Optionally disallow statement expressions, this is an experiment

mypy/checker.py

@@ -3110,7 +3110,15 @@ def try_infer_partial_type_from_indexed_assignment(
                         del partial_types[var]
 
     def visit_expression_stmt(self, s: ExpressionStmt) -> None:
-        self.expr_checker.accept(s.expr, allow_none_return=True, always_allow_any=True)
+        type_ = self.expr_checker.accept(s.expr, allow_none_return=True, always_allow_any=True)
+        proper_type = get_proper_type(type_)
+        if (self.options.disallow_unused_expressions and
+            not (self.current_node_deferred or
+                    isinstance(s.expr, (EllipsisExpr,)) or
+                    isinstance(proper_type, (NoneType, UninhabitedType)) or
+                    isinstance(proper_type, Instance) and proper_type.last_known_value
+                )):
+            self.fail("Unused expression of type '{}'".format(proper_type), s)
 
     def visit_return_stmt(self, s: ReturnStmt) -> None:
         """Type check a return statement."""

mypy/main.py

@@ -609,6 +609,11 @@ def add_invertible_flag(flag: str,
                              " non-overlapping types",
                         group=strictness_group)
 
+    add_invertible_flag('--disallow-unused-expressions', default=True, strict_flag=False,
+                        help="Prohibit using expressions in statement context without using their"
+                             "value",
+                        group=strictness_group)
+
     strict_help = "Strict mode; enables the following flags: {}".format(
         ", ".join(strict_flag_names))
     strictness_group.add_argument(

mypy/options.py

@@ -35,6 +35,7 @@ class BuildType:
     "disallow_untyped_calls",
     "disallow_untyped_decorators",
     "disallow_untyped_defs",
+    "disallow_unused_expressions",
     "follow_imports",
     "follow_imports_for_stubs",
     "ignore_errors",
@@ -111,6 +112,8 @@ def __init__(self) -> None:
         # Disallow subclassing values of type 'Any'
         self.disallow_subclassing_any = False
 
+        self.disallow_unused_expressions = False
+
         # Also check typeshed for missing annotations
         self.warn_incomplete_stub = False