Dispatch: Better error when defining duplicate signature

runtype/dispatch.py

@@ -1,6 +1,6 @@
 from collections import defaultdict
 from functools import wraps
-from typing import Any, Dict, Callable, Sequence
+from typing import Any, Dict, Callable, Sequence, List
 from operator import itemgetter
 import warnings
 
@@ -14,6 +14,22 @@ class DispatchError(Exception):
     "Thrown whenever a dispatch fails. Contains text describing the conflict."
 
 
+@dataclass
+class DuplicateSignatureError(Exception):
+    signature: List[type]
+    first: Callable
+    second: Callable
+
+    def __str__(self):
+        code1 = self.first.__code__
+        code2 = self.second.__code__
+        return (f"Duplicate signature defined for '{self.first.__name__}'\n"
+                 f" - Signature: {self.signature}.\n"
+                 f" - Definition 1: {code1.co_filename}:{code1.co_firstlineno}\n"
+                 f" - Definition 2: {code2.co_filename}:{code2.co_firstlineno}\n"
+        )
+
+
 # TODO: Remove test_subtypes, replace with support for Type[], like isa(t, Type[t])
 class MultiDispatch:
     """Creates a dispatch group for multiple dispatch
@@ -180,10 +196,8 @@ def define_function(self, f):
                 node = node.follow_type[t]
 
             if node.func is not None:
-                code_obj = node.func[0].__code__
-                raise ValueError(
-                    f"Function {f.__name__} at {code_obj.co_filename}:{code_obj.co_firstlineno} matches existing signature: {signature}!"
-                )
+                raise DuplicateSignatureError(signature, node.func[0], f)
+
             node.func = f, signature
 
     def choose_most_specific_function(self, args, *funcs):

tests/test_basic.py

@@ -12,7 +12,7 @@
 logging.basicConfig(level=logging.INFO)
 
 from runtype import Dispatch, DispatchError, dataclass, isa, is_subtype, issubclass, assert_isa, String, Int, validate_func, cv_type_checking, multidispatch
-from runtype.dispatch import MultiDispatch
+from runtype.dispatch import MultiDispatch, DuplicateSignatureError
 from runtype.dataclass import Configuration
 
 try:
@@ -291,7 +291,7 @@ def f(s:str):
             @dy
             def f(x: int):
                 return NotImplemented
-        except ValueError:
+        except DuplicateSignatureError:
             pass
         else:
             assert False, f
@@ -512,7 +512,7 @@ def f(i:int, j:object, k:object):
             @dy
             def f(i:int, j:object, k:object):
                 return "Oops"
-        except ValueError:
+        except DuplicateSignatureError:
             pass
         else:
             assert False
@@ -536,7 +536,7 @@ def f(x: types[0]):
                     def f(x):
                         pass
                     assert False
-                except ValueError:
+                except DuplicateSignatureError:
                     pass
 
             for t in types[1:]:
@@ -545,7 +545,7 @@ def f(x):
                     def f(x: t):
                         pass
                     assert False, t
-                except ValueError:
+                except DuplicateSignatureError:
                     pass
 
         _test_canon(object, Union[object], include_none=True)
@@ -688,7 +688,7 @@ def f(a: int):
         def f(a: int):
             return 'a'
         f.__module__ = 'a'
-        self.assertRaises(ValueError, multidispatch, f)
+        self.assertRaises(DuplicateSignatureError, multidispatch, f)
 
     def test_none(self):
         dp = Dispatch()