More verbose error message on invalid return value

Author: ecprice

mypy/checker.py

@@ -1192,6 +1192,7 @@ def visit_return_stmt(self, s: ReturnStmt) -> Type:
                         self.check_subtype(
                             typ, self.return_types[-1], s,
                             messages.INCOMPATIBLE_RETURN_VALUE_TYPE
+                             + ": expected {}, got {}".format(self.return_types[-1], typ)
                             )
             else:
                 # Return without a value. It's valid in a generator function.

mypy/test/data/check-basic.test

@@ -193,7 +193,7 @@ class A: pass
 class B: pass
 [out]
 main: In function "f":
-main, line 3: Incompatible return value type
+main, line 3: Incompatible return value type: expected __main__.A, got __main__.B
 
 [case testTopLevelContextAndInvalidReturn]
 import typing
@@ -204,7 +204,7 @@ class A: pass
 class B: pass
 [out]
 main: In function "f":
-main, line 3: Incompatible return value type
+main, line 3: Incompatible return value type: expected __main__.A, got __main__.B
 main: At top level:
 main, line 4: Incompatible types in assignment (expression has type "B", variable has type "A")
 
@@ -313,7 +313,7 @@ def f(x): # type: (int) -> str
 f('')
 [out]
 main: In function "f":
-main, line 2: Incompatible return value type
+main, line 2: Incompatible return value type: expected builtins.str, got builtins.int
 main: At top level:
 main, line 3: Argument 1 to "f" has incompatible type "str"; expected "int"
 
@@ -327,7 +327,7 @@ A().f('') # Fail
 [out]
 main: In member "f" of class "A":
 main, line 4: Argument 1 to "f" of "A" has incompatible type "str"; expected "int"
-main, line 5: Incompatible return value type
+main, line 5: Incompatible return value type: expected builtins.str, got builtins.int
 main: At top level:
 main, line 6: Argument 1 to "f" of "A" has incompatible type "str"; expected "int"
 

mypy/test/data/check-classes.test

@@ -96,7 +96,7 @@ class A:
 class B: pass
 [out]
 main: In member "f" of class "A":
-main, line 4: Incompatible return value type
+main, line 4: Incompatible return value type: expected __main__.A, got __main__.B
 
 [case testSelfArgument]
 import typing

mypy/test/data/check-expressions.test

@@ -828,7 +828,7 @@ f = lambda: ''.x
 f = lambda: ''
 [out]
 main, line 3: "str" has no attribute "x"
-main, line 4: Incompatible return value type
+main, line 4: Incompatible return value type: expected builtins.int, got builtins.str
 main, line 4: Incompatible types in assignment (expression has type , variable has type )
 
 

mypy/test/data/check-functions.test

@@ -424,9 +424,9 @@ class A: pass
 class B: pass
 [out]
 main: In function "g":
-main, line 4: Incompatible return value type
+main, line 4: Incompatible return value type: expected __main__.B, got __main__.A
 main: In function "f":
-main, line 6: Incompatible return value type
+main, line 6: Incompatible return value type: expected __main__.A, got __main__.B
 
 [case testDynamicallyTypedNestedFunction]
 import typing

mypy/test/data/check-generics.test

@@ -476,7 +476,7 @@ class p(Generic[T, S]):
 def f(s: S, t: T) -> p[T, A]:
     a = t # type: S # E: Incompatible types in assignment (expression has type "T", variable has type "S")
     s = t           # E: Incompatible types in assignment (expression has type "T", variable has type "S")
-    return p[S, A](s, A()) # E: Incompatible return value type
+    return p[S, A](s, A()) # E: Incompatible return value type: expected __main__.p[T`-2, __main__.A], got __main__.p[S`-1, __main__.A*]
     b = t # type: T
     c = s # type: S
     return p[T, A](t, A())
@@ -492,8 +492,8 @@ class p(Generic[T, S]):
 class A(Generic[T]):
     def f(self, s: S, t: T) -> p[S, T]:
         s = t # E: Incompatible types in assignment (expression has type "T", variable has type "S")
-        return p[S, S](s, s) # E: Incompatible return value type
-        return p[T, T](t, t) # E: Incompatible return value type
+        return p[S, S](s, s) # E: Incompatible return value type: expected __main__.p[S`-1, T`1], got __main__.p[S`-1, S`-1]
+        return p[T, T](t, t) # E: Incompatible return value type: expected __main__.p[S`-1, T`1], got __main__.p[T`1, T`1]
         t = t
         s = s
         return p[S, T](s, t)

mypy/test/data/check-statements.test

@@ -15,9 +15,9 @@ class B:
     pass
 [out]
 main: In function "f":
-main, line 4: Incompatible return value type
+main, line 4: Incompatible return value type: expected __main__.A, got __main__.B
 main: In function "g":
-main, line 6: Incompatible return value type
+main, line 6: Incompatible return value type: expected __main__.B, got __main__.A
 
 [case testReturnSubtype]
 import typing
@@ -31,7 +31,7 @@ class B(A):
     pass
 [out]
 main: In function "f":
-main, line 3: Incompatible return value type
+main, line 3: Incompatible return value type: expected __main__.B, got __main__.A
 
 [case testReturnWithoutAValue]
 import typing

mypy/test/data/check-typevar-values.test

@@ -70,9 +70,9 @@ def g(x: AB) -> AB:
     return x.g() # Error
 [out]
 main: In function "f":
-main, line 10: Incompatible return value type
+main, line 10: Incompatible return value type: expected __main__.B*, got __main__.A
 main: In function "g":
-main, line 12: Incompatible return value type
+main, line 12: Incompatible return value type: expected __main__.A*, got __main__.B
 
 [case testTypeInferenceAndTypeVarValues]
 from typing import typevar
@@ -88,7 +88,7 @@ def f(x: AB) -> AB:
     if y:
         return y.f()
     else:
-        return y.g() # E: Incompatible return value type
+        return y.g() # E: Incompatible return value type: expected __main__.A*, got __main__.B
 [out]
 main: In function "f":
 
@@ -98,7 +98,7 @@ T = typevar('T', values=(int, str))
 def f(x: T) -> List[T]:
     return List[T]()
 def g(x: T) -> List[T]:
-    return List[int]() # E: Incompatible return value type
+    return List[int]() # E: Incompatible return value type: expected builtins.list[builtins.str*], got builtins.list[builtins.int*]
 [builtins fixtures/list.py]
 [out]
 main: In function "g":
@@ -127,7 +127,7 @@ def g(x: T) -> T:
         return ''
 def h(x: T) -> T:
     if isinstance(x, int):
-        return '' # E: Incompatible return value type
+        return '' # E: Incompatible return value type: expected builtins.int*, got builtins.str
     return x
 [builtins fixtures/isinstance.py]
 [out]
@@ -143,9 +143,9 @@ def f(x: T) -> T:
         return ''
 def g(x: T) -> T:
     if isinstance(x, int):
-        return '' # E: Incompatible return value type
+        return '' # E: Incompatible return value type: expected builtins.int*, got builtins.str
     else:
-        return 2  # E: Incompatible return value type
+        return 2  # E: Incompatible return value type: expected builtins.str*, got builtins.int
     return x
 [builtins fixtures/isinstance.py]
 [out]
@@ -170,7 +170,7 @@ def f(x: T) -> T:
         y = 1
     else:
         y = object()
-    return y # E: Incompatible return value type
+    return y # E: Incompatible return value type: expected builtins.str*, got builtins.object
 [builtins fixtures/isinstance.py]
 [out]
 main: In function "f":
@@ -183,7 +183,7 @@ def f(x: T) -> T:
         y = object()
     else:
         y = ''
-    return y # E: Incompatible return value type
+    return y # E: Incompatible return value type: expected builtins.int*, got builtins.object
 [builtins fixtures/isinstance.py]
 [out]
 main: In function "f":