Add interactions between Literal and Final (#6081)

This pull request adds logic to handle interactions between Literal and
Final. In short, if the user were to define a variable like `x: Final = 3` and
latter do `some_func(x)`, mypy will attempt to type-check the code almost
as if the user had done `some_func(3)` instead.

This normally does not make a difference, except when type-checking code
using literal types. For example, if `some_func` accepts a `Literal[3]` up
above, the code would type-check since `x` cannot be anything other
then a `3`.

Or to put it another way, this pull request makes variables that use `Final`
with the type omitted context-sensitive.

mypy/checker.py

@@ -1810,8 +1810,11 @@ def check_assignment(self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type
                 self.check_indexed_assignment(index_lvalue, rvalue, lvalue)
 
             if inferred:
-                self.infer_variable_type(inferred, lvalue, self.expr_checker.accept(rvalue),
-                                         rvalue)
+                rvalue_type = self.expr_checker.accept(
+                    rvalue,
+                    in_final_declaration=inferred.is_final,
+                )
+                self.infer_variable_type(inferred, lvalue, rvalue_type, rvalue)
 
     def check_compatibility_all_supers(self, lvalue: RefExpr, lvalue_type: Optional[Type],
                                        rvalue: Expression) -> bool:

mypy/checkexpr.py

@@ -18,7 +18,7 @@
 from mypy.types import (
     Type, AnyType, CallableType, Overloaded, NoneTyp, TypeVarDef,
     TupleType, TypedDictType, Instance, TypeVarType, ErasedType, UnionType,
-    PartialType, DeletedType, UninhabitedType, TypeType, TypeOfAny, LiteralType,
+    PartialType, DeletedType, UninhabitedType, TypeType, TypeOfAny, LiteralType, LiteralValue,
     true_only, false_only, is_named_instance, function_type, callable_type, FunctionLike,
     StarType, is_optional, remove_optional, is_generic_instance
 )
@@ -139,6 +139,16 @@ def __init__(self,
         self.msg = msg
         self.plugin = plugin
         self.type_context = [None]
+
+        # Set to 'True' whenever we are checking the expression in some 'Final' declaration.
+        # For example, if we're checking the "3" in a statement like "var: Final = 3".
+        #
+        # This flag changes the type that eventually gets inferred for "var". Instead of
+        # inferring *just* a 'builtins.int' instance, we infer an instance that keeps track
+        # of the underlying literal value. See the comments in Instance's constructors for
+        # more details.
+        self.in_final_declaration = False
+
         # Temporary overrides for expression types. This is currently
         # used by the union math in overloads.
         # TODO: refactor this to use a pattern similar to one in
@@ -210,10 +220,12 @@ def analyze_ref_expr(self, e: RefExpr, lvalue: bool = False) -> Type:
 
     def analyze_var_ref(self, var: Var, context: Context) -> Type:
         if var.type:
-            if is_literal_type_like(self.type_context[-1]) and var.name() in {'True', 'False'}:
-                return LiteralType(var.name() == 'True', self.named_type('builtins.bool'))
-            else:
-                return var.type
+            if isinstance(var.type, Instance):
+                if self.is_literal_context() and var.type.final_value is not None:
+                    return var.type.final_value
+                if var.name() in {'True', 'False'}:
+                    return self.infer_literal_expr_type(var.name() == 'True', 'builtins.bool')
+            return var.type
         else:
             if not var.is_ready and self.chk.in_checked_function():
                 self.chk.handle_cannot_determine_type(var.name(), context)
@@ -691,7 +703,8 @@ def check_call(self,
         elif isinstance(callee, Instance):
             call_function = analyze_member_access('__call__', callee, context,
                                                   False, False, False, self.msg,
-                                                  original_type=callee, chk=self.chk)
+                                                  original_type=callee, chk=self.chk,
+                                                  in_literal_context=self.is_literal_context())
             return self.check_call(call_function, args, arg_kinds, context, arg_names,
                                    callable_node, arg_messages)
         elif isinstance(callee, TypeVarType):
@@ -1755,7 +1768,8 @@ def analyze_ordinary_member_access(self, e: MemberExpr,
             original_type = self.accept(e.expr)
             member_type = analyze_member_access(
                 e.name, original_type, e, is_lvalue, False, False,
-                self.msg, original_type=original_type, chk=self.chk)
+                self.msg, original_type=original_type, chk=self.chk,
+                in_literal_context=self.is_literal_context())
             return member_type
 
     def analyze_external_member_access(self, member: str, base_type: Type,
@@ -1765,35 +1779,57 @@ def analyze_external_member_access(self, member: str, base_type: Type,
         """
         # TODO remove; no private definitions in mypy
         return analyze_member_access(member, base_type, context, False, False, False,
-                                     self.msg, original_type=base_type, chk=self.chk)
+                                     self.msg, original_type=base_type, chk=self.chk,
+                                     in_literal_context=self.is_literal_context())
+
+    def is_literal_context(self) -> bool:
+        return is_literal_type_like(self.type_context[-1])
+
+    def infer_literal_expr_type(self, value: LiteralValue, fallback_name: str) -> Type:
+        """Analyzes the given literal expression and determines if we should be
+        inferring an Instance type, a Literal[...] type, or an Instance that
+        remembers the original literal. We...
+
+        1. ...Infer a normal Instance in most circumstances.
+
+        2. ...Infer a Literal[...] if we're in a literal context. For example, if we
+           were analyzing the "3" in "foo(3)" where "foo" has a signature of
+           "def foo(Literal[3]) -> None", we'd want to infer that the "3" has a
+           type of Literal[3] instead of Instance.
+
+        3. ...Infer an Instance that remembers the original Literal if we're declaring
+           a Final variable with an inferred type -- for example, "bar" in "bar: Final = 3"
+           would be assigned an Instance that remembers it originated from a '3'. See
+           the comments in Instance's constructor for more details.
+        """
+        typ = self.named_type(fallback_name)
+        if self.is_literal_context():
+            return LiteralType(value=value, fallback=typ)
+        elif self.in_final_declaration:
+            return typ.copy_modified(final_value=LiteralType(
+                value=value,
+                fallback=typ,
+                line=typ.line,
+                column=typ.column,
+            ))
+        else:
+            return typ
 
     def visit_int_expr(self, e: IntExpr) -> Type:
         """Type check an integer literal (trivial)."""
-        typ = self.named_type('builtins.int')
-        if is_literal_type_like(self.type_context[-1]):
-            return LiteralType(value=e.value, fallback=typ)
-        return typ
+        return self.infer_literal_expr_type(e.value, 'builtins.int')
 
     def visit_str_expr(self, e: StrExpr) -> Type:
         """Type check a string literal (trivial)."""
-        typ = self.named_type('builtins.str')
-        if is_literal_type_like(self.type_context[-1]):
-            return LiteralType(value=e.value, fallback=typ)
-        return typ
+        return self.infer_literal_expr_type(e.value, 'builtins.str')
 
     def visit_bytes_expr(self, e: BytesExpr) -> Type:
         """Type check a bytes literal (trivial)."""
-        typ = self.named_type('builtins.bytes')
-        if is_literal_type_like(self.type_context[-1]):
-            return LiteralType(value=e.value, fallback=typ)
-        return typ
+        return self.infer_literal_expr_type(e.value, 'builtins.bytes')
 
     def visit_unicode_expr(self, e: UnicodeExpr) -> Type:
         """Type check a unicode literal (trivial)."""
-        typ = self.named_type('builtins.unicode')
-        if is_literal_type_like(self.type_context[-1]):
-            return LiteralType(value=e.value, fallback=typ)
-        return typ
+        return self.infer_literal_expr_type(e.value, 'builtins.unicode')
 
     def visit_float_expr(self, e: FloatExpr) -> Type:
         """Type check a float literal (trivial)."""
@@ -1930,7 +1966,8 @@ def check_method_call_by_name(self,
         """
         local_errors = local_errors or self.msg
         method_type = analyze_member_access(method, base_type, context, False, False, True,
-                                            local_errors, original_type=base_type, chk=self.chk)
+                                            local_errors, original_type=base_type, chk=self.chk,
+                                            in_literal_context=self.is_literal_context())
         return self.check_method_call(
             method, base_type, method_type, args, arg_kinds, context, local_errors)
 
@@ -1994,6 +2031,7 @@ def lookup_operator(op_name: str, base_type: Type) -> Optional[Type]:
                 context=context,
                 msg=local_errors,
                 chk=self.chk,
+                in_literal_context=self.is_literal_context()
             )
             if local_errors.is_errors():
                 return None
@@ -2950,7 +2988,8 @@ def analyze_super(self, e: SuperExpr, is_lvalue: bool) -> Type:
                                                  override_info=base,
                                                  context=e,
                                                  msg=self.msg,
-                                                 chk=self.chk)
+                                                 chk=self.chk,
+                                                 in_literal_context=self.is_literal_context())
             assert False, 'unreachable'
         else:
             # Invalid super. This has been reported by the semantic analyzer.
@@ -3117,13 +3156,16 @@ def accept(self,
                type_context: Optional[Type] = None,
                allow_none_return: bool = False,
                always_allow_any: bool = False,
+               in_final_declaration: bool = False,
                ) -> Type:
         """Type check a node in the given type context.  If allow_none_return
         is True and this expression is a call, allow it to return None.  This
         applies only to this expression and not any subexpressions.
         """
         if node in self.type_overrides:
             return self.type_overrides[node]
+        old_in_final_declaration = self.in_final_declaration
+        self.in_final_declaration = in_final_declaration
         self.type_context.append(type_context)
         try:
             if allow_none_return and isinstance(node, CallExpr):
@@ -3136,6 +3178,7 @@ def accept(self,
             report_internal_error(err, self.chk.errors.file,
                                   node.line, self.chk.errors, self.chk.options)
         self.type_context.pop()
+        self.in_final_declaration = old_in_final_declaration
         assert typ is not None
         self.chk.store_type(node, typ)
 

mypy/checkmember.py

@@ -71,7 +71,8 @@ def analyze_member_access(name: str,
                           msg: MessageBuilder, *,
                           original_type: Type,
                           chk: 'mypy.checker.TypeChecker',
-                          override_info: Optional[TypeInfo] = None) -> Type:
+                          override_info: Optional[TypeInfo] = None,
+                          in_literal_context: bool = False) -> Type:
     """Return the type of attribute 'name' of 'typ'.
 
     The actual implementation is in '_analyze_member_access' and this docstring
@@ -96,7 +97,11 @@ def analyze_member_access(name: str,
                        context,
                        msg,
                        chk=chk)
-    return _analyze_member_access(name, typ, mx, override_info)
+    result = _analyze_member_access(name, typ, mx, override_info)
+    if in_literal_context and isinstance(result, Instance) and result.final_value is not None:
+        return result.final_value
+    else:
+        return result
 
 
 def _analyze_member_access(name: str,

mypy/fixup.py

@@ -155,6 +155,8 @@ def visit_instance(self, inst: Instance) -> None:
                 base.accept(self)
         for a in inst.args:
             a.accept(self)
+        if inst.final_value is not None:
+            inst.final_value.accept(self)
 
     def visit_any(self, o: Any) -> None:
         pass  # Nothing to descend into.

mypy/sametypes.py

@@ -77,7 +77,8 @@ def visit_deleted_type(self, left: DeletedType) -> bool:
     def visit_instance(self, left: Instance) -> bool:
         return (isinstance(self.right, Instance) and
                 left.type == self.right.type and
-                is_same_types(left.args, self.right.args))
+                is_same_types(left.args, self.right.args) and
+                left.final_value == self.right.final_value)
 
     def visit_type_var(self, left: TypeVarType) -> bool:
         return (isinstance(self.right, TypeVarType) and

mypy/semanal.py

@@ -65,7 +65,7 @@
 from mypy.messages import CANNOT_ASSIGN_TO_TYPE, MessageBuilder
 from mypy.types import (
     FunctionLike, UnboundType, TypeVarDef, TupleType, UnionType, StarType, function_type,
-    CallableType, Overloaded, Instance, Type, AnyType,
+    CallableType, Overloaded, Instance, Type, AnyType, LiteralType, LiteralValue,
     TypeTranslator, TypeOfAny, TypeType, NoneTyp,
 )
 from mypy.nodes import implicit_module_attrs
@@ -1760,9 +1760,9 @@ def final_cb(keep_final: bool) -> None:
                     self.type and self.type.is_protocol and not self.is_func_scope()):
                 self.fail('All protocol members must have explicitly declared types', s)
             # Set the type if the rvalue is a simple literal (even if the above error occurred).
-            if len(s.lvalues) == 1 and isinstance(s.lvalues[0], NameExpr):
+            if len(s.lvalues) == 1 and isinstance(s.lvalues[0], RefExpr):
                 if s.lvalues[0].is_inferred_def:
-                    s.type = self.analyze_simple_literal_type(s.rvalue)
+                    s.type = self.analyze_simple_literal_type(s.rvalue, s.is_final_def)
         if s.type:
             # Store type into nodes.
             for lvalue in s.lvalues:
@@ -1900,8 +1900,10 @@ def unbox_literal(self, e: Expression) -> Optional[Union[int, float, bool, str]]
             return True if e.name == 'True' else False
         return None
 
-    def analyze_simple_literal_type(self, rvalue: Expression) -> Optional[Type]:
-        """Return builtins.int if rvalue is an int literal, etc."""
+    def analyze_simple_literal_type(self, rvalue: Expression, is_final: bool) -> Optional[Type]:
+        """Return builtins.int if rvalue is an int literal, etc.
+
+        If this is a 'Final' context, we return "Literal[...]" instead."""
         if self.options.semantic_analysis_only or self.function_stack:
             # Skip this if we're only doing the semantic analysis pass.
             # This is mostly to avoid breaking unit tests.
@@ -1910,16 +1912,31 @@ def analyze_simple_literal_type(self, rvalue: Expression) -> Optional[Type]:
             # inside type variables with value restrictions (like
             # AnyStr).
             return None
-        if isinstance(rvalue, IntExpr):
-            return self.named_type_or_none('builtins.int')
         if isinstance(rvalue, FloatExpr):
             return self.named_type_or_none('builtins.float')
+
+        value = None  # type: LiteralValue
+        type_name = None  # type: Optional[str]
+        if isinstance(rvalue, IntExpr):
+            value, type_name = rvalue.value, 'builtins.int'
         if isinstance(rvalue, StrExpr):
-            return self.named_type_or_none('builtins.str')
+            value, type_name = rvalue.value, 'builtins.str'
         if isinstance(rvalue, BytesExpr):
-            return self.named_type_or_none('builtins.bytes')
+            value, type_name = rvalue.value, 'builtins.bytes'
         if isinstance(rvalue, UnicodeExpr):
-            return self.named_type_or_none('builtins.unicode')
+            value, type_name = rvalue.value, 'builtins.unicode'
+
+        if type_name is not None:
+            typ = self.named_type_or_none(type_name)
+            if typ and is_final:
+                return typ.copy_modified(final_value=LiteralType(
+                    value=value,
+                    fallback=typ,
+                    line=typ.line,
+                    column=typ.column,
+                ))
+            return typ
+
         return None
 
     def analyze_alias(self, rvalue: Expression) -> Tuple[Optional[Type], List[str],

mypy/server/astdiff.py

@@ -284,7 +284,8 @@ def visit_deleted_type(self, typ: DeletedType) -> SnapshotItem:
     def visit_instance(self, typ: Instance) -> SnapshotItem:
         return ('Instance',
                 typ.type.fullname(),
-                snapshot_types(typ.args))
+                snapshot_types(typ.args),
+                None if typ.final_value is None else snapshot_type(typ.final_value))
 
     def visit_type_var(self, typ: TypeVarType) -> SnapshotItem:
         return ('TypeVar',

mypy/server/astmerge.py

@@ -342,6 +342,8 @@ def visit_instance(self, typ: Instance) -> None:
         typ.type = self.fixup(typ.type)
         for arg in typ.args:
             arg.accept(self)
+        if typ.final_value:
+            typ.final_value.accept(self)
 
     def visit_any(self, typ: AnyType) -> None:
         pass

mypy/server/deps.py

@@ -882,6 +882,8 @@ def visit_instance(self, typ: Instance) -> List[str]:
         triggers = [trigger]
         for arg in typ.args:
             triggers.extend(self.get_type_triggers(arg))
+        if typ.final_value:
+            triggers.extend(self.get_type_triggers(typ.final_value))
         return triggers
 
     def visit_any(self, typ: AnyType) -> List[str]:

mypy/type_visitor.py

@@ -13,7 +13,7 @@
 
 from abc import abstractmethod
 from collections import OrderedDict
-from typing import Generic, TypeVar, cast, Any, List, Callable, Iterable
+from typing import Generic, TypeVar, cast, Any, List, Callable, Iterable, Optional
 from mypy_extensions import trait
 
 T = TypeVar('T')
@@ -159,7 +159,18 @@ def visit_deleted_type(self, t: DeletedType) -> Type:
         return t
 
     def visit_instance(self, t: Instance) -> Type:
-        return Instance(t.type, self.translate_types(t.args), t.line, t.column)
+        final_value = None  # type: Optional[LiteralType]
+        if t.final_value is not None:
+            raw_final_value = t.final_value.accept(self)
+            assert isinstance(raw_final_value, LiteralType)
+            final_value = raw_final_value
+        return Instance(
+            typ=t.type,
+            args=self.translate_types(t.args),
+            line=t.line,
+            column=t.column,
+            final_value=final_value,
+        )
 
     def visit_type_var(self, t: TypeVarType) -> Type:
         return t

mypy/typeanal.py

@@ -697,6 +697,9 @@ def analyze_literal_param(self, idx: int, arg: Type, ctx: Context) -> Optional[L
         elif isinstance(arg, (NoneTyp, LiteralType)):
             # Types that we can just add directly to the literal/potential union of literals.
             return [arg]
+        elif isinstance(arg, Instance) and arg.final_value is not None:
+            # Types generated from declarations like "var: Final = 4".
+            return [arg.final_value]
         elif isinstance(arg, UnionType):
             out = []
             for union_arg in arg.items: