Create a dedicated get_model_changes function in beets/util/diff.py

- Add `get_model_changes` as the public API for computing diffs
- Remove tests for `show_model_changes` as test_diff.py fully covers them.

beets/ui/__init__.py

@@ -46,7 +46,7 @@ from beets.util.color import (
     uncolorize,
 )
 from beets.util.deprecation import deprecate_for_maintainers
-from beets.util.diff import _field_diff
+from beets.util.diff import get_model_changes
 from beets.util.functemplate import template
 
 if TYPE_CHECKING:
@@ -802,32 +802,14 @@ def show_model_changes(
     always: bool = False,
     print_obj: bool = True,
 ) -> bool:
-    """Given a Model object, print a list of changes from its pristine
-    version stored in the database. Return a boolean indicating whether
-    any changes were found.
+    """Print a diff of changes between two library model states.
 
-    `old` may be the "original" object to avoid using the pristine
-    version from the database. `fields` may be a list of fields to
-    restrict the detection to. `always` indicates whether the object is
-    always identified, regardless of whether any changes are present.
+    Optionally prints the original object label before listing field-level
+    changes when `print_obj` is enabled. When `always` is set, the object
+    label is printed even if no changes are detected. Returns whether any
+    changes were found.
     """
-    old = old or new.get_fresh_from_db()
-
-    # Keep the formatted views around instead of re-creating them in each
-    # iteration step
-    old_fmt = old.formatted()
-    new_fmt = new.formatted()
-
-    # Build up lines showing changed fields.
-    diff_fields = (set(old) | set(new)) - {"mtime"}
-    if allowed_fields := set(fields or {}):
-        diff_fields &= allowed_fields
-
-    changes = [
-        d
-        for f in sorted(diff_fields)
-        if (d := _field_diff(f, old_fmt, new_fmt))
-    ]
+    changes = get_model_changes(new, old, fields)
 
     # Print changes.
     if print_obj and (changes or always):

beets/util/diff.py

@@ -6,7 +6,10 @@ from typing import TYPE_CHECKING
 from .color import colorize
 
 if TYPE_CHECKING:
+    from collections.abc import Iterable
+
     from beets.dbcore.db import FormattedMapping
+    from beets.library.models import LibModel
 
 
 def colordiff(a: str, b: str) -> tuple[str, str]:
@@ -79,3 +82,33 @@ def _field_diff(
         newstr = colorize("text_diff_added", newstr)
 
     return f"{field}: {oldstr} -> {newstr}"
+
+
+def get_model_changes(
+    new: LibModel,
+    old: LibModel | None,
+    fields: Iterable[str] | None,
+) -> list[str]:
+    """Compute human-readable diff lines for changed fields between two models.
+
+    Compares `new` against `old`, falling back to the database version of
+    `new` when `old` is not provided. When `fields` is given, only those
+    fields are considered. The `mtime` field is always excluded.
+    """
+    old = old or new.get_fresh_from_db()
+
+    # Keep the formatted views around instead of re-creating them in each
+    # iteration step
+    old_fmt = old.formatted()
+    new_fmt = new.formatted()
+
+    # Build up lines showing changed fields.
+    diff_fields = (set(old) | set(new)) - {"mtime"}
+    if allowed_fields := set(fields or {}):
+        diff_fields &= allowed_fields
+
+    return [
+        d
+        for f in sorted(diff_fields)
+        if (d := _field_diff(f, old_fmt, new_fmt))
+    ]

test/ui/test_ui.py

@@ -329,57 +329,6 @@ class ConfigTest(IOMixin, TestPluginTestCase):
         assert config["statefile"].as_path() == self.beetsdir / "state"
 
 
-class ShowModelChangeTest(IOMixin, unittest.TestCase):
-    def setUp(self):
-        super().setUp()
-        self.a = _common.item()
-        self.b = _common.item()
-        self.a.path = self.b.path
-
-    def _show(self, **kwargs):
-        change = ui.show_model_changes(self.a, self.b, **kwargs)
-        out = self.io.getoutput()
-        return change, out
-
-    def test_identical(self):
-        change, out = self._show()
-        assert not change
-        assert out == ""
-
-    def test_string_fixed_field_change(self):
-        self.b.title = "x"
-        change, out = self._show()
-        assert change
-        assert "title" in out
-
-    def test_int_fixed_field_change(self):
-        self.b.track = 9
-        change, out = self._show()
-        assert change
-        assert "track" in out
-
-    def test_floats_close_to_identical(self):
-        self.a.length = 1.00001
-        self.b.length = 1.00005
-        change, out = self._show()
-        assert not change
-        assert out == ""
-
-    def test_floats_different(self):
-        self.a.length = 1.00001
-        self.b.length = 2.00001
-        change, out = self._show()
-        assert change
-        assert "length" in out
-
-    def test_both_values_shown(self):
-        self.a.title = "foo"
-        self.b.title = "bar"
-        _, out = self._show()
-        assert "foo" in out
-        assert "bar" in out
-
-
 class PathFormatTest(unittest.TestCase):
     def test_custom_paths_prepend(self):
         default_formats = ui.get_path_formats()