MIDI-175: Support empty MidiPiece trims (#13)

* Support empty trims

* fix tests

Author: roszcz

fortepyan/midi/structures.py

@@ -113,6 +113,12 @@ def time_shift(self, shift_s: float) -> "MidiPiece":
         new_piece.df.end += shift_s
         return new_piece
 
+    @classmethod
+    def empty(cls) -> "MidiPiece":
+        df = pd.DataFrame(columns=["start", "duration", "velocity", "pitch"])
+        piece = cls(df=df)
+        return piece
+
     def trim(
         self,
         start: float,
@@ -122,7 +128,7 @@ def trim(
 
         idx = np.where(ids)[0]
         if len(idx) == 0:
-            raise IndexError("No notes found in the specified range.")
+            return MidiPiece.empty()
 
         start_idx = idx[0]
         finish_idx = idx[-1] + 1

tests/midi/test_structures.py

@@ -87,14 +87,16 @@ def test_midi_piece_duration_calculation(sample_df):
 
 
 def test_trim_out_of_bounds(sample_midi_piece):
-    with pytest.raises(IndexError):
-        _ = sample_midi_piece.trim(5.5, 8)  # Out of bounds, should raise an error
+    # Out of bounds, should return empty piece
+    piece = sample_midi_piece.trim(5.5, 8)
+    assert piece.df.empty
 
 
 def test_trim_with_invalid_range(sample_midi_piece):
-    # Assuming the behavior is to raise an error with invalid range
-    with pytest.raises(IndexError):
-        _ = sample_midi_piece.trim(4, 2)  # Invalid range, start is greater than finish
+    # Invalid range, start is greater than finish
+    # Should return empty piece
+    piece = sample_midi_piece.trim(4, 2)
+    assert piece.df.empty
 
 
 def test_to_midi(sample_midi_piece):