Fix: function-redefined not triggered for functions with leading underscores (close #9894) (#10606)

Author: Akshay9715

doc/whatsnew/fragments/9894.false_negative

@@ -0,0 +1,3 @@
+Fix false negative where function-redefined (E0102) was not reported for functions with a leading underscore.
+
+Closes #9894

pylint/checkers/base/basic_error_checker.py

@@ -95,6 +95,71 @@ def x(self, value): self._x = value
     return False
 
 
+def _extract_register_target(dec: nodes.NodeNG) -> nodes.NodeNG | None:
+    """
+    If decorator `dec` looks like `@func.register(...)` or `@func.register`,
+    return the `func` target node (Name or Attribute). Otherwise return None.
+    """
+    if isinstance(dec, nodes.Call):
+        func_part = dec.func
+        if isinstance(func_part, nodes.Attribute) and func_part.attrname == "register":
+            return func_part.expr
+        return None
+
+    if isinstance(dec, nodes.Attribute) and dec.attrname == "register":
+        return dec.expr
+
+    return None
+
+
+def _inferred_has_singledispatchmethod(target: nodes.NodeNG) -> bool:
+    """
+    Infer `target` and return True if the inferred object has a
+    @singledispatchmethod decorator.
+    """
+    inferred = utils.safe_infer(target)
+    if not inferred:
+        return False
+
+    if isinstance(inferred, (nodes.FunctionDef, nodes.AsyncFunctionDef)):
+        decorators = inferred.decorators
+        if isinstance(decorators, nodes.Decorators):
+            for dec in decorators.nodes:
+                inferred_dec = utils.safe_infer(dec)
+                if (
+                    inferred_dec
+                    and inferred_dec.qname() == "functools.singledispatchmethod"
+                ):
+                    return True
+
+    return False
+
+
+def _is_singledispatchmethod_registration(node: nodes.FunctionDef) -> bool:
+    """
+    Return True if `node` is a function decorated like:
+
+        @func.register(...)
+        def _(…): ...
+
+    where `func` is a singledispatchmethod (i.e. its base was decorated
+    with @singledispatchmethod).
+    """
+    decorators = node.decorators
+    if not decorators:
+        return False
+
+    for dec in decorators.nodes:
+        target = _extract_register_target(dec)
+        if target is None:
+            continue
+
+        if _inferred_has_singledispatchmethod(target):
+            return True
+
+    return False
+
+
 class BasicErrorChecker(_BasicChecker):
     msgs = {
         "E0100": (
@@ -536,6 +601,9 @@ def _check_redefinition(
             ):
                 return
 
+            if _is_singledispatchmethod_registration(node):
+                return
+
             # Skip typing.overload() functions.
             if utils.is_overload_stub(node):
                 return
@@ -572,9 +640,6 @@ def _check_redefinition(
                     ):
                         return
 
-            dummy_variables_rgx = self.linter.config.dummy_variables_rgx
-            if dummy_variables_rgx and dummy_variables_rgx.match(node.name):
-                return
             self.add_message(
                 "function-redefined",
                 node=node,

tests/functional/f/function_redefined.py

@@ -80,7 +80,7 @@ def dummy_func():
     """First dummy function"""
     pass
 
-def dummy_func():
+def dummy_func2():
     """Second dummy function, don't emit function-redefined message
     because of the dummy name"""
     pass
@@ -94,7 +94,7 @@ def math(): # [function-redefined]
     pass
 
 import math as _
-def _():
+def fun():
     pass
 
 # pylint: disable=too-few-public-methods

tests/functional/f/function_redefined.txt

@@ -1,7 +1,7 @@
-function-redefined:18:4:18:15:AAAA.method2:method already defined line 15:UNDEFINED
-function-redefined:21:0:21:10:AAAA:class already defined line 8:UNDEFINED
-function-redefined:35:0:35:9:func2:function already defined line 32:UNDEFINED
-redefined-outer-name:37:4:37:16:func2:Redefining name '__revision__' from outer scope (line 7):UNDEFINED
-function-redefined:54:4:54:23:exclusive_func2:function already defined line 48:UNDEFINED
-function-redefined:89:0:89:8:ceil:function already defined line 88:UNDEFINED
-function-redefined:93:0:93:8:math:function already defined line 92:UNDEFINED
+function-redefined:18:4:18:15:AAAA.method2:method already defined line 15:UNDEFINED
+function-redefined:21:0:21:10:AAAA:class already defined line 8:UNDEFINED
+function-redefined:35:0:35:9:func2:function already defined line 32:UNDEFINED
+redefined-outer-name:37:4:37:16:func2:Redefining name '__revision__' from outer scope (line 7):UNDEFINED
+function-redefined:54:4:54:23:exclusive_func2:function already defined line 48:UNDEFINED
+function-redefined:89:0:89:8:ceil:function already defined line 88:UNDEFINED
+function-redefined:93:0:93:8:math:function already defined line 92:UNDEFINED

tests/functional/f/function_redefined_private_function.py

@@ -0,0 +1,6 @@
+# pylint: disable=missing-module-docstring, missing-function-docstring
+def _my_func():
+    pass
+
+def _my_func():  # [function-redefined]
+    pass

tests/functional/f/function_redefined_private_function.txt

@@ -0,0 +1 @@
+function-redefined:5:0:5:12:_my_func:function already defined line 2:UNDEFINED

tests/functional/s/singledispatch/singledispatch_function_redefined.py

@@ -0,0 +1,30 @@
+# pylint: disable=missing-module-docstring, missing-class-docstring, missing-function-docstring, too-few-public-methods
+
+from functools import singledispatch, singledispatchmethod
+
+# --- singledispatch function case ---
+@singledispatch
+def process(value):
+    return f"default handler for {value!r}"
+
+@process.register(int)
+def _(value):
+    return f"int handler for {value}"
+
+@process.register(str)
+def _(value):
+    return f"str handler for {value}"
+
+# --- singledispatchmethod case ---
+class Handler:
+    @singledispatchmethod
+    def handle(self, value):
+        return f"default method handler for {value!r}"
+
+    @handle.register(int)
+    def _(self, value):
+        return f"int method handler for {value}"
+
+    @handle.register(str)
+    def _(self, value):
+        return f"str method handler for {value}"