Fluent to Fluent Migrations

When migrating existing Fluent messages, it’s possible to copy a source directly with COPY_PATTERN, or to apply string replacements and other changes by extending the 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 alt attribute.

Original message:

about-logins-icon = Warning icon
    .title = Breached website

New message:

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:

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: about-logins-icon.title matches the title attribute of the message with identifier about-logins-icon, while about-logins-icon alone matches the value of the message.

Warning

The second argument of COPY_PATTERN and 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 TransformPattern class to create your transformation. This is a powerful general-purpose tool, of which 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:

videocontrols-label =
    { $position }<span data-l10n-name="duration"> / { $duration }</span>

New message:

videocontrols-scrubber =
    .aria-valuetext = { $position } / { $duration }

A migration may be applied to create this new message with:

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 STRIP_SPAN is not supported by the transforms_from utility, so the list of transforms needs to be defined explicitly.

Internally, TransformPattern extends the fluent.syntax Transformer, which defines the FTL AST used here. As a specific convenience, pattern element visitors such as visit_TextElement are allowed to return a FTL.Pattern to replace themselves with more than one node.