Validate tone name at construction time (fixes #39)

Tone("X") now raises ValueError immediately instead of silently
storing an invalid name and only failing on .frequency access.

Closes #39

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pytheory/tones.py

@@ -31,6 +31,7 @@ class Tone:
         alt_names: Optional[list[str]] = None,
         octave: Optional[int] = None,
         system: Union[str, object] = "western",
+        _validate: bool = True,
     ) -> None:
         """Initialize a Tone with a name, optional octave, and musical system.
 
@@ -68,6 +69,13 @@ class Tone:
             self.system_name = None
             self._system = system
 
+        # Validate tone name against the system early (fixes #39).
+        if _validate and self.system.resolve_name(name) is None:
+            raise ValueError(
+                f"Unknown tone name: {name!r}. "
+                f"Not found in the {system!r} system."
+            )
+
     @property
     def exists(self) -> bool:
         """True if this tone's name is found in the associated system."""
@@ -345,7 +353,7 @@ class Tone:
         if system:
             return klass(name=tone, octave=octave, system=system)
         else:
-            return klass(name=tone, octave=octave)
+            return klass(name=tone, octave=octave, _validate=False)
 
     @classmethod
     def from_tuple(klass, t: tuple[str, ...]) -> Tone:

test_pytheory.py

@@ -68,9 +68,16 @@ def test_tone_system():
 
 def test_tone_exists():
     c4 = Tone(name="C", octave=4, system="western")
-    invalid_tone = Tone(name="H", octave=4, system="western")
     assert c4.exists is True
-    assert invalid_tone.exists is False
+
+
+def test_tone_invalid_raises():
+    """Invalid tone names raise ValueError at construction time (fixes #39)."""
+    import pytest
+    with pytest.raises(ValueError, match="Unknown tone name"):
+        Tone(name="H", octave=4, system="western")
+    with pytest.raises(ValueError, match="Unknown tone name"):
+        Tone("X")
 
 
 def test_tone_names_method():
@@ -4839,10 +4846,11 @@ def test_solfege_no_octave():
     assert t.solfege == "Do"
 
 
-def test_solfege_unknown_returns_name():
-    """A non-standard name should be returned unchanged."""
-    t = Tone(name="X", system="western")
-    assert t.solfege == "X"
+def test_solfege_unknown_raises():
+    """A non-standard name should raise ValueError at construction (fixes #39)."""
+    import pytest
+    with pytest.raises(ValueError, match="Unknown tone name"):
+        Tone(name="X", system="western")
 
 
 # ── Rhythm / Duration system ────────────────────────────────────────────────