Disallow assignments to methods

mypy/checker.py

@@ -396,6 +396,14 @@ def infer_local_variable_type(self, x, y, z):
                 lvalue_types.append(None)
                 index_lvalues.append(ilv)
                 inferred.append(None)
+            elif isinstance(lv, MemberExpr):
+                mlv = (MemberExpr)lv
+                lvalue_types.append(
+                    self.expr_checker.analyse_ordinary_member_access(mlv,
+                                                                     True))
+                self.store_type(mlv, lvalue_types[-1])
+                index_lvalues.append(None)
+                inferred.append(None)
             else:
                 lvalue_types.append(self.accept(lv))
                 index_lvalues.append(None)

mypy/checkmember.py

@@ -43,13 +43,10 @@
             info = override_info
 
         # Look up the member. First look up the method dictionary.
-        FuncBase method = None
-        if not is_lvalue:
-            method = info.get_method(name)
-
+        method = info.get_method(name)
         if method:
-            # Found a method. The call below has a unique result for all valid
-            # programs.
+            if is_lvalue:
+                msg.fail(messages.CANNOT_ASSIGN_TO_METHOD, node)
             itype = map_instance_to_supertype(itype, method.info)
             return expand_type_by_instance(method_type(method), itype)
         else:
@@ -65,10 +62,11 @@
         return analyse_member_access(name, basic_types.tuple, node, is_lvalue,
                                      is_super, basic_types, msg)
     elif isinstance(typ, FunctionLike) and ((FunctionLike)typ).is_type_obj():
-        # TODO super? lvalue?
+        # TODO super?
         sig = (FunctionLike)typ
         itype = (Instance)sig.items()[0].ret_type
-        result = analyse_class_attribute_access(itype, name, node, msg)
+        result = analyse_class_attribute_access(itype, name, node, is_lvalue,
+                                                msg)
         if result:
             return result
         # Look up from the 'type' type.
@@ -148,9 +146,11 @@
 
 
 Type analyse_class_attribute_access(Instance itype, str name, Context context,
-                                    MessageBuilder msg):
+                                    bool is_lvalue, MessageBuilder msg):
     node = itype.type.get(name)
     if node:
+        if is_lvalue and isinstance(node.node, FuncDef):
+            msg.fail(messages.CANNOT_ASSIGN_TO_METHOD, context)
         t = node.type()
         if t:
             return add_class_tvars(t, itype.type)

mypy/messages.py

@@ -47,6 +47,7 @@
 INVALID_SLICE_INDEX = 'Slice index must be an integer or None'
 CANNOT_INFER_LAMBDA_TYPE = 'Cannot infer type of lambda'
 CANNOT_ACCESS_INIT = 'Cannot access "__init__" directly'
+CANNOT_ASSIGN_TO_METHOD = 'Cannot assign to a method'
 
 
 class MessageBuilder:

test/data/check-classes.test

@@ -104,6 +104,11 @@ main: In member "f" of class "A":
 main, line 3: Incompatible types in assignment
 main, line 4: "A" has no attribute "g"
 
+[case testAssignToMethodViaInstance]
+class A:
+    def f(self): pass
+A().f = None # E: Cannot assign to a method
+
 
 -- Attributes
 -- ----------
@@ -460,6 +465,11 @@ A.__init__(a, b)
 A.__init__(b, b) # E: Argument 1 to "__init__" of "A" has incompatible type "B"
 A.__init__(a, a) # E: Argument 2 to "__init__" of "A" has incompatible type "A"
 
+[case testAssignToMethodViaClass]
+class A:
+    def f(self): pass
+A.f = None # E: Cannot assign to a method
+
 
 -- Nested classes
 -- --------------