Improved error message for incompatible default argument

mypy/checker.py

@@ -740,9 +740,7 @@ def is_implicit_any(t: Type) -> bool:
 
                 # Type check initialization expressions.
                 for arg in item.arguments:
-                    init = arg.initialization_statement
-                    if init:
-                        self.accept(init)
+                    self.expr_checker.check_default_arg(arg)
 
             # Type check body in a new scope.
             with self.binder.top_frame_context():

mypy/checkexpr.py

@@ -3,7 +3,7 @@
 from collections import OrderedDict
 from typing import cast, Dict, Set, List, Tuple, Callable, Union, Optional
 
-from mypy.errors import report_internal_error
+from mypy.errors import Errors, report_internal_error
 from mypy.typeanal import has_any_from_unimported_type, check_for_explicit_any, set_any_tvars
 from mypy.types import (
     Type, AnyType, CallableType, Overloaded, NoneTyp, TypeVarDef,
@@ -1011,6 +1011,20 @@ def check_arg(self, caller_type: Type, original_caller_type: Type,
             messages.incompatible_argument(n, m, callee, original_caller_type,
                                            caller_kind, context)
 
+    def check_default_arg(self, arg: mypy.nodes.Argument) -> None:
+        rvalue = arg.initializer
+        lval_type = arg.variable.type
+        if rvalue and lval_type and not (self.chk.is_stub and isinstance(rvalue, EllipsisExpr)):
+            single = CallableType([lval_type], [ARG_POS], [None],
+                                  AnyType(), self.named_type('builtins.function'))
+            msg = MessageBuilder(Errors(), self.chk.modules)
+            self.check_call(single, [rvalue], [ARG_POS], arg, arg_messages=msg)
+            if msg.is_errors():
+                rval_type = self.accept(rvalue, lval_type)
+                varname = arg.variable.name()
+                self.msg.incompatible_default_argument(
+                    varname, lval_type, rval_type, arg)
+
     def overload_call_target(self, arg_types: List[Type], arg_kinds: List[int],
                              arg_names: List[str],
                              overload: Overloaded, context: Context,

mypy/messages.py

@@ -492,6 +492,16 @@ def untyped_function_call(self, callee: CallableType, context: Context) -> Type:
         self.fail('Call to untyped function {} in typed context'.format(name), context)
         return AnyType()
 
+    def incompatible_default_argument(self, name: str, lvalue: Type, rvalue: Type,
+                                      context: Context) -> None:
+        if name.startswith('__tuple_arg_'):
+            fmt = "Incompatible default for tuple argument no' {}".format(name[12:])
+        else:
+            fmt = 'Incompatible default for argument "{}"'.format(name)
+        fmt += ' (argument has type {}, default has type {})'
+        lvalt, rvalt = self.format_distinctly(lvalue, rvalue)
+        self.fail(fmt.format(lvalt, rvalt), context)
+
     def incompatible_argument(self, n: int, m: int, callee: CallableType, arg_type: Type,
                               arg_kind: int, context: Context) -> None:
         """Report an error about an incompatible argument type.

test-data/unit/check-functions.test

@@ -388,17 +388,42 @@ class A: pass
 
 [case testDefaultArgumentExpressions2]
 import typing
-def f(x: 'A' = B()) -> None: # E: Incompatible types in assignment (expression has type "B", variable has type "A")
+def f(x: 'A' = B()) -> None: # E: Incompatible default for argument "x" (argument has type "A", default has type "B")
     b = x # type: B      # E: Incompatible types in assignment (expression has type "A", variable has type "B")
     a = x # type: A
 
 class B: pass
 class A: pass
-[out]
+
+[case testDefaultArgumentExpressionsGeneric]
+from typing import TypeVar
+T = TypeVar('T', bound='A')
+def f(x: T = B()) -> None: # E: Incompatible default for argument "x" (argument has type "T", default has type "B")
+    b = x # type: B      # E: Incompatible types in assignment (expression has type "T", variable has type "B")
+    a = x # type: A
+
+class B: pass
+class A: pass
+
+[case testDefaultArgumentExpressionsPython2]
+# flags: --python-version 2.7
+from typing import Tuple
+def f(x = B()): # E: Incompatible default for argument "x" (argument has type "A", default has type "B")
+    # type: (A) -> None
+    b = x # type: B      # E: Incompatible types in assignment (expression has type "A", variable has type "B")
+    a = x # type: A
+
+def g((x, y) = (A(), B())): # E: Incompatible default for tuple argument no' 1 (argument has type "Tuple[B, B]", default has type "Tuple[A, B]")
+    # type: ( Tuple[B, B] ) -> None
+    b = x # type: B
+    a = x # type: A      # E: Incompatible types in assignment (expression has type "B", variable has type "A")
+
+class B: pass
+class A: pass
 
 [case testDefaultArgumentsWithSubtypes]
 import typing
-def f(x: 'B' = A()) -> None: # E: Incompatible types in assignment (expression has type "A", variable has type "B")
+def f(x: 'B' = A()) -> None: # E: Incompatible default for argument "x" (argument has type "B", default has type "A")
     pass
 def g(x: 'A' = B()) -> None:
     pass
@@ -409,7 +434,7 @@ class B(A): pass
 
 [case testMultipleDefaultArgumentExpressions]
 import typing
-def f(x: 'A' = B(), y: 'B' = B()) -> None: # E: Incompatible types in assignment (expression has type "B", variable has type "A")
+def f(x: 'A' = B(), y: 'B' = B()) -> None: # E: Incompatible default for argument "x" (argument has type "A", default has type "B")
     pass
 def h(x: 'A' = A(), y: 'B' = B()) -> None:
     pass
@@ -420,7 +445,7 @@ class B: pass
 
 [case testMultipleDefaultArgumentExpressions2]
 import typing
-def g(x: 'A' = A(), y: 'B' = A()) -> None: # E: Incompatible types in assignment (expression has type "A", variable has type "B")
+def g(x: 'A' = A(), y: 'B' = A()) -> None: # E: Incompatible default for argument "y" (argument has type "B", default has type "A")
     pass
 
 class A: pass

test-data/unit/check-inference.test

@@ -1119,7 +1119,7 @@ from typing import Callable
 def f(a: Callable[..., None] = lambda *a, **k: None):
     pass
 
-def g(a: Callable[..., None] = lambda *a, **k: 1):  # E: Incompatible types in assignment (expression has type Callable[[VarArg(Any), KwArg(Any)], int], variable has type Callable[..., None])
+def g(a: Callable[..., None] = lambda *a, **k: 1):  # E: Incompatible default for argument "a" (argument has type Callable[..., None], default has type Callable[[VarArg(Any), KwArg(Any)], int])
     pass
 [builtins fixtures/dict.pyi]
 

test-data/unit/check-modules.test

@@ -475,11 +475,11 @@ def f(x: int = ...) -> None: pass
 [file m.pyi]
 def g(x: int = '') -> None: pass
 [out]
-tmp/m.pyi:1: error: Incompatible types in assignment (expression has type "str", variable has type "int")
-main:2: error: Incompatible types in assignment (expression has type "ellipsis", variable has type "int")
+tmp/m.pyi:1: error: Incompatible default for argument "x" (argument has type "int", default has type "str")
+main:2: error: Incompatible default for argument "x" (argument has type "int", default has type "ellipsis")
 
 [case testEllipsisDefaultArgValueInNonStub]
-def f(x: int = ...) -> None: pass # E: Incompatible types in assignment (expression has type "ellipsis", variable has type "int")
+def f(x: int = ...) -> None: pass # E: Incompatible default for argument "x" (argument has type "int", default has type "ellipsis")
 [out]
 
 [case testStarImportOverlapping]

test-data/unit/check-optional.test

@@ -127,7 +127,7 @@ f(None)
 
 [case testNoInferOptionalFromDefaultNone]
 # flags: --no-implicit-optional
-def f(x: int = None) -> None:  # E: Incompatible types in assignment (expression has type None, variable has type "int")
+def f(x: int = None) -> None:  # E: Incompatible default for argument "x" (argument has type "int", default has type None)
   pass
 [out]
 
@@ -140,7 +140,7 @@ f(None)
 
 [case testNoInferOptionalFromDefaultNoneComment]
 # flags: --no-implicit-optional
-def f(x=None):  # E: Incompatible types in assignment (expression has type None, variable has type "int")
+def f(x=None):  # E: Incompatible default for argument "x" (argument has type "int", default has type None)
   # type: (int) -> None
   pass
 [out]