function-redefined` exempts function redefined on a condition.

Close #2410

ChangeLog

@@ -15,6 +15,10 @@ Release date: TBA
 
   Close #2874
 
+* ``function-redefined`` exempts function redefined on a condition.
+
+   Close #2410
+
 * ``typing.overload`` functions are exempted from docstring checks
 
   Close #3350

pylint/checkers/base.py

@@ -845,7 +845,6 @@ def _check_redefinition(self, redeftype, node):
             node,
         )
         if defined_self is not node and not astroid.are_exclusive(node, defined_self):
-
             # Additional checks for methods which are not considered
             # redefined, since they are already part of the base API.
             if (
@@ -854,9 +853,32 @@ def _check_redefinition(self, redeftype, node):
             ):
                 return
 
+            # Skip typing.overload() functions.
             if utils.is_overload_stub(node):
                 return
 
+            # Exempt functions redefined on a condition.
+            if isinstance(node.parent, astroid.If):
+                # Exempt "if not <func>" cases
+                if (
+                    isinstance(node.parent.test, astroid.UnaryOp)
+                    and node.parent.test.op == "not"
+                    and isinstance(node.parent.test.operand, astroid.Name)
+                    and node.parent.test.operand.name == node.name
+                ):
+                    return
+
+                # Exempt "if <func> is not None" cases
+                if (
+                    isinstance(node.parent.test, astroid.Compare)
+                    and isinstance(node.parent.test.left, astroid.Name)
+                    and node.parent.test.left.name == node.name
+                    and node.parent.test.ops[0][0] == "is"
+                    and isinstance(node.parent.test.ops[0][1], astroid.Const)
+                    and node.parent.test.ops[0][1].value is None
+                ):
+                    return
+
             # Check if we have forward references for this node.
             try:
                 redefinition_index = redefinitions.index(node)

tests/functional/f/function_redefined.py

@@ -107,3 +107,14 @@ def __module__(self):
     @property
     def __doc__(self):
         return "Docstring"
+
+
+# Do not emit the error for conditional definitions
+def func(callback1=None, callback2=None):
+    if not callback1:
+        def callback1():
+            return 42
+    if callback2 is None:
+        def callback2():
+            return 24
+    return callback1(), callback2()