Turn --warn-no-return on by default (#2462)

Author: ddfisher

mypy/checker.py

@@ -27,7 +27,7 @@
     Type, AnyType, CallableType, Void, FunctionLike, Overloaded, TupleType, TypedDictType,
     Instance, NoneTyp, ErrorType, strip_type, TypeType,
     UnionType, TypeVarId, TypeVarType, PartialType, DeletedType, UninhabitedType, TypeVarDef,
-    true_only, false_only, function_type
+    true_only, false_only, function_type, is_named_instance
 )
 from mypy.sametypes import is_same_type, is_same_types
 from mypy.messages import MessageBuilder
@@ -606,20 +606,24 @@ def is_implicit_any(t: Type) -> bool:
                     self.accept(item.body)
                 unreachable = self.binder.is_unreachable()
 
-            if (self.options.warn_no_return and not unreachable
-                    and not isinstance(self.return_types[-1], (Void, NoneTyp, AnyType))
-                    and (defn.is_coroutine or not defn.is_generator)):
-                # Control flow fell off the end of a function that was
-                # declared to return a non-None type.
-                # Allow functions that are entirely pass/Ellipsis.
-                if self.is_trivial_body(defn.body):
-                    pass
+            if (self.options.warn_no_return and not unreachable):
+                if (defn.is_generator or
+                        is_named_instance(self.return_types[-1], 'typing.AwaitableGenerator')):
+                    return_type = self.get_generator_return_type(self.return_types[-1],
+                                                                 defn.is_coroutine)
                 else:
-                    if isinstance(self.return_types[-1], UninhabitedType):
+                    return_type = self.return_types[-1]
+
+                if (not isinstance(return_type, (Void, NoneTyp, AnyType))
+                        and not self.is_trivial_body(defn.body)):
+                    # Control flow fell off the end of a function that was
+                    # declared to return a non-None type and is not
+                    # entirely pass/Ellipsis.
+                    if isinstance(return_type, UninhabitedType):
                         # This is a NoReturn function
                         self.msg.note(messages.INVALID_IMPLICIT_RETURN, defn)
                     else:
-                        self.msg.note(messages.MISSING_RETURN_STATEMENT, defn)
+                        self.msg.fail(messages.MISSING_RETURN_STATEMENT, defn)
 
             self.return_types.pop()
 

mypy/main.py

@@ -219,8 +219,8 @@ def add_invertible_flag(flag: str,
                         " --check-untyped-defs enabled")
     add_invertible_flag('--warn-redundant-casts', default=False, strict_flag=True,
                         help="warn about casting an expression to its inferred type")
-    add_invertible_flag('--warn-no-return', default=False,
-                        help="warn about functions that end without returning")
+    add_invertible_flag('--no-warn-no-return', dest='warn_no_return', default=True,
+                        help="do not warn about functions that end without returning")
     add_invertible_flag('--warn-unused-ignores', default=False, strict_flag=True,
                         help="warn about unneeded '# type: ignore' comments")
     add_invertible_flag('--show-error-context', default=True,

mypy/options.py

@@ -63,7 +63,7 @@ def __init__(self) -> None:
         self.warn_redundant_casts = False
 
         # Warn about falling off the end of a function returning non-None
-        self.warn_no_return = False
+        self.warn_no_return = True
 
         # Warn about unused '# type: ignore' comments
         self.warn_unused_ignores = False

test-data/samples/crawl.py

@@ -319,7 +319,7 @@ def close(self, recycle: bool = False) -> None:
             self.conn = None
 
     @asyncio.coroutine
-    def putline(self, line: str) -> Generator[Any, None, None]:
+    def putline(self, line: str) -> None:
         """Write a line to the connection.
 
         Used for the request line and headers.

test-data/stdlib-samples/3.2/subprocess.py

@@ -784,6 +784,7 @@ def __exit__(self, type: type, value: BaseException,
             self.stdin.close()
         # Wait for the process to terminate, to avoid zombies.
         self.wait()
+        return False
 
     def __del__(self, _maxsize: int = sys.maxsize,
                 _active: List['Popen'] = _active) -> None:

test-data/stdlib-samples/3.2/tempfile.py

@@ -101,6 +101,7 @@ def __stat(fn: str) -> object:
         except IOError:
             raise _os.error()
         f.close()
+        return None
     _stat = __stat
 
 def _exists(fn: str) -> bool:
@@ -421,6 +422,7 @@ def __exit__(self, exc: _Type[BaseException], value: BaseException,
         def __exit__(self, exc: _Type[BaseException], value: BaseException,
                      tb: _Optional[_TracebackType]) -> bool:
             self.file.__exit__(exc, value, tb)
+            return False
 
 
 def NamedTemporaryFile(mode: str = 'w+b', buffering: int = -1,
@@ -554,6 +556,7 @@ def __enter__(self) -> 'SpooledTemporaryFile':
     def __exit__(self, exc: type, value: BaseException,
                  tb: _TracebackType) -> bool:
         self._file.close()
+        return False
 
     # file protocol
     def __iter__(self) -> _Iterable[_Any]:
@@ -690,6 +693,7 @@ def cleanup(self, _warn: bool = False) -> None:
     def __exit__(self, exc: type, value: BaseException,
                  tb: _TracebackType) -> bool:
         self.cleanup()
+        return False
 
     def __del__(self) -> None:
         # Issue a ResourceWarning if implicit cleanup needed

test-data/unit/check-async-await.test

@@ -20,7 +20,7 @@ async def f() -> int:
     make_this_not_trivial = 1
 [builtins fixtures/async_await.pyi]
 [out]
-main:2: note: Missing return statement
+main:2: error: Missing return statement
 
 [case testAsyncDefReturnWithoutValue]
 # flags: --fast-parser
@@ -69,6 +69,7 @@ T = TypeVar('T')
 async def f(x: T) -> T:
     y = await f(x)  # type: int
     reveal_type(y)
+    return x
 [out]
 main:5: error: Argument 1 to "f" has incompatible type "T"; expected "int"
 main:6: error: Revealed type is 'builtins.int'
@@ -113,6 +114,7 @@ async def g() -> int:
     return 0
 async def f() -> str:
     x = await g()  # type: str
+    return x
 [builtins fixtures/async_await.pyi]
 [out]
 main:5: error: Incompatible types in assignment (expression has type "int", variable has type "str")

test-data/unit/check-classes.test

@@ -838,6 +838,7 @@ class A:
     def f(self) -> str:
         self.x = 1
         self.x = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+        return ''
 [builtins fixtures/property.pyi]
 [out]
 

test-data/unit/check-generics.test

@@ -250,6 +250,7 @@ class A(Generic[T]):
         a = object()    # Fail
         b = self.f() # type: object
         b = self.f()
+        return None
 [out]
 main:5: error: Incompatible types in assignment (expression has type "object", variable has type "T")
 main:6: error: Incompatible types in assignment (expression has type "object", variable has type "T")

test-data/unit/check-incremental.test

@@ -1553,6 +1553,7 @@ from funcs import callee
 from classes import Outer
 def caller(a: Outer.Inner) -> int:
     callee(a)
+    return 0
 
 [case testIncrementalLoadsParentAfterChild]
 # cmd: mypy -m r.s

test-data/unit/check-optional.test

@@ -493,7 +493,7 @@ def f() -> None:
 def g() -> int:
   1 + 1  #
 [out]
-main:5: note: Missing return statement
+main:5: error: Missing return statement
 
 [case testGenericTypeAliasesOptional]
 from typing import TypeVar, Generic, Optional

test-data/unit/check-statements.test

@@ -61,6 +61,12 @@ def f() -> Generator[int, None, str]:
     return  # E: Return value expected
 [out]
 
+[case testNoReturnInGenerator]
+from typing import Generator
+def f() -> Generator[int, None, str]:  # E: Missing return statement
+    yield 1
+[out]
+
 [case testEmptyReturnInNoneTypedGenerator]
 from typing import Generator
 def f() -> Generator[int, None, None]:
@@ -722,6 +728,7 @@ def f(*a: BaseException) -> int:
     except BaseException as err: pass
     try: pass
     except BaseException as err: f(err)
+    return 0
 x = f()
 [builtins fixtures/exception.pyi]
 
@@ -732,6 +739,7 @@ def f(*a: BaseException) -> int:
     x
     try: pass
     except BaseException as err: f(err)
+    return 0
 x = f()
 [builtins fixtures/exception.pyi]
 
@@ -742,6 +750,7 @@ def f(*a: BaseException) -> int:
     try: pass
     except BaseException as err: f(err)
     x
+    return 0
 x = f()
 [builtins fixtures/exception.pyi]
 
@@ -762,6 +771,7 @@ def f(*arg: BaseException) -> int:
         f(err)
         b = err.b
         reveal_type(b)
+    return 0
 x = f()
 [builtins fixtures/exception.pyi]
 [out]
@@ -785,6 +795,7 @@ def f(*arg: BaseException) -> int:
         f(err)
         b = err.b
         reveal_type(b)
+    return 0
 x = f()
 [builtins fixtures/exception.pyi]
 [out]
@@ -808,6 +819,7 @@ def f(*arg: BaseException) -> int:
         b = err.b
         reveal_type(b)
     x
+    return 0
 x = f()
 [builtins fixtures/exception.pyi]
 [out]

test-data/unit/check-super.test

@@ -15,6 +15,7 @@ class A(B):
     a = super().f() # E: Incompatible types in assignment (expression has type "B", variable has type "A")
     a = super().g() # E: "g" undefined in superclass
     b = super().f()
+    return a
 [out]
 
 [case testAccessingSuperTypeMethodWithArgs]
@@ -85,7 +86,7 @@ class A:
 class B(A):
     def __new__(cls, x: int, y: str = '') -> 'A':
         super().__new__(cls, 1)
-        super().__new__(cls, 1, '')  # E: Too many arguments for "__new__" of "A"
+        return super().__new__(cls, 1, '')  # E: Too many arguments for "__new__" of "A"
 B('')  # E: Argument 1 to "B" has incompatible type "str"; expected "int"
 B(1)
 B(1, 'x')

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

@@ -99,6 +99,7 @@ def f(x: T) -> T:
     b = x
     a = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
     b = 1  # E: Incompatible types in assignment (expression has type "int", variable has type "str")
+    return x
 [out]
 
 [case testIsinstanceAndTypeVarValues]
@@ -107,9 +108,11 @@ T = TypeVar('T', int, str)
 def f(x: T) -> T:
     if isinstance(x, int):
         return 2
+    return x
 def g(x: T) -> T:
     if isinstance(x, str):
         return ''
+    return x
 def h(x: T) -> T:
     if isinstance(x, int):
         return '' # E: Incompatible return value type (got "str", expected "int")

test-data/unit/check-warnings.test

@@ -71,7 +71,7 @@ def g() -> int:
         return 1
 [builtins fixtures/list.pyi]
 [out]
-main:5: note: Missing return statement
+main:5: error: Missing return statement
 
 [case testNoReturnWhile]
 # flags: --warn-no-return
@@ -95,7 +95,7 @@ def j() -> int:
             continue
 [builtins fixtures/list.pyi]
 [out]
-main:7: note: Missing return statement
+main:7: error: Missing return statement
 
 [case testNoReturnExcept]
 # flags: --warn-no-return
@@ -122,7 +122,7 @@ def h() -> int:
         return 1
 [builtins fixtures/exception.pyi]
 [out]
-main:2: note: Missing return statement
+main:2: error: Missing return statement
 
 [case testNoReturnEmptyBodyWithDocstring]
 def f() -> int:

test-data/unit/typexport-basic.test

@@ -314,7 +314,7 @@ from typing import TypeVar, Generic
 T = TypeVar('T')
 class A(Generic[T]):
   def f(self) -> T:
-    self.f()
+    return self.f()
 [out]
 CallExpr(5) : T`1
 MemberExpr(5) : def () -> T`1