Tweaks to traceback.format_list and from_list

Author: nathanielmanistaatgoogle

stdlib/2and3/traceback.pyi

@@ -1,6 +1,6 @@
 # Stubs for traceback
 
-from typing import Any, Dict, Generator, IO, Iterator, List, Mapping, Optional, Tuple, Type, Iterable
+from typing import Any, Dict, Generator, IO, Iterable, Iterator, List, Mapping, Optional, Tuple, Type, Union
 from types import FrameType, TracebackType
 import sys
 
@@ -35,6 +35,8 @@ if sys.version_info >= (3, 5):
     def extract_tb(tb: Optional[TracebackType], limit: Optional[int] = ...) -> StackSummary: ...
     def extract_stack(f: Optional[FrameType] = ...,
                       limit: Optional[int] = ...) -> StackSummary: ...
+    # TODO(https://bugs.python.org/issue34648): relax the type of
+    # extracted_list to Iterable[FrameSummary].
     def format_list(extracted_list: List[FrameSummary]) -> List[str]: ...
 else:
     def extract_tb(tb: Optional[TracebackType], limit: Optional[int] = ...) -> List[_PT]: ...
@@ -111,6 +113,9 @@ if sys.version_info >= (3, 5):
                     frame_gen: Generator[Tuple[FrameType, int], None, None],
                     *, limit: Optional[int] = ..., lookup_lines: bool = ...,
                     capture_locals: bool = ...) -> StackSummary: ...
+        # TODO(https://bugs.python.org/issue34648): relax "a_list" to
+        # Iterable[Union[_PT, FrameSummary]]... and then at some later point
+        # drop support for _PT.
         @classmethod
-        def from_list(cls, a_list: List[_PT]) -> StackSummary: ...
+        def from_list(cls, a_list: List[Union[_PT, FrameSummary]]) -> StackSummary: ...
         def format(self) -> List[str]: ...