Propagate type narrowing to nested functions (#15133)

Fixes #2608.

Use the heuristic suggested in #2608 and allow narrowed types of
variables (but not attributes) to be propagated to nested functions if
the variable is not assigned to after the definition of the nested
function in the outer function.

Since we don't have a full control flow graph, we simply look for
assignments that are textually after the nested function in the outer
function. This can result in false negatives (at least in loops) and
false positives (in if statements, and if the assigned type is narrow
enough), but I expect these to be rare and not a significant issue. Type
narrowing is already unsound, and the additional unsoundness seems
minor, while the usability benefit is big.

This doesn't do the right thing for nested classes yet. I'll create an
issue to track that.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

mypy/binder.py

@@ -51,6 +51,9 @@ def __init__(self, id: int, conditional_frame: bool = False) -> None:
         # need this field.
         self.suppress_unreachable_warnings = False
 
+    def __repr__(self) -> str:
+        return f"Frame({self.id}, {self.types}, {self.unreachable}, {self.conditional_frame})"
+
 
 Assigns = DefaultDict[Expression, List[Tuple[Type, Optional[Type]]]]
 
@@ -63,7 +66,7 @@ class ConditionalTypeBinder:
 
     ```
     class A:
-        a = None          # type: Union[int, str]
+        a: Union[int, str] = None
     x = A()
     lst = [x]
     reveal_type(x.a)      # Union[int, str]
@@ -446,6 +449,7 @@ def top_frame_context(self) -> Iterator[Frame]:
         assert len(self.frames) == 1
         yield self.push_frame()
         self.pop_frame(True, 0)
+        assert len(self.frames) == 1
 
 
 def get_declaration(expr: BindableExpression) -> Type | None:

mypy/checker.py

@@ -26,7 +26,7 @@
 
 import mypy.checkexpr
 from mypy import errorcodes as codes, message_registry, nodes, operators
-from mypy.binder import ConditionalTypeBinder, get_declaration
+from mypy.binder import ConditionalTypeBinder, Frame, get_declaration
 from mypy.checkmember import (
     MemberContext,
     analyze_decorator_or_funcbase_access,
@@ -41,7 +41,7 @@
 from mypy.errors import Errors, ErrorWatcher, report_internal_error
 from mypy.expandtype import expand_self_type, expand_type, expand_type_by_instance
 from mypy.join import join_types
-from mypy.literals import Key, literal, literal_hash
+from mypy.literals import Key, extract_var_from_literal_hash, literal, literal_hash
 from mypy.maptype import map_instance_to_supertype
 from mypy.meet import is_overlapping_erased_types, is_overlapping_types
 from mypy.message_registry import ErrorMessage
@@ -134,6 +134,7 @@
     is_final_node,
 )
 from mypy.options import Options
+from mypy.patterns import AsPattern, StarredPattern
 from mypy.plugin import CheckerPluginInterface, Plugin
 from mypy.scope import Scope
 from mypy.semanal import is_trivial_body, refers_to_fullname, set_callable_name
@@ -151,7 +152,7 @@
     restrict_subtype_away,
     unify_generic_callable,
 )
-from mypy.traverser import all_return_statements, has_return_statement
+from mypy.traverser import TraverserVisitor, all_return_statements, has_return_statement
 from mypy.treetransform import TransformVisitor
 from mypy.typeanal import check_for_explicit_any, has_any_from_unimported_type, make_optional_type
 from mypy.typeops import (
@@ -1207,6 +1208,20 @@ def check_func_def(
 
             # Type check body in a new scope.
             with self.binder.top_frame_context():
+                # Copy some type narrowings from an outer function when it seems safe enough
+                # (i.e. we can't find an assignment that might change the type of the
+                # variable afterwards).
+                new_frame: Frame | None = None
+                for frame in old_binder.frames:
+                    for key, narrowed_type in frame.types.items():
+                        key_var = extract_var_from_literal_hash(key)
+                        if key_var is not None and not self.is_var_redefined_in_outer_context(
+                            key_var, defn.line
+                        ):
+                            # It seems safe to propagate the type narrowing to a nested scope.
+                            if new_frame is None:
+                                new_frame = self.binder.push_frame()
+                            new_frame.types[key] = narrowed_type
                 with self.scope.push_function(defn):
                     # We suppress reachability warnings when we use TypeVars with value
                     # restrictions: we only want to report a warning if a certain statement is
@@ -1218,6 +1233,8 @@ def check_func_def(
                         self.binder.suppress_unreachable_warnings()
                     self.accept(item.body)
                 unreachable = self.binder.is_unreachable()
+                if new_frame is not None:
+                    self.binder.pop_frame(True, 0)
 
             if not unreachable:
                 if defn.is_generator or is_named_instance(
@@ -1310,6 +1327,23 @@ def check_func_def(
 
             self.binder = old_binder
 
+    def is_var_redefined_in_outer_context(self, v: Var, after_line: int) -> bool:
+        """Can the variable be assigned to at module top level or outer function?
+
+        Note that this doesn't do a full CFG analysis but uses a line number based
+        heuristic that isn't correct in some (rare) cases.
+        """
+        outers = self.tscope.outer_functions()
+        if not outers:
+            # Top-level function -- outer context is top level, and we can't reason about
+            # globals
+            return True
+        for outer in outers:
+            if isinstance(outer, FuncDef):
+                if find_last_var_assignment_line(outer.body, v) >= after_line:
+                    return True
+        return False
+
     def check_unbound_return_typevar(self, typ: CallableType) -> None:
         """Fails when the return typevar is not defined in arguments."""
         if isinstance(typ.ret_type, TypeVarType) and typ.ret_type in typ.variables:
@@ -7629,3 +7663,80 @@ def collapse_walrus(e: Expression) -> Expression:
     if isinstance(e, AssignmentExpr):
         return e.target
     return e
+
+
+def find_last_var_assignment_line(n: Node, v: Var) -> int:
+    """Find the highest line number of a potential assignment to variable within node.
+
+    This supports local and global variables.
+
+    Return -1 if no assignment was found.
+    """
+    visitor = VarAssignVisitor(v)
+    n.accept(visitor)
+    return visitor.last_line
+
+
+class VarAssignVisitor(TraverserVisitor):
+    def __init__(self, v: Var) -> None:
+        self.last_line = -1
+        self.lvalue = False
+        self.var_node = v
+
+    def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
+        self.lvalue = True
+        for lv in s.lvalues:
+            lv.accept(self)
+        self.lvalue = False
+
+    def visit_name_expr(self, e: NameExpr) -> None:
+        if self.lvalue and e.node is self.var_node:
+            self.last_line = max(self.last_line, e.line)
+
+    def visit_member_expr(self, e: MemberExpr) -> None:
+        old_lvalue = self.lvalue
+        self.lvalue = False
+        super().visit_member_expr(e)
+        self.lvalue = old_lvalue
+
+    def visit_index_expr(self, e: IndexExpr) -> None:
+        old_lvalue = self.lvalue
+        self.lvalue = False
+        super().visit_index_expr(e)
+        self.lvalue = old_lvalue
+
+    def visit_with_stmt(self, s: WithStmt) -> None:
+        self.lvalue = True
+        for lv in s.target:
+            if lv is not None:
+                lv.accept(self)
+        self.lvalue = False
+        s.body.accept(self)
+
+    def visit_for_stmt(self, s: ForStmt) -> None:
+        self.lvalue = True
+        s.index.accept(self)
+        self.lvalue = False
+        s.body.accept(self)
+        if s.else_body:
+            s.else_body.accept(self)
+
+    def visit_assignment_expr(self, e: AssignmentExpr) -> None:
+        self.lvalue = True
+        e.target.accept(self)
+        self.lvalue = False
+        e.value.accept(self)
+
+    def visit_as_pattern(self, p: AsPattern) -> None:
+        if p.pattern is not None:
+            p.pattern.accept(self)
+        if p.name is not None:
+            self.lvalue = True
+            p.name.accept(self)
+            self.lvalue = False
+
+    def visit_starred_pattern(self, p: StarredPattern) -> None:
+        if p.capture is not None:
+            self.lvalue = True
+            p.capture.accept(self)
+            self.lvalue = False

mypy/fastparse.py

@@ -1766,7 +1766,8 @@ def visit_MatchStar(self, n: MatchStar) -> StarredPattern:
         if n.name is None:
             node = StarredPattern(None)
         else:
-            node = StarredPattern(NameExpr(n.name))
+            name = self.set_line(NameExpr(n.name), n)
+            node = StarredPattern(name)
 
         return self.set_line(node, n)
 

mypy/literals.py

@@ -139,6 +139,16 @@ def literal_hash(e: Expression) -> Key | None:
     return e.accept(_hasher)
 
 
+def extract_var_from_literal_hash(key: Key) -> Var | None:
+    """If key refers to a Var node, return it.
+
+    Return None otherwise.
+    """
+    if len(key) == 2 and key[0] == "Var" and isinstance(key[1], Var):
+        return key[1]
+    return None
+
+
 class _Hasher(ExpressionVisitor[Optional[Key]]):
     def visit_int_expr(self, e: IntExpr) -> Key:
         return ("Literal", e.value)

mypy/scope.py

@@ -21,6 +21,7 @@ def __init__(self) -> None:
         self.module: str | None = None
         self.classes: list[TypeInfo] = []
         self.function: FuncBase | None = None
+        self.functions: list[FuncBase] = []
         # Number of nested scopes ignored (that don't get their own separate targets)
         self.ignored = 0
 
@@ -65,19 +66,24 @@ def module_scope(self, prefix: str) -> Iterator[None]:
 
     @contextmanager
     def function_scope(self, fdef: FuncBase) -> Iterator[None]:
+        self.functions.append(fdef)
         if not self.function:
             self.function = fdef
         else:
             # Nested functions are part of the topmost function target.
             self.ignored += 1
         yield
+        self.functions.pop()
         if self.ignored:
             # Leave a scope that's included in the enclosing target.
             self.ignored -= 1
         else:
             assert self.function
             self.function = None
 
+    def outer_functions(self) -> list[FuncBase]:
+        return self.functions[:-1]
+
     def enter_class(self, info: TypeInfo) -> None:
         """Enter a class target scope."""
         if not self.function: