Warn about redundant casts

A cast is considered redundant if the target type of the cast is the
same as the inferred type of the expression. A cast to a supertype
like `cast(object, 1)` is not considered redundant because such a cast
could be needed to work around deficiencies in type inference.

Fixes python#958.

mypy/build.py

@@ -68,6 +68,8 @@
 CHECK_UNTYPED_DEFS = 'check-untyped-defs'
 # Also check typeshed for missing annotations
 WARN_INCOMPLETE_STUB = 'warn-incomplete-stub'
+# Warn about casting an expression to its inferred type
+WARN_REDUNDANT_CASTS = 'warn-redundant-casts'
 
 PYTHON_EXTENSIONS = ['.pyi', '.py']
 
@@ -386,7 +388,8 @@ def __init__(self, data_dir: str,
                                         DISALLOW_UNTYPED_CALLS in self.flags,
                                         DISALLOW_UNTYPED_DEFS in self.flags,
                                         check_untyped_defs,
-                                        WARN_INCOMPLETE_STUB in self.flags)
+                                        WARN_INCOMPLETE_STUB in self.flags,
+                                        WARN_REDUNDANT_CASTS in self.flags)
         self.missing_modules = set()  # type: Set[str]
 
     def all_imported_modules_in_file(self,

mypy/checker.py

@@ -380,12 +380,14 @@ class TypeChecker(NodeVisitor[Type]):
     # Should we check untyped function defs?
     check_untyped_defs = False
     warn_incomplete_stub = False
+    warn_redundant_casts = False
     is_typeshed_stub = False
 
     def __init__(self, errors: Errors, modules: Dict[str, MypyFile],
                  pyversion: Tuple[int, int] = defaults.PYTHON3_VERSION,
                  disallow_untyped_calls=False, disallow_untyped_defs=False,
-                 check_untyped_defs=False, warn_incomplete_stub=False) -> None:
+                 check_untyped_defs=False, warn_incomplete_stub=False,
+                 warn_redundant_casts=False) -> None:
         """Construct a type checker.
 
         Use errors to report type check errors.
@@ -411,6 +413,7 @@ def __init__(self, errors: Errors, modules: Dict[str, MypyFile],
         self.disallow_untyped_defs = disallow_untyped_defs
         self.check_untyped_defs = check_untyped_defs
         self.warn_incomplete_stub = warn_incomplete_stub
+        self.warn_redundant_casts = warn_redundant_casts
 
     def visit_file(self, file_node: MypyFile, path: str) -> None:
         """Type check a mypy file with the given path."""

mypy/checkexpr.py

@@ -1229,6 +1229,8 @@ def visit_cast_expr(self, expr: CastExpr) -> Type:
         """Type check a cast expression."""
         source_type = self.accept(expr.expr, context=AnyType())
         target_type = expr.type
+        if self.chk.warn_redundant_casts and is_same_type(source_type, target_type):
+            self.msg.redundant_cast(target_type, expr)
         if not self.is_valid_cast(source_type, target_type):
             self.msg.invalid_cast(target_type, source_type, expr)
         return target_type

mypy/main.py

@@ -153,6 +153,8 @@ def parse_version(v):
     parser.add_argument('--warn-incomplete-stub', action='store_true',
                         help="warn if missing type annotation in typeshed, only relevant with"
                         " --check-untyped-defs enabled")
+    parser.add_argument('--warn-redundant-casts', action='store_true',
+                        help="warn about casting an expression to its inferred type")
     parser.add_argument('--fast-parser', action='store_true',
                         help="enable experimental fast parser")
     parser.add_argument('-i', '--incremental', action='store_true',
@@ -251,6 +253,8 @@ def parse_version(v):
 
     if args.warn_incomplete_stub:
         options.build_flags.append(build.WARN_INCOMPLETE_STUB)
+    if args.warn_redundant_casts:
+        options.build_flags.append(build.WARN_REDUNDANT_CASTS)
 
     # experimental
     if args.fast_parser:

mypy/messages.py

@@ -841,6 +841,9 @@ def reveal_type(self, typ: Type, context: Context) -> None:
     def unsupported_type_type(self, item: Type, context: Context) -> None:
         self.fail('Unsupported type Type[{}]'.format(self.format(item)), context)
 
+    def redundant_cast(self, typ: Type, context: Context) -> None:
+        self.note('Redundant cast to {}'.format(self.format(typ)), context)
+
 
 def capitalize(s: str) -> str:
     """Capitalize the first character of a string."""

mypy/test/testcheck.py

@@ -60,6 +60,7 @@
     'check-bound.test',
     'check-optional.test',
     'check-fastparse.test',
+    'check-warnings.test',
 ]
 
 

test-data/unit/check-warnings.test

@@ -0,0 +1,36 @@
+-- Test cases for warning generation.
+
+-- Redundant casts
+-- ---------------
+
+[case testRedundantCast]
+# flags: warn-redundant-casts
+from typing import cast
+a = 1
+b = cast(str, a)
+c = cast(int, a)
+[out]
+main:5: note: Redundant cast to "int"
+
+[case testRedundantCastWithIsinstance]
+# flags: warn-redundant-casts
+from typing import cast, Union
+x = 1  # type: Union[int, str]
+if isinstance(x, str):
+    cast(str, x)
+[builtins fixtures/isinstance.py]
+[out]
+main:5: note: Redundant cast to "str"
+
+[case testCastToSuperclassNotRedundant]
+# flags: warn-redundant-casts
+from typing import cast, TypeVar, List
+T = TypeVar('T')
+def add(xs: List[T], ys: List[T]) -> List[T]: pass
+class A: pass
+class B(A): pass
+a = A()
+b = B()
+# Without the cast, the following line would fail to type check.
+c = add([cast(A, b)], [a])
+[builtins fixtures/list.py]