Raise warning when a method is overwritten (#161)

* Raise warning when a method is overwritten

* Test MethodRedefintionWarning

* Explicity resolve registrations

plum/resolver.py

@@ -1,5 +1,6 @@
 import pydoc
 import sys
+import warnings
 from functools import wraps
 from typing import Callable, Optional, Tuple, Union
 
@@ -14,6 +15,10 @@
 __all__ = ["AmbiguousLookupError", "NotFoundLookupError"]
 
 
+class MethodRedefinitionWarning(Warning):
+    """A method is redefined."""
+
+
 def _render_function_call(f: str, target: Union[Tuple, Signature]) -> str:
     """Render a function call.
 
@@ -260,6 +265,12 @@ def register(self, method: Method) -> None:
                     f"The added method `{method}` is equal to {sum(existing)} "
                     f"existing methods. This should never happen."
                 )
+            previous_method = self.methods[existing.index(True)]
+            warnings.warn(
+                f"`{method}` overwrites the earlier definition `{previous_method}`.",
+                category=MethodRedefinitionWarning,
+                stacklevel=0,
+            )
             self.methods[existing.index(True)] = method
         else:
             self.methods.append(method)

tests/test_resolver.py

@@ -1,13 +1,16 @@
 import sys
 import textwrap
 import typing
+import warnings
 
 import pytest
 
 import plum.resolver
+from plum.dispatcher import Dispatcher
 from plum.method import Method
 from plum.resolver import (
     AmbiguousLookupError,
+    MethodRedefinitionWarning,
     NotFoundLookupError,
     Resolver,
     _document,
@@ -242,3 +245,29 @@ def f(x):
     assert r.resolve(m_c1.signature) == m_b1
     m_b2.signature.precedence = 2
     assert r.resolve(m_c1.signature) == m_b2
+
+
+def test_redefinition_warning():
+    dispatch = Dispatcher()
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+
+        @dispatch
+        def f(x: int):
+            pass
+
+        @dispatch
+        def f(x: str):
+            pass
+
+        # Warnings are only emitted when all registrations are resolved.
+        f._resolve_pending_registrations()
+
+    with pytest.warns(MethodRedefinitionWarning):
+
+        @dispatch
+        def f(x: int):
+            pass
+
+        f._resolve_pending_registrations()