python · JuanJoseAra · Mar 6, 2020 · Feb 28, 2020 · Mar 9, 2020
diff --git a/mypy/checker.py b/mypy/checker.py
@@ -3924,10 +3924,12 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
         elif is_false_literal(node):
             return None, {}
         elif isinstance(node, CallExpr):
+            if len(node.args) == 0:
+                return {}, {}
+            expr = collapse_walrus(node.args[0])
             if refers_to_fullname(node.callee, 'builtins.isinstance'):
                 if len(node.args) != 2:  # the error will be reported elsewhere
                     return {}, {}
-                expr = node.args[0]
                 if literal(expr) == LITERAL_TYPE:
                     return self.conditional_type_map_with_intersection(
                         expr,
@@ -3937,13 +3939,11 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
             elif refers_to_fullname(node.callee, 'builtins.issubclass'):
                 if len(node.args) != 2:  # the error will be reported elsewhere
                     return {}, {}
-                expr = node.args[0]
                 if literal(expr) == LITERAL_TYPE:
                     return self.infer_issubclass_maps(node, expr, type_map)
             elif refers_to_fullname(node.callee, 'builtins.callable'):
                 if len(node.args) != 1:  # the error will be reported elsewhere
                     return {}, {}
-                expr = node.args[0]
                 if literal(expr) == LITERAL_TYPE:
                     vartype = type_map[expr]
                     return self.conditional_callable_type_map(expr, vartype)
@@ -3952,7 +3952,7 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
             # narrow their types. (For example, we shouldn't try narrowing the
             # types of literal string or enum expressions).
 
-            operands = node.operands
+            operands = [collapse_walrus(x) for x in node.operands]
             operand_types = []
             narrowable_operand_index_to_hash = {}
             for i, expr in enumerate(operands):
@@ -5742,3 +5742,14 @@ def has_bool_item(typ: ProperType) -> bool:
         return any(is_named_instance(item, 'builtins.bool')
                    for item in typ.items)
     return False
+
+
+def collapse_walrus(e: Expression) -> Expression:
+    """If an expression is an AssignmentExpr, pull out the assignment target.
+
+    We don't make any attempt to pull out all the targets in code like `x := (y := z)`.
+    We could support narrowing those if that sort of code turns out to be common.
+    """
+    if isinstance(e, AssignmentExpr):
+        return e.target
+    return e
diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
@@ -2409,7 +2409,16 @@ def make_local_errors() -> MessageBuilder:
         def lookup_operator(op_name: str, base_type: Type) -> Optional[Type]:
             """Looks up the given operator and returns the corresponding type,
             if it exists."""
+
+            # This check is an important performance optimization,
+            # even though it is mostly a subset of
+            # analyze_member_access.
+            # TODO: Find a way to remove this call without performance implications.
+            if not self.has_member(base_type, op_name):
+                return None
+
             local_errors = make_local_errors()
+
             member = analyze_member_access(
                 name=op_name,
                 typ=base_type,
@@ -3790,6 +3799,45 @@ def is_valid_keyword_var_arg(self, typ: Type) -> bool:
                     [self.named_type('builtins.unicode'),
                      AnyType(TypeOfAny.special_form)])))
 
+    def has_member(self, typ: Type, member: str) -> bool:
+        """Does type have member with the given name?"""
+        # TODO: refactor this to use checkmember.analyze_member_access, otherwise
+        # these two should be carefully kept in sync.
+        # This is much faster than analyze_member_access, though, and so using
+        # it first as a filter is important for performance.
+        typ = get_proper_type(typ)
+
+        if isinstance(typ, TypeVarType):
+            typ = get_proper_type(typ.upper_bound)
+        if isinstance(typ, TupleType):
+            typ = tuple_fallback(typ)
+        if isinstance(typ, LiteralType):
+            typ = typ.fallback
+        if isinstance(typ, Instance):
+            return typ.type.has_readable_member(member)
+        if isinstance(typ, CallableType) and typ.is_type_obj():
+            return typ.fallback.type.has_readable_member(member)
+        elif isinstance(typ, AnyType):
+            return True
+        elif isinstance(typ, UnionType):
+            result = all(self.has_member(x, member) for x in typ.relevant_items())
+            return result
+        elif isinstance(typ, TypeType):
+            # Type[Union[X, ...]] is always normalized to Union[Type[X], ...],
+            # so we don't need to care about unions here.
+            item = typ.item
+            if isinstance(item, TypeVarType):
+                item = get_proper_type(item.upper_bound)
+            if isinstance(item, TupleType):
+                item = tuple_fallback(item)
+            if isinstance(item, Instance) and item.type.metaclass_type is not None:
+                return self.has_member(item.type.metaclass_type, member)
+            if isinstance(item, AnyType):
+                return True
+            return False
+        else:
+            return False
+
     def not_ready_callback(self, name: str, context: Context) -> None:
         """Called when we can't infer the type of a variable because it's not ready yet.
 

diff --git a/mypy/typeops.py b/mypy/typeops.py
@@ -719,5 +719,5 @@ def custom_special_method(typ: Type, name: str, check_all: bool = False) -> bool
     if isinstance(typ, AnyType):
         # Avoid false positives in uncertain cases.
         return True
-    # TODO: support other types (see analyze_member_access)?
+    # TODO: support other types (see ExpressionChecker.has_member())?
     return False
diff --git a/mypy/version.py b/mypy/version.py
@@ -5,7 +5,7 @@
 # - Release versions have the form "0.NNN".
 # - Dev versions have the form "0.NNN+dev" (PLUS sign to conform to PEP 440).
 # - For 1.0 we'll switch back to 1.2.3 form.
-__version__ = '0.770+dev'
+__version__ = '0.770'
 base_version = __version__
 
 mypy_dir = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))

diff --git a/test-data/unit/check-classes.test b/test-data/unit/check-classes.test
@@ -6650,7 +6650,7 @@ reveal_type(0.5 + C)  # N: Revealed type is 'Any'
 
 reveal_type(0.5 + D())  # N: Revealed type is 'Any'
 reveal_type(D() + 0.5)  # N: Revealed type is 'Any'
-reveal_type("str" + D())  # N: Revealed type is 'Any'
+reveal_type("str" + D())  # N: Revealed type is 'builtins.str'
 reveal_type(D() + "str")  # N: Revealed type is 'Any'
 
 

diff --git a/test-data/unit/check-python38.test b/test-data/unit/check-python38.test
@@ -189,7 +189,7 @@ def f(p1: bytes, p2: float, /) -> None:
 
 [case testWalrus]
 # flags: --strict-optional
-from typing import NamedTuple, Optional
+from typing import NamedTuple, Optional, List
 from typing_extensions import Final
 
 if a := 2:
@@ -288,10 +288,23 @@ def check_partial() -> None:
 
     reveal_type(x)  # N: Revealed type is 'Union[builtins.int, None]'
 
-def check_narrow(x: Optional[int]) -> None:
+def check_narrow(x: Optional[int], s: List[int]) -> None:
     if (y := x):
         reveal_type(y)  # N: Revealed type is 'builtins.int'
-[builtins fixtures/f_string.pyi]
+
+    if (y := x) is not None:
+        reveal_type(y)  # N: Revealed type is 'builtins.int'
+
+    if (y := x) == 10:
+        reveal_type(y)  # N: Revealed type is 'builtins.int'
+
+    if (y := x) in s:
+        reveal_type(y)  # N: Revealed type is 'builtins.int'
+
+    if isinstance((y := x), int):
+        reveal_type(y)  # N: Revealed type is 'builtins.int'
+
+[builtins fixtures/isinstancelist.pyi]
 
 [case testWalrusPartialTypes]
 from typing import List