fix ignored KeyError exceptions raised in diff handlers

Author: mr-mixas

nested_diff/__init__.py

@@ -154,13 +154,15 @@ def diff(self, a, b):
         if a is b:
             return True, {'U': a} if self.op_u else {}
 
+        differ = self.default_differ
+
         if a.__class__ is b.__class__:
             try:
-                return self._differs[a.__class__](self, a, b)
+                differ = self._differs[a.__class__]
             except KeyError:
                 pass
 
-        return self.default_differ(self, a, b)
+        return differ(self, a, b)
 
     def set_handler(self, handler):
         """Set handler.

tests/test_handlers.py

@@ -1,3 +1,5 @@
+import pytest
+
 from nested_diff import Differ, handlers
 
 
@@ -25,6 +27,15 @@ def diff(self, differ, a, b):
     assert got == expected
 
 
+def test_diff_handler_exceptions_propagated():
+    class IntHandler(handlers.IntHandler):
+        def diff(*args):  # noqa: ARG002
+            raise KeyError
+
+    with pytest.raises(KeyError):
+        Differ(handlers=[IntHandler()]).diff(1, 2)
+
+
 def test_scalar_handler_diff_equal():
     a = 0
     b = 0