Handle legacy fields in modify

[snejus]

Handle legacy singular field names in modify

Fixes: #6483

The modify command now accepts legacy singular field names (genre, composer, lyricist, remixer, arranger) as assignment targets, automatically rewrites them to their plural multi-value equivalents, and emits a deprecation warning directing users to the correct names.

What changed

• beets/util/deprecation.py: Two new constants — ALBUM_LIST_FIELD_BY_LEGACY_FIELD and ITEM_LIST_FIELD_BY_LEGACY_FIELD — act as a single source of truth for the legacy-to-plural field mappings. Previously these mappings were scattered across hooks.py.
• beets/autotag/hooks.py: AlbumInfo and TrackInfo now reference the centralised constants instead of defining their own inline mappings.
• beets/ui/commands/modify.py: modify_parse_args gains an is_album flag, selects the appropriate mapping, and — when a legacy field is detected — rewrites the key and warns the user via deprecate_for_user.
• Tests: Existing modify_parse_args call-sites updated to pass is_album; new parametrised tests cover the rewrite and warning behaviour.

Impact

Users who run commands like beet modify genre="rock" will now get a working command plus a clear prompt to switch to genres="rock", rather than silently writing to a non-standard field or failing. No breaking change — purely additive compatibility layer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.17%. Comparing base (06512c9) to head (03728f2).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6494      +/-   ##
==========================================
+ Coverage   70.16%   70.17%   +0.01%     
==========================================
  Files         147      147              
  Lines       18651    18660       +9     
  Branches     3039     3040       +1     
==========================================
+ Hits        13086    13095       +9     
  Misses       4923     4923              
  Partials      642      642              

Files with missing lines	Coverage Δ
beets/autotag/hooks.py	98.85% <100.00%> (+<0.01%)	⬆️
beets/ui/commands/modify.py	94.28% <100.00%> (+0.53%)	⬆️
beets/util/deprecation.py	86.95% <100.00%> (+1.24%)	⬆️

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Copilot]

Pull request overview

PR make beet modify accept old singular multi-value field names, then rewrite to new plural field names and warn user, so user no longer make wrong custom field by accident.

Changes:

• Add central legacy->plural list field maps in beets/util/deprecation.py
• Reuse those maps from autotag AlbumInfo / TrackInfo instead of inline maps
• Update modify_parse_args to rewrite legacy assignment keys + emit user-facing deprecation warning; add tests + docs

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
beets/util/deprecation.py	Add shared legacy singular → plural list-field mapping constants.
beets/autotag/hooks.py	Switch Info subclasses to use shared mapping constants.
beets/ui/commands/modify.py	Rewrite legacy assignment keys to plural fields + warn user.
test/ui/commands/test_modify.py	Update call sites for new arg + add rewrite/warn tests.
test/autotag/test_hooks.py	Stop using Info directly in legacy-field tests; use TrackInfo.
docs/reference/cli.rst	Document legacy singular field support in modify.
docs/changelog.rst	Add changelog entry for modify legacy-field rewrite behavior.

[Copilot]

grug see docs say legacy singular fields (composer/lyricist/remixer/arranger) translate too. but here when is_album True you pick ALBUM_LIST_FIELD_BY_LEGACY_FIELD (only genre). so beet modify -a composer=... still make new composer flex field and not touch composers, so tag write still wrong. grug think modify_parse_args should use item legacy->list map even in album mode (or union maps) so album modify can still rewrite these fields when inherit push to items.

Suggested change

	list_field_by_legacy_field = (
	ALBUM_LIST_FIELD_BY_LEGACY_FIELD
	if is_album
	else ITEM_LIST_FIELD_BY_LEGACY_FIELD
	)
	for arg in args:
	if arg.endswith("!") and "=" not in arg and ":" not in arg:
	dels.append(arg[:-1]) # Strip trailing !.
	list_field_by_legacy_field = dict(ITEM_LIST_FIELD_BY_LEGACY_FIELD)
	if is_album:
	list_field_by_legacy_field.update(ALBUM_LIST_FIELD_BY_LEGACY_FIELD)
	for arg in args:
	if arg.endswith("!") and "=" not in arg and ":" not in arg:
	key = arg[:-1] # Strip trailing !.
	if list_field := list_field_by_legacy_field.get(key):
	deprecate_for_user(
	log,
	f"The '{key}' field",
	f"'{list_field}' (separate values by '; ')",
	)
	key = list_field
	dels.append(key)

[Copilot]

grug see base Info declare LIST_BY_LEGACY_FIELD but no value. if someone ever make Info() and set any key, self.LIST_BY_LEGACY_FIELD lookup can crash with AttributeError. grug think set safe default (empty dict or genre->genres) on Info class so no surprise crash.

Suggested change

	LIST_BY_LEGACY_FIELD: ClassVar[dict[str, str]]
	LIST_BY_LEGACY_FIELD: ClassVar[dict[str, str]] = {}