:js:`about-logins-icon`, while :js:`about-logins-icon` alone matches the value
of the message.
.. warning::
The second argument of :python:`COPY_PATTERN` and :python:`TransformPattern`
identifies a pattern, so using the message identifier will not
migrate the message as a whole, with all its attributes, only its value.
Transforming Fluent Patterns
----------------------------
To apply changes to Fluent messages, you may extend the
:python:`TransformPattern` class to create your transformation.
This is a powerful general-purpose tool, of which :python:`COPY_PATTERN` is the
simplest extension that applies no transformation to the source.
Consider for example a patch copying an existing message to strip out its HTML
content to use as an ARIA value.
Original message:
.. code-block:: fluent
videocontrols-label =
{ $position }<span data-l10n-name="duration"> / { $duration }</span>
New message:
.. code-block:: fluent
videocontrols-scrubber =
.aria-valuetext = { $position } / { $duration }
A migration may be applied to create this new message with:
.. code-block:: python
from fluent.migrate.transforms import TransformPattern
import fluent.syntax.ast as FTL
class STRIP_SPAN(TransformPattern):
def visit_TextElement(self, node):
node.value = re.sub("</?span[^>]*>", "", node.value)
return node
def migrate(ctx):
path = "toolkit/toolkit/global/videocontrols.ftl"
ctx.add_transforms(
path,
path,
[
FTL.Message(
id=FTL.Identifier("videocontrols-scrubber"),
attributes=[
FTL.Attribute(
id=FTL.Identifier("aria-valuetext"),
value=STRIP_SPAN(path, "videocontrols-label"),
),
],
),
],
)
Note that a custom extension such as :python:`STRIP_SPAN` is not supported by
the :python:`transforms_from` utility, so the list of transforms needs to be
defined explicitly.
Internally, :python:`TransformPattern` extends the `fluent.syntax`__
:python:`Transformer`, which defines the :python:`FTL` AST used here.
As a specific convenience, pattern element visitors such as
:python:`visit_TextElement` are allowed to return a :python:`FTL.Pattern`
to replace themselves with more than one node.
__
https://projectfluent.org/python-fluent/fluent.syntax/stable/
