Better error message for incompatible default argument (#3783)

mypy/checker.py

@@ -740,9 +740,15 @@ 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)
+                    if arg.initializer is not None:
+                        name = arg.variable.name()
+                        msg = 'Incompatible default for '
+                        if name.startswith('__tuple_arg_'):
+                            msg += "tuple argument {}".format(name[12:])
+                        else:
+                            msg += 'argument "{}"'.format(name)
+                        self.check_simple_assignment(arg.variable.type, arg.initializer,
+                            context=arg, msg=msg, lvalue_name='argument', rvalue_name='default')
 
             # Type check body in a new scope.
             with self.binder.top_frame_context():

test-data/unit/check-functions.test

@@ -388,17 +388,54 @@ 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" (default has type "B", argument has type "A")
     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" (default has type "B", argument has type "T")
+    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" (default has type "B", argument has type "A")
+    # type: (A) -> None
+    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
+
+[case testDefaultTupleArgumentExpressionsPython2]
+# flags: --python-version 2.7
+from typing import Tuple
+def f((x, y) = (A(), B())): # E: Incompatible default for tuple argument 1 (default has type "Tuple[A, B]", argument has type "Tuple[B, 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")
+def g(a, (x, y) = (A(),)): # E: Incompatible default for tuple argument 2 (default has type "Tuple[A]", argument has type "Tuple[B, B]")
+    # type: (int, Tuple[B, B]) -> None
+    pass
+def h((x, y) = (A(), B(), A())): # E: Incompatible default for tuple argument 1 (default has type "Tuple[A, B, A]", argument has type "Tuple[B, B]")
+    # type: (Tuple[B, B]) -> None
+    pass
+
+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" (default has type "A", argument has type "B")
     pass
 def g(x: 'A' = B()) -> None:
     pass
@@ -409,7 +446,7 @@ 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" (default has type "B", argument has type "A")
     pass
 def h(x: 'A' = A(), y: 'B' = B()) -> None:
     pass
@@ -420,7 +457,7 @@ 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" (default has type "A", argument has type "B")
     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" (default has type Callable[[VarArg(Any), KwArg(Any)], int], argument has type Callable[..., None])
     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" (default has type "str", argument has type "int")
+main:2: error: Incompatible default for argument "x" (default has type "ellipsis", argument has type "int")
 
 [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" (default has type "ellipsis", argument has type "int")
 [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" (default has type None, argument has type "int")
   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" (default has type None, argument has type "int")
   # type: (int) -> None
   pass
 [out]