More precise return types for open(), Path.open(), bz2.open(), etc. (#3371)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

stdlib/2/__builtin__.pyi

@@ -5,12 +5,13 @@ from typing import (
     TypeVar, Iterator, Iterable, NoReturn, overload, Container,
     Sequence, MutableSequence, Mapping, MutableMapping, Tuple, List, Any, Dict, Callable, Generic,
     Set, AbstractSet, FrozenSet, MutableSet, Sized, Reversible, SupportsInt, SupportsFloat, SupportsAbs,
-    SupportsComplex, IO, BinaryIO, Union,
+    SupportsComplex, IO, BinaryIO, TextIO, Union,
     ItemsView, KeysView, ValuesView, ByteString, Optional, AnyStr, Type, Text,
     Protocol,
 )
 from abc import abstractmethod, ABCMeta
 from ast import mod, AST
+from io import _OpenBinaryMode, _OpenTextMode
 from types import TracebackType, CodeType
 import sys
 
@@ -1357,14 +1358,47 @@ def next(__i: Iterator[_T]) -> _T: ...
 def next(__i: Iterator[_T], default: _VT) -> Union[_T, _VT]: ...
 def oct(__number: Union[int, _SupportsIndex]) -> str: ...
 
-if sys.version_info >= (3, 6):
-    def open(file: Union[str, bytes, int, _PathLike[Any]], mode: str = ..., buffering: int = ..., encoding: Optional[str] = ...,
-             errors: Optional[str] = ..., newline: Optional[str] = ..., closefd: bool = ...,
-             opener: Optional[Callable[[str, int], int]] = ...) -> IO[Any]: ...
-elif sys.version_info >= (3,):
-    def open(file: Union[str, bytes, int], mode: str = ..., buffering: int = ..., encoding: Optional[str] = ...,
-             errors: Optional[str] = ..., newline: Optional[str] = ..., closefd: bool = ...,
-             opener: Optional[Callable[[str, int], int]] = ...) -> IO[Any]: ...
+if sys.version_info >= (3,):
+    if sys.version_info >= (3, 6):
+        # Changed in version 3.6: Support added to accept objects implementing os.PathLike.
+        _OpenFile = Union[str, bytes, int, _PathLike[Any]]
+    else:
+        _OpenFile = Union[str, bytes, int]
+
+    @overload
+    def open(
+        file: _OpenFile,
+        mode: _OpenTextMode = ...,
+        buffering: int = ...,
+        encoding: Optional[str] = ...,
+        errors: Optional[str] = ...,
+        newline: Optional[str] = ...,
+        closefd: bool = ...,
+        opener: Optional[Callable[[str, int], int]] = ...,
+    ) -> TextIO: ...
+    @overload
+    def open(
+        file: _OpenFile,
+        mode: _OpenBinaryMode,
+        buffering: int = ...,
+        encoding: None = ...,
+        errors: None = ...,
+        newline: None = ...,
+        closefd: bool = ...,
+        opener: Optional[Callable[[str, int], int]] = ...,
+    ) -> BinaryIO: ...
+    @overload
+    def open(
+        file: _OpenFile,
+        mode: str,
+        buffering: int = ...,
+        encoding: Optional[str] = ...,
+        errors: Optional[str] = ...,
+        newline: Optional[str] = ...,
+        closefd: bool = ...,
+        opener: Optional[Callable[[str, int], int]] = ...,
+    ) -> IO[Any]: ...
+
 else:
     def open(name: Union[unicode, int], mode: unicode = ..., buffering: int = ...) -> BinaryIO: ...
 

stdlib/2/io.pyi

@@ -5,6 +5,7 @@
 # Only a subset of functionality is included.
 
 from typing import List, BinaryIO, TextIO, IO, overload, Iterator, Iterable, Any, Union, Optional
+from typing_extensions import Literal
 import _io
 
 from _io import BlockingIOError as BlockingIOError
@@ -21,6 +22,19 @@ from _io import TextIOWrapper as TextIOWrapper
 from _io import UnsupportedOperation as UnsupportedOperation
 from _io import open as open
 
+_OpenTextMode = Literal[
+    'r', 'r+', '+r', 'rt', 'tr', 'rt+', 'r+t', '+rt', 'tr+', 't+r', '+tr',
+    'w', 'w+', '+w', 'wt', 'tw', 'wt+', 'w+t', '+wt', 'tw+', 't+w', '+tw',
+    'a', 'a+', '+a', 'at', 'ta', 'at+', 'a+t', '+at', 'ta+', 't+a', '+ta',
+    'U', 'rU', 'Ur', 'rtU', 'rUt', 'Urt', 'trU', 'tUr', 'Utr',
+]
+_OpenBinaryMode = Literal[
+    'rb', 'br', 'rb+', 'r+b', '+rb', 'br+', 'b+r', '+br',
+    'wb', 'bw', 'wb+', 'w+b', '+wb', 'bw+', 'b+w', '+bw',
+    'ab', 'ba', 'ab+', 'a+b', '+ab', 'ba+', 'b+a', '+ba',
+    'rbU', 'rUb', 'Urb', 'brU', 'bUr', 'Ubr',
+]
+
 def _OpenWrapper(file: Union[str, unicode, int],
                  mode: unicode = ..., buffering: int = ..., encoding: unicode = ...,
                  errors: unicode = ..., newline: unicode = ...,