Support rename=True in collections.namedtuple (#17247)

A pretty marginal feature but it's now tested in the typing conformance
suite,
and it was easy to add support.

For reference:
https://github.com/python/typing/blob/9f7f400bb7c4c79f1fb938402e0bb3198dac0054/conformance/tests/namedtuples_define_functional.py#L46,
https://github.com/python/cpython/blob/7d8725ac6f3304677d71dabdb7c184e98a62d864/Lib/collections/__init__.py#L389

mypy/semanal_namedtuple.py

@@ -5,9 +5,11 @@
 
 from __future__ import annotations
 
+import keyword
 from contextlib import contextmanager
-from typing import Final, Iterator, List, Mapping, cast
+from typing import Container, Final, Iterator, List, Mapping, cast
 
+from mypy.errorcodes import ARG_TYPE, ErrorCode
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.messages import MessageBuilder
 from mypy.nodes import (
@@ -352,6 +354,7 @@ def parse_namedtuple_args(
             self.fail(f'Too few arguments for "{type_name}()"', call)
             return None
         defaults: list[Expression] = []
+        rename = False
         if len(args) > 2:
             # Typed namedtuple doesn't support additional arguments.
             if fullname in TYPED_NAMEDTUPLE_NAMES:
@@ -370,7 +373,17 @@ def parse_namedtuple_args(
                             "{}()".format(type_name),
                             arg,
                         )
-                    break
+                elif arg_name == "rename":
+                    arg = args[i]
+                    if isinstance(arg, NameExpr) and arg.name in ("True", "False"):
+                        rename = arg.name == "True"
+                    else:
+                        self.fail(
+                            'Boolean literal expected as the "rename" argument to '
+                            f"{type_name}()",
+                            arg,
+                            code=ARG_TYPE,
+                        )
         if call.arg_kinds[:2] != [ARG_POS, ARG_POS]:
             self.fail(f'Unexpected arguments to "{type_name}()"', call)
             return None
@@ -417,17 +430,28 @@ def parse_namedtuple_args(
                     return [], [], [], typename, [], False
         if not types:
             types = [AnyType(TypeOfAny.unannotated) for _ in items]
-        underscore = [item for item in items if item.startswith("_")]
-        if underscore:
-            self.fail(
-                f'"{type_name}()" field names cannot start with an underscore: '
-                + ", ".join(underscore),
-                call,
-            )
+        processed_items = []
+        seen_names: set[str] = set()
+        for i, item in enumerate(items):
+            problem = self.check_namedtuple_field_name(item, seen_names)
+            if problem is None:
+                processed_items.append(item)
+                seen_names.add(item)
+            else:
+                if not rename:
+                    self.fail(f'"{type_name}()" {problem}', call)
+                # Even if rename=False, we pretend that it is True.
+                # At runtime namedtuple creation would throw an error;
+                # applying the rename logic means we create a more sensible
+                # namedtuple.
+                new_name = f"_{i}"
+                processed_items.append(new_name)
+                seen_names.add(new_name)
+
         if len(defaults) > len(items):
             self.fail(f'Too many defaults given in call to "{type_name}()"', call)
             defaults = defaults[: len(items)]
-        return items, types, defaults, typename, tvar_defs, True
+        return processed_items, types, defaults, typename, tvar_defs, True
 
     def parse_namedtuple_fields_with_types(
         self, nodes: list[Expression], context: Context
@@ -666,5 +690,17 @@ def save_namedtuple_body(self, named_tuple_info: TypeInfo) -> Iterator[None]:
 
     # Helpers
 
-    def fail(self, msg: str, ctx: Context) -> None:
-        self.api.fail(msg, ctx)
+    def check_namedtuple_field_name(self, field: str, seen_names: Container[str]) -> str | None:
+        """Return None for valid fields, a string description for invalid ones."""
+        if field in seen_names:
+            return f'has duplicate field name "{field}"'
+        elif not field.isidentifier():
+            return f'field name "{field}" is not a valid identifier'
+        elif field.startswith("_"):
+            return f'field name "{field}" starts with an underscore'
+        elif keyword.iskeyword(field):
+            return f'field name "{field}" is a keyword'
+        return None
+
+    def fail(self, msg: str, ctx: Context, code: ErrorCode | None = None) -> None:
+        self.api.fail(msg, ctx, code=code)

test-data/unit/check-namedtuple.test

@@ -22,10 +22,13 @@ a, b, c = x # E: Need more than 2 values to unpack (3 expected)
 x[2] # E: Tuple index out of range
 [builtins fixtures/tuple.pyi]
 
-[case testNamedTupleNoUnderscoreFields]
+[case testNamedTupleInvalidFields]
 from collections import namedtuple
 
-X = namedtuple('X', 'x, _y, _z')  # E: "namedtuple()" field names cannot start with an underscore: _y, _z
+X = namedtuple('X', 'x, _y')  # E: "namedtuple()" field name "_y" starts with an underscore
+Y = namedtuple('Y', ['x', '1'])  # E: "namedtuple()" field name "1" is not a valid identifier
+Z = namedtuple('Z', ['x', 'def'])  # E: "namedtuple()" field name "def" is a keyword
+A = namedtuple('A', ['x', 'x'])  # E: "namedtuple()" has duplicate field name "x"
 [builtins fixtures/tuple.pyi]
 
 [case testNamedTupleAccessingAttributes]
@@ -125,6 +128,8 @@ E = namedtuple('E', 'a b', 0)
 [builtins fixtures/bool.pyi]
 
 [out]
+main:4: error: Boolean literal expected as the "rename" argument to namedtuple()
+main:5: error: Boolean literal expected as the "rename" argument to namedtuple()
 main:5: error: Argument "rename" to "namedtuple" has incompatible type "str"; expected "int"
 main:6: error: Unexpected keyword argument "unrecognized_arg" for "namedtuple"
 <ROOT>/test-data/unit/lib-stub/collections.pyi:3: note: "namedtuple" defined here
@@ -145,6 +150,23 @@ Z = namedtuple('Z', ['x', 'y'], defaults='not a tuple')  # E: List or tuple lite
 
 [builtins fixtures/list.pyi]
 
+[case testNamedTupleRename]
+from collections import namedtuple
+
+X = namedtuple('X', ['abc', 'def'], rename=False)  # E: "namedtuple()" field name "def" is a keyword
+Y = namedtuple('Y', ['x', 'x', 'def', '42', '_x'], rename=True)
+y = Y(x=0, _1=1, _2=2, _3=3, _4=4)
+reveal_type(y.x)  # N: Revealed type is "Any"
+reveal_type(y._1)  # N: Revealed type is "Any"
+reveal_type(y._2)  # N: Revealed type is "Any"
+reveal_type(y._3)  # N: Revealed type is "Any"
+reveal_type(y._4)  # N: Revealed type is "Any"
+y._0  # E: "Y" has no attribute "_0"
+y._5  # E: "Y" has no attribute "_5"
+y._x  # E: "Y" has no attribute "_x"
+
+[builtins fixtures/list.pyi]
+
 [case testNamedTupleWithItemTypes]
 from typing import NamedTuple
 N = NamedTuple('N', [('a', int),

test-data/unit/semanal-namedtuple.test

@@ -165,7 +165,7 @@ N = namedtuple('N', ['x', 1]) # E: String literal expected as "namedtuple()" ite
 
 [case testNamedTupleWithUnderscoreItemName]
 from collections import namedtuple
-N = namedtuple('N', ['_fallback']) # E: "namedtuple()" field names cannot start with an underscore: _fallback
+N = namedtuple('N', ['_fallback']) # E: "namedtuple()" field name "_fallback" starts with an underscore
 [builtins fixtures/tuple.pyi]
 
 -- NOTE: The following code works at runtime but is not yet supported by mypy.
@@ -197,7 +197,7 @@ N = NamedTuple('N', 1) # E: List or tuple literal expected as the second argumen
 
 [case testTypingNamedTupleWithUnderscoreItemName]
 from typing import NamedTuple
-N = NamedTuple('N', [('_fallback', int)]) # E: "NamedTuple()" field names cannot start with an underscore: _fallback
+N = NamedTuple('N', [('_fallback', int)]) # E: "NamedTuple()" field name "_fallback" starts with an underscore
 [builtins fixtures/tuple.pyi]
 
 [case testTypingNamedTupleWithUnexpectedNames]