1 files changed, 154 insertions, 3 deletions
diff --git a/Lib/fontTools/varLib/errors.py b/Lib/fontTools/varLib/errors.py
index b73f1886..5840070f 100644
--- a/Lib/fontTools/varLib/errors.py
+++ b/Lib/fontTools/varLib/errors.py
@@ -1,3 +1,6 @@
+import textwrap
+
+
 class VarLibError(Exception):
     """Base exception for the varLib module."""
 
@@ -9,8 +12,144 @@ class VarLibValidationError(VarLibError):
 class VarLibMergeError(VarLibError):
     """Raised when input data cannot be merged into a variable font."""
 
+    def __init__(self, merger, **kwargs):
+        self.merger = merger
+        if not kwargs:
+            kwargs = {}
+        if "stack" in kwargs:
+            self.stack = kwargs["stack"]
+            del kwargs["stack"]
+        else:
+            self.stack = []
+        self.cause = kwargs
+
+    @property
+    def reason(self):
+        return self.__doc__
+
+    def _master_name(self, ix):
+        ttf = self.merger.ttfs[ix]
+        if (
+            "name" in ttf
+            and ttf["name"].getDebugName(1)
+            and ttf["name"].getDebugName(2)
+        ):
+            return ttf["name"].getDebugName(1) + " " + ttf["name"].getDebugName(2)
+        elif hasattr(ttf.reader, "file") and hasattr(ttf.reader.file, "name"):
+            return ttf.reader.file.name
+        else:
+            return "master number %i" % ix
+
+    @property
+    def offender(self):
+        if "expected" in self.cause and "got" in self.cause:
+            index = [x == self.cause["expected"] for x in self.cause["got"]].index(
+                False
+            )
+            return index, self._master_name(index)
+        return None, None
+
+    @property
+    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 ShouldBeConstant(VarLibMergeError):
+    """some values were different, but should have been the same"""
+
+    @property
+    def details(self):
+        if self.stack[0] != ".FeatureCount":
+            return super().details
+        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[self.stack[-1]].table.FeatureList.FeatureRecord
+        ]
+        bad_features = [
+            x.FeatureTag
+            for x in bad_ttf[self.stack[-1]].table.FeatureList.FeatureRecord
+        ]
+        return (
+            "\nIncompatible features between masters.\n"
+            f"Expected: {', '.join(good_features)}.\n"
+            f"Got: {', '.join(bad_features)}.\n"
+        )
+
+
+class FoundANone(VarLibMergeError):
+    """one of the values in a list was empty when it shouldn't have been"""
+
+    @property
+    def offender(self):
+        cause = self.argv[0]
+        index = [x is None for x in cause["got"]].index(True)
+        return index, self._master_name(index)
+
+    @property
+    def details(self):
+        cause, stack = self.args[0], self.args[1:]
+        return f"{stack[0]}=={cause['got']}\n"
+
+
+class MismatchedTypes(VarLibMergeError):
+    """data had inconsistent types"""
+
+
+class LengthsDiffer(VarLibMergeError):
+    """a list of objects had inconsistent lengths"""
 
-class VarLibCFFDictMergeError(VarLibMergeError):
+
+class KeysDiffer(VarLibMergeError):
+    """a list of objects had different keys"""
+
+
+class InconsistentGlyphOrder(VarLibMergeError):
+    """the glyph order was inconsistent between masters"""
+
+
+class InconsistentExtensions(VarLibMergeError):
+    """the masters use extension lookups in inconsistent ways"""
+
+
+class UnsupportedFormat(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 UnsupportedFormat(UnsupportedFormat):
+    """an OpenType subtable (%s) had inconsistent formats between masters"""
+
+
+class VarLibCFFMergeError(VarLibError):
+    pass
+
+
+class VarLibCFFDictMergeError(VarLibCFFMergeError):
     """Raised when a CFF PrivateDict cannot be merged."""
 
     def __init__(self, key, value, values):
@@ -23,8 +162,8 @@ class VarLibCFFDictMergeError(VarLibMergeError):
         self.args = (error_msg,)
 
 
-class VarLibCFFPointTypeMergeError(VarLibMergeError):
-    """Raised when a CFF glyph cannot be merged."""
+class VarLibCFFPointTypeMergeError(VarLibCFFMergeError):
+    """Raised when a CFF glyph cannot be merged because of point type differences."""
 
     def __init__(self, point_type, pt_index, m_index, default_type, glyph_name):
         error_msg = (
@@ -35,5 +174,17 @@ class VarLibCFFPointTypeMergeError(VarLibMergeError):
         self.args = (error_msg,)
 
 
+class VarLibCFFHintTypeMergeError(VarLibCFFMergeError):
+    """Raised when a CFF glyph cannot be merged because of hint type differences."""
+
+    def __init__(self, hint_type, cmd_index, m_index, default_type, glyph_name):
+        error_msg = (
+            f"Glyph '{glyph_name}': '{hint_type}' at index {cmd_index} in "
+            f"master index {m_index} differs from the default font hint type "
+            f"'{default_type}'"
+        )
+        self.args = (error_msg,)
+
+
 class VariationModelError(VarLibError):
     """Raised when a variation model is faulty."""