Merge pull request #3503 from Glandos/patch-2

Don't try to hash value if enum is empty

src/PIL/TiffTags.py

@@ -29,7 +29,10 @@ class TagInfo(namedtuple("_TagInfo", "value name type length enum")):
             cls, value, name, type, length, enum or {})
 
     def cvt_enum(self, value):
-        return self.enum.get(value, value)
+        # Using get will call hash(value), which can be expensive
+        # for some types (e.g. Fraction). Since self.enum is rarely
+        # used, it's usually better to test it first.
+        return self.enum.get(value, value) if self.enum else value
 
 
 def lookup(tag):