Use individual exception classes instead of enum

Lib/fontTools/varLib/errors.py

@@ -1,22 +1,6 @@
-from enum import Enum, auto
 import textwrap
 
 
-class VarLibMergeFailure(Enum):
-    ShouldBeConstant = "some values were different, but should have been the same"
-    MismatchedTypes = "data had inconsistent types"
-    LengthsDiffer = "a list of objects had inconsistent lengths"
-    KeysDiffer = "a list of objects had different keys"
-    InconsistentGlyphOrder = "the glyph order was inconsistent between masters"
-    FoundANone = "one of the values in a list was empty when it shouldn't have been"
-    NotANone = "one of the values in a list was not empty when it should have been"
-    UnsupportedFormat = "an OpenType subtable (%s) had a format I didn't expect"
-    InconsistentFormat = (
-        "an OpenType subtable (%s) had inconsistent formats between masters"
-    )
-    InconsistentExtensions = "the masters use extension lookups in inconsistent ways"
-
-
 class VarLibError(Exception):
     """Base exception for the varLib module."""
 
@@ -32,6 +16,18 @@ class VarLibMergeError(VarLibError):
         self.merger = merger
         self.args = args
 
+    @property
+    def cause(self):
+        return self.args[0]
+
+    @property
+    def stack(self):
+        return self.args[1:]
+
+    @property
+    def reason(self):
+        return self.__doc__
+
     def _master_name(self, ix):
         ttf = self.merger.ttfs[ix]
         if (
@@ -45,26 +41,55 @@ class VarLibMergeError(VarLibError):
         else:
             return "master number %i" % ix
 
-    def _offender(self, cause):
+    @property
-        reason = cause["reason"].value
+    def offender(self):
-        if "expected" in cause and "got" in cause:
+        if "expected" in self.cause and "got" in self.cause:
-            index = [x == cause["expected"] for x in cause["got"]].index(False)
+            index = [x == self.cause["expected"] for x in self.cause["got"]].index(
-            return index, self._master_name(index)
+                False
-        if reason is VarLibMergeFailure.FoundANone:
+            )
-            index = [x is None for x in cause["got"]].index(True)
             return index, self._master_name(index)
         return None, None
 
-    def _incompatible_features(self, offender_index):
+    @property
-        cause, stack = self.args[0], self.args[1:]
+    def details(self):
+        if "expected" in self.cause and "got" in self.cause:
+            offender_index, offender = self.offender
+            got = self.cause["got"][offender_index]
+            return f"Expected to see {self.stack[0]}=={self.cause['expected']}, instead saw {got}\n"
+        return ""
+
+    def __str__(self):
+        offender_index, offender = self.offender
+        location = ""
+        if offender:
+            location = f"\n\nThe problem is likely to be in {offender}:\n"
+        context = "".join(reversed(self.stack))
+        basic = textwrap.fill(
+            f"Couldn't merge the fonts, because {self.reason}. "
+            f"This happened while performing the following operation: {context}",
+            width=78,
+        )
+        return "\n\n" + basic + location + self.details
+
+
+class VarLibMergeFailureShouldBeConstant(VarLibMergeError):
+    """some values were different, but should have been the same"""
+
+    @property
+    def details(self):
+        if self.stack[0] != ".FeatureCount":
+            return super()
+        offender_index, offender = self.offender
         bad_ttf = self.merger.ttfs[offender_index]
         good_ttf = self.merger.ttfs[offender_index - 1]
 
         good_features = [
-            x.FeatureTag for x in good_ttf[stack[-1]].table.FeatureList.FeatureRecord
+            x.FeatureTag
+            for x in good_ttf[self.stack[-1]].table.FeatureList.FeatureRecord
         ]
         bad_features = [
-            x.FeatureTag for x in bad_ttf[stack[-1]].table.FeatureList.FeatureRecord
+            x.FeatureTag
+            for x in bad_ttf[self.stack[-1]].table.FeatureList.FeatureRecord
         ]
         return (
             "\nIncompatible features between masters.\n"
@@ -72,39 +97,65 @@ class VarLibMergeError(VarLibError):
             f"Got: {', '.join(bad_features)}.\n"
         )
 
-    def __str__(self):
-        cause, stack = self.args[0], self.args[1:]
-        context = "".join(reversed(stack))
-        details = ""
-        reason = cause["reason"].value
-        offender_index, offender = self._offender(cause)
-        if offender:
-            details = f"\n\nThe problem is likely to be in {offender}:\n"
-        if cause["reason"] is VarLibMergeFailure.FoundANone:
-            details = details + f"{stack[0]}=={cause['got']}\n"
-        elif (
-            cause["reason"] is VarLibMergeFailure.ShouldBeConstant
-            and stack[0] == ".FeatureCount"
-        ):
-            # Common case
-            details = details + self._incompatible_features(offender_index)
-        elif "expected" in cause and "got" in cause:
-            got = cause["got"][offender_index]
-            details = details + (
-                f"Expected to see {stack[0]}=={cause['expected']}, instead saw {got}\n"
-            )
 
-        if (
+class VarLibMergeFailureFoundANone(VarLibMergeError):
-            cause["reason"] is VarLibMergeFailure.UnsupportedFormat
+    """one of the values in a list was empty when it shouldn't have been"""
-            or cause["reason"] is VarLibMergeFailure.InconsistentFormat
+
-        ):
+    @property
-            reason = reason % cause["subtable"]
+    def offender(self):
-        basic = textwrap.fill(
+        cause = self.argv[0]
-            f"Couldn't merge the fonts, because {reason}. "
+        index = [x is None for x in cause["got"]].index(True)
-            f"This happened while performing the following operation: {context}",
+        return index, self._master_name(index)
-            width=78,
+
-        )
+    @property
-        return "\n\n" + basic + details
+    def details(self):
+        cause, stack = self.args[0], self.args[1:]
+        return f"{stack[0]}=={cause['got']}\n"
+
+
+class VarLibMergeFailureMismatchedTypes(VarLibMergeError):
+    """data had inconsistent types"""
+
+    pass
+
+
+class VarLibMergeFailureLengthsDiffer(VarLibMergeError):
+    """a list of objects had inconsistent lengths"""
+
+    pass
+
+
+class VarLibMergeFailureKeysDiffer(VarLibMergeError):
+    """a list of objects had different keys"""
+
+    pass
+
+
+class VarLibMergeFailureInconsistentGlyphOrder(VarLibMergeError):
+    """the glyph order was inconsistent between masters"""
+
+    pass
+
+
+class VarLibMergeFailureInconsistentExtensions(VarLibMergeError):
+    """the masters use extension lookups in inconsistent ways"""
+
+    pass
+
+
+class VarLibMergeFailureUnsupportedFormat(VarLibMergeError):
+    """an OpenType subtable (%s) had a format I didn't expect"""
+
+    @property
+    def reason(self):
+        cause, stack = self.args[0], self.args[1:]
+        return self.__doc__ % cause["subtable"]
+
+
+class VarLibMergeFailureUnsupportedFormat(VarLibMergeFailureUnsupportedFormat):
+    """an OpenType subtable (%s) had inconsistent formats between masters"""
+
+    pass
 
 
 class VarLibCFFDictMergeError(VarLibMergeError):

Lib/fontTools/varLib/merger.py

@@ -14,8 +14,18 @@ from fontTools.varLib.varStore import VarStoreInstancer
 from functools import reduce
 from fontTools.otlLib.builder import buildSinglePos
 
-from .errors import VarLibMergeError, VarLibMergeFailure
+from .errors import (
-
+    VarLibMergeFailureShouldBeConstant,
+    VarLibMergeFailureFoundANone,
+    VarLibMergeFailureMismatchedTypes,
+    VarLibMergeFailureLengthsDiffer,
+    VarLibMergeFailureKeysDiffer,
+    VarLibMergeFailureInconsistentGlyphOrder,
+    VarLibMergeFailureInconsistentExtensions,
+    VarLibMergeFailureUnsupportedFormat,
+    VarLibMergeFailureUnsupportedFormat,
+    VarLibMergeError,
+)
 
 class Merger(object):
 
@@ -69,8 +79,7 @@ class Merger(object):
 				item.ensureDecompiled()
 		keys = sorted(vars(out).keys())
 		if not all(keys == sorted(vars(v).keys()) for v in lst):
-			raise VarLibMergeError(self, ({
+			raise VarLibMergeFailureKeysDiffer(self, ({
-						"reason": VarLibMergeFailure.KeysDiffer,
 						"expected": keys,
 						"got": [sorted(vars(v).keys()) for v in lst]},))
 		mergers = self.mergersFor(out)
@@ -88,8 +97,7 @@ class Merger(object):
 
 	def mergeLists(self, out, lst):
 		if not allEqualTo(out, lst, len):
-			raise VarLibMergeError(self, ({
+			raise VarLibMergeFailureLengthsDiffer(self, ({
-						"reason": VarLibMergeFailure.LengthsDiffer,
 						"expected": len(out),
 						"got": [len(x) for x in lst]},))
 		for i,(value,values) in enumerate(zip(out, zip(*lst))):
@@ -101,8 +109,7 @@ class Merger(object):
 
 	def mergeThings(self, out, lst):
 		if not allEqualTo(out, lst, type):
-			raise VarLibMergeError(self, ({
+			raise VarLibMergeFailureMismatchedTypes(self, ({
-					"reason": VarLibMergeFailure.MismatchedTypes,
 					"expected": type(out),
 					"got": [type(x) for x in lst]},))
 		mergerFunc = self.mergersFor(out).get(None, None)
@@ -114,8 +121,7 @@ class Merger(object):
 			self.mergeLists(out, lst)
 		else:
 			if not allEqualTo(out, lst):
-				raise VarLibMergeError(self, ({
+				raise VarLibMergeFailureShouldBeConstant(self, ({
-					"reason": VarLibMergeFailure.ShouldBeConstant,
 					"expected": out,
 					"got": lst},))
 
@@ -140,8 +146,7 @@ class AligningMerger(Merger):
 def merge(merger, self, lst):
 	if self is None:
 		if not allNone(lst):
-			raise VarLibMergeError(self, ({
+			raise VarLibMergeFailureNotANone(self, ({
-				"reason": VarLibMergeFailure.NotANone,
 				"expected": None,
 				"got": lst
 			},))
@@ -157,8 +162,7 @@ def merge(merger, self, lst):
 	for k in allKeys:
 		allValues = nonNone(l.get(k) for l in lst)
 		if not allEqual(allValues):
-			raise VarLibMergeError(self, ({
+			raise VarLibMergeFailureShouldBeConstant(self, ({
-				"reason": VarLibMergeFailure.ShouldBeConstant,
 				"expected": allValues[0],
 				"got": lst,
 			}, "."+k))
@@ -198,8 +202,7 @@ def _merge_GlyphOrders(font, lst, values_lst=None, default=None):
 	order = sorted(combined, key=sortKey)
 	# Make sure all input glyphsets were in proper order
 	if not all(sorted(vs, key=sortKey) == vs for vs in lst):
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureInconsistentGlyphOrder(self, ({
-			"reason" : VarLibMergeFailure.InconsistentGlyphOrder
 		},))
 	del combined
 
@@ -227,8 +230,7 @@ def _Lookup_SinglePos_get_effective_value(subtables, glyph):
 		elif self.Format == 2:
 			return self.Value[self.Coverage.glyphs.index(glyph)]
 		else:
-			raise VarLibMergeError(self, ({
+			raise VarLibMergeFailureUnsupportedFormat(self, ({
-				"reason": VarLibMergeFailure.UnsupportedFormat,
 				"subtable": "single positioning lookup"
 			},))
 	return None
@@ -252,8 +254,7 @@ def _Lookup_PairPos_get_effective_value_pair(subtables, firstGlyph, secondGlyph)
 			klass2 = self.ClassDef2.classDefs.get(secondGlyph, 0)
 			return self.Class1Record[klass1].Class2Record[klass2]
 		else:
-			raise VarLibMergeError(self, ({
+			raise VarLibMergeFailureUnsupportedFormat(self, ({
-				"reason": VarLibMergeFailure.UnsupportedFormat,
 				"subtable": "pair positioning lookup"
 			},))
 	return None
@@ -262,8 +263,7 @@ def _Lookup_PairPos_get_effective_value_pair(subtables, firstGlyph, secondGlyph)
 def merge(merger, self, lst):
 	self.ValueFormat = valueFormat = reduce(int.__or__, [l.ValueFormat for l in lst], 0)
 	if not (len(lst) == 1 or (valueFormat & ~0xF == 0)):
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureUnsupportedFormat(self, ({
-				"reason": VarLibMergeFailure.UnsupportedFormat,
 				"subtable": "single positioning lookup"
 			},))
 
@@ -552,8 +552,7 @@ def merge(merger, self, lst):
 	elif self.Format == 2:
 		_PairPosFormat2_merge(self, lst, merger)
 	else:
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureUnsupportedFormat(self, ({
-				"reason": VarLibMergeFailure.UnsupportedFormat,
 				"subtable": "pair positioning lookup"
 			},))
 
@@ -621,8 +620,7 @@ def _MarkBasePosFormat1_merge(self, lst, merger, Mark='Mark', Base='Base'):
 		# input masters.
 
 		if not allEqual(allClasses):
-			raise VarLibMergeError(self, allClasses)
+			raise allClasses(self, allClasses)
-		if not allClasses:
 			rec = None
 		else:
 			rec = ot.MarkRecord()
@@ -671,32 +669,28 @@ def _MarkBasePosFormat1_merge(self, lst, merger, Mark='Mark', Base='Base'):
 @AligningMerger.merger(ot.MarkBasePos)
 def merge(merger, self, lst):
 	if not allEqualTo(self.Format, (l.Format for l in lst)):
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureInconsistentFormats(self, ({
-			"reason": VarLibMergeFailure.InconsistentFormats,
 			"subtable": "mark-to-base positioning lookup",
 			"expected": self.Format,
 			"got": [l.Format for l in lst]},))
 	if self.Format == 1:
 		_MarkBasePosFormat1_merge(self, lst, merger)
 	else:
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureUnsupportedFormat(self, ({
-			"reason": VarLibMergeFailure.UnsupportedFormat,
 			"subtable": "mark-to-base positioning lookup",
 		}))
 
 @AligningMerger.merger(ot.MarkMarkPos)
 def merge(merger, self, lst):
 	if not allEqualTo(self.Format, (l.Format for l in lst)):
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureInconsistentFormats(self, ({
-			"reason": VarLibMergeFailure.InconsistentFormats,
 			"subtable": "mark-to-mark positioning lookup",
 			"expected": self.Format,
 			"got": [l.Format for l in lst]},))
 	if self.Format == 1:
 		_MarkBasePosFormat1_merge(self, lst, merger, 'Mark1', 'Mark2')
 	else:
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureUnsupportedFormat(self, ({
-			"reason": VarLibMergeFailure.UnsupportedFormat,
 			"subtable": "mark-to-mark positioning lookup",
 		}))
 
@@ -828,14 +822,12 @@ def merge(merger, self, lst):
 			continue
 		if sts[0].__class__.__name__.startswith('Extension'):
 			if not allEqual([st.__class__ for st in sts]):
-				raise VarLibMergeError(self, ({
+				raise VarLibMergeFailureInconsistentExtensions(self, ({
-					"reason": VarLibMergeFailure.InconsistentExtensions,
 					"expected": "Extension",
 					"got": [st.__class__.__name__ for st in sts]
 				},))
 			if not allEqual([st.ExtensionLookupType for st in sts]):
-				raise VarLibMergeError(self, ({
+				raise VarLibMergeFailureInconsistentExtensions(self, ({
-					"reason": VarLibMergeFailure.InconsistentExtensions,
 				},))
 			l.LookupType = sts[0].ExtensionLookupType
 			new_sts = [st.ExtSubTable for st in sts]
@@ -1065,8 +1057,7 @@ class VariationMerger(AligningMerger):
 		if None in lst:
 			if allNone(lst):
 				if out is not None:
-					raise VarLibMergeError(self, ({
+					raise VarLibMergeFailureFoundANone(self, ({
-						"reason": VarLibMergeFailure.FoundANone,
 						"got": lst
 					},))
 				return
@@ -1089,8 +1080,7 @@ def buildVarDevTable(store_builder, master_values):
 @VariationMerger.merger(ot.BaseCoord)
 def merge(merger, self, lst):
 	if self.Format != 1:
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureUnsupportedFormat(self, ({
-			"cause": VarLibMergeFailure.UnsupportedFormat,
 			"subtable": "a baseline coordinate"
 		},))
 	self.Coordinate, DeviceTable = buildVarDevTable(merger.store_builder, [a.Coordinate for a in lst])
@@ -1101,8 +1091,7 @@ def merge(merger, self, lst):
 @VariationMerger.merger(ot.CaretValue)
 def merge(merger, self, lst):
 	if self.Format != 1:
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureUnsupportedFormat(self, ({
-			"cause": VarLibMergeFailure.UnsupportedFormat,
 			"subtable": "a caret"
 		},))
 	self.Coordinate, DeviceTable = buildVarDevTable(merger.store_builder, [a.Coordinate for a in lst])
@@ -1113,8 +1102,7 @@ def merge(merger, self, lst):
 @VariationMerger.merger(ot.Anchor)
 def merge(merger, self, lst):
 	if self.Format != 1:
-		raise VarLibMergeError(self, ({
+		raise VarLibMergeFailureUnsupportedFormat(self, ({
-			"cause": VarLibMergeFailure.UnsupportedFormat,
 			"subtable": "an anchor"
 		},))
 	self.XCoordinate, XDeviceTable = buildVarDevTable(merger.store_builder, [a.XCoordinate for a in lst])