Support NamedTuple class syntax in all stubs (#7646)

Previously, it was not supported in stubs when checking in Python 2.7 or
3.5 mode.

Closes: #3464

Author: srittau

mypy/semanal.py

@@ -1109,7 +1109,8 @@ def analyze_namedtuple_classdef(self, defn: ClassDef) -> bool:
             # in the named tuple class body.
             is_named_tuple, info = True, defn.info  # type: bool, Optional[TypeInfo]
         else:
-            is_named_tuple, info = self.named_tuple_analyzer.analyze_namedtuple_classdef(defn)
+            is_named_tuple, info = self.named_tuple_analyzer.analyze_namedtuple_classdef(
+                defn, self.is_stub_file)
         if is_named_tuple:
             if info is None:
                 self.mark_incomplete(defn.name, defn)

mypy/semanal_namedtuple.py

@@ -41,7 +41,8 @@ def __init__(self, options: Options, api: SemanticAnalyzerInterface) -> None:
         self.options = options
         self.api = api
 
-    def analyze_namedtuple_classdef(self, defn: ClassDef) -> Tuple[bool, Optional[TypeInfo]]:
+    def analyze_namedtuple_classdef(self, defn: ClassDef, is_stub_file: bool
+                                    ) -> Tuple[bool, Optional[TypeInfo]]:
         """Analyze if given class definition can be a named tuple definition.
 
         Return a tuple where first item indicates whether this can possibly be a named tuple,
@@ -52,7 +53,7 @@ def analyze_namedtuple_classdef(self, defn: ClassDef) -> Tuple[bool, Optional[Ty
             if isinstance(base_expr, RefExpr):
                 self.api.accept(base_expr)
                 if base_expr.fullname == 'typing.NamedTuple':
-                    result = self.check_namedtuple_classdef(defn)
+                    result = self.check_namedtuple_classdef(defn, is_stub_file)
                     if result is None:
                         # This is a valid named tuple, but some types are incomplete.
                         return True, None
@@ -68,8 +69,10 @@ def analyze_namedtuple_classdef(self, defn: ClassDef) -> Tuple[bool, Optional[Ty
         # This can't be a valid named tuple.
         return False, None
 
-    def check_namedtuple_classdef(
-            self, defn: ClassDef) -> Optional[Tuple[List[str], List[Type], Dict[str, Expression]]]:
+    def check_namedtuple_classdef(self, defn: ClassDef, is_stub_file: bool
+                                  ) -> Optional[Tuple[List[str],
+                                                List[Type],
+                                                Dict[str, Expression]]]:
         """Parse and validate fields in named tuple class definition.
 
         Return a three tuple:
@@ -78,7 +81,7 @@ def check_namedtuple_classdef(
           * field default values
         or None, if any of the types are not ready.
         """
-        if self.options.python_version < (3, 6):
+        if self.options.python_version < (3, 6) and not is_stub_file:
             self.fail('NamedTuple class syntax is only supported in Python 3.6', defn)
             return [], [], {}
         if len(defn.base_type_exprs) > 1:

test-data/unit/check-namedtuple.test

@@ -44,6 +44,24 @@ x.x
 x.y
 x.z # E: "X" has no attribute "z"
 
+[case testNamedTupleClassPython35]
+# flags: --python-version 3.5
+from typing import NamedTuple
+
+class A(NamedTuple):
+    x = 3  # type: int
+[out]
+main:4: error: NamedTuple class syntax is only supported in Python 3.6
+
+[case testNamedTupleClassInStubPython35]
+# flags: --python-version 3.5
+import foo
+
+[file foo.pyi]
+from typing import NamedTuple
+
+class A(NamedTuple):
+    x: int
 
 [case testNamedTupleAttributesAreReadOnly]
 from collections import namedtuple