bpo-40280: Change subprocess imports for cleaner error on wasm32 (GH-30620)

Author: tiran

Lib/subprocess.py

@@ -65,16 +65,11 @@
            # NOTE: We intentionally exclude list2cmdline as it is
            # considered an internal implementation detail.  issue10838.
 
-try:
+_mswindows = sys.platform == "win32"
+
+if _mswindows:
     import msvcrt
     import _winapi
-    _mswindows = True
-except ModuleNotFoundError:
-    _mswindows = False
-    import _posixsubprocess
-    import select
-    import selectors
-else:
     from _winapi import (CREATE_NEW_CONSOLE, CREATE_NEW_PROCESS_GROUP,
                          STD_INPUT_HANDLE, STD_OUTPUT_HANDLE,
                          STD_ERROR_HANDLE, SW_HIDE,
@@ -95,6 +90,10 @@
                     "NORMAL_PRIORITY_CLASS", "REALTIME_PRIORITY_CLASS",
                     "CREATE_NO_WINDOW", "DETACHED_PROCESS",
                     "CREATE_DEFAULT_ERROR_MODE", "CREATE_BREAKAWAY_FROM_JOB"])
+else:
+    import _posixsubprocess
+    import select
+    import selectors
 
 
 # Exception classes used by this module.

Misc/NEWS.d/next/Library/2022-01-16-14-07-14.bpo-40280.LtFHfF.rst

@@ -0,0 +1,4 @@
+:mod:`subprocess` now imports Windows-specific imports when
+``sys.platform == "win32"`` and POSIX-specific imports on all other
+platforms. This gives a clean exception when ``_posixsubprocess`` is not
+available (e.g. Emscripten browser target) and it's slightly faster, too.