Fix MetronomeMark objects silently dropped from Lilypond output

music21/lily/lilyObjects.py

@@ -867,23 +867,34 @@ class LyTempoEvent(LyObject):
     r'''
     tempo_event: "\tempo" steno_duration '=' tempo_range
                | "\tempo" scalar steno_duration '=' tempo_range
+               | "\tempo" steno_duration '=' scalar
                | "\tempo" scalar
 
     >>> lte = lily.lilyObjects.LyTempoEvent(scalar='40')
     >>> str(lte)
     '\\tempo 40'
 
-    More complex:
+    A steno_duration paired with a single bpm scalar (and no tempoRange)
+    is the common case for a music21 MetronomeMark, e.g. quarter = 87:
 
-    >>> steno = lily.lilyObjects.LyStenoDuration('quarter')
+    >>> steno = lily.lilyObjects.LyStenoDuration('4')
+    >>> lte = lily.lilyObjects.LyTempoEvent(stenoDuration=steno, scalar=87)
+    >>> str(lte)
+    '\\tempo 4  = 87'
+
+    More complex, with a tempo range. Note that steno_duration takes a
+    Lilypond duration number such as '4' for a quarter note, not the
+    English name 'quarter':
+
+    >>> steno = lily.lilyObjects.LyStenoDuration('4')
     >>> tempoRange = lily.lilyObjects.LyTempoRange(70, 100)
     >>> lte = lily.lilyObjects.LyTempoEvent(tempoRange=tempoRange, stenoDuration=steno)
     >>> str(lte)
-    '\\tempo quarter  = 70~100 '
+    '\\tempo 4  = 70~100 '
 
     >>> lte.scalar = 85
     >>> str(lte)
-    '\\tempo 85 quarter  = 70~100 '
+    '\\tempo 85 4  = 70~100 '
     '''
 
     def __init__(self, tempoRange=None, stenoDuration=None, scalar=None):
@@ -906,6 +917,11 @@ def stringOutput(self):
             else:
                 return ' '.join([base, self.stenoDuration.stringOutput(),
                                  '=', self.tempoRange.stringOutput()])
+        elif self.stenoDuration is not None:
+            if self.scalar is None:  # pragma: no cover
+                raise LilyObjectsException(
+                    'If tempoRange is not defined but stenoDuration is, need a scalar')
+            return ' '.join([base, self.stenoDuration.stringOutput(), '=', str(self.scalar)])
         elif self.scalar is None:  # pragma: no cover
             raise LilyObjectsException('If tempoRange is not defined then need scalar')
 

music21/lily/translate.py

@@ -33,6 +33,7 @@
 from music21 import key
 from music21 import note
 from music21 import stream
+from music21 import tempo
 from music21 import variant
 
 from music21.lily import lilyObjects as lyo
@@ -1096,6 +1097,11 @@ def appendM21ObjectToContext(self, thisObject):
             lyObject = self.lyEmbeddedScmFromTimeSignature(thisObject)
             currentMusicList.append(lyObject)
             lyObject.setParent(contextObject)
+        elif 'MetronomeMark' in c:
+            lyObject = self.lyEmbeddedScmFromMetronomeMark(thisObject)
+            if lyObject is not None:
+                currentMusicList.append(lyObject)
+                lyObject.setParent(contextObject)
         elif 'Variant' in c:
             self.appendContextFromVariant(thisObject, coloredVariants=self.coloredVariants)
         elif 'SystemLayout' in c:
@@ -1680,6 +1686,38 @@ def lyEmbeddedScmFromTimeSignature(self, ts):
         lpEmbeddedScm.content = keyScheme
         return lpEmbeddedScm
 
+    def lyEmbeddedScmFromMetronomeMark(self, mm: tempo.MetronomeMark) -> lyo.LyEmbeddedScm|None:
+        # noinspection PyShadowingNames
+        r'''
+        convert a :class:`~music21.tempo.MetronomeMark` object
+        to a lilyObjects.LyEmbeddedScm object
+
+        * New in v11: MetronomeMark objects are now written out when
+          converting a Stream to Lilypond; previously they were silently dropped.
+
+        >>> mm = tempo.MetronomeMark(number=87, referent=note.Note(type='quarter'))
+        >>> conv = lily.translate.LilypondConverter()
+        >>> print(conv.lyEmbeddedScmFromMetronomeMark(mm))
+        \tempo 4  = 87
+
+        A MetronomeMark without a number produces no output:
+
+        >>> mm = tempo.MetronomeMark()
+        >>> mm.number is None
+        True
+        >>> conv.lyEmbeddedScmFromMetronomeMark(mm) is None
+        True
+        '''
+        if mm.number is None:
+            return None
+
+        multipliedDuration = self.lyMultipliedDurationFromDuration(mm.referent)
+        tempoEvent = lyo.LyTempoEvent(stenoDuration=multipliedDuration.stenoDur, scalar=mm.number)
+
+        lpEmbeddedScm = lyo.LyEmbeddedScm()
+        lpEmbeddedScm.content = tempoEvent.stringOutput() + lpEmbeddedScm.newlineIndent
+        return lpEmbeddedScm
+
     def setContextForTupletStart(self, inObj):
         r'''
         if the inObj has tuplets then we set a new context
@@ -2596,6 +2634,27 @@ def testColors(self):
             "c' 4  "
         )
 
+    def testMetronomeMark(self):
+        mm = tempo.MetronomeMark(number=87, referent=note.Note(type='quarter'))
+        lpEmbeddedScm = LilypondConverter().lyEmbeddedScmFromMetronomeMark(mm)
+        self.assertEqual(str(lpEmbeddedScm).strip(), r'\tempo 4  = 87')
+
+        # a MetronomeMark with no number produces no output
+        textOnly = tempo.MetronomeMark()
+        self.assertIsNone(LilypondConverter().lyEmbeddedScmFromMetronomeMark(textOnly))
+
+    def testMetronomeMarkWrittenInStream(self):
+        # https://github.com/cuthbertLab/music21/issues/1852
+        from music21 import key
+        from music21 import meter
+        keysig = key.Key('a-')
+        mm = tempo.MetronomeMark(number=87, referent=note.Note(type='quarter'))
+        timesig = meter.TimeSignature('3/4')
+        s = stream.Stream([keysig, mm, timesig, note.Note()])
+        lpc = LilypondConverter()
+        lpc.loadObjectFromScore(s, makeNotation=False)
+        self.assertIn(r'\tempo 4  = 87', str(lpc.topLevelObject))
+
 class TestExternal(unittest.TestCase):
     show = True