gh-155254: Avoid warning about missing stdlib when using _pth files (GH-156015)

Author: zooba

Doc/library/sys_path_init.rst

@@ -105,6 +105,8 @@ Please refer to :mod:`site`'s
    mechanism, such as :mod:`venv`, that many virtual environment implementations
    follow.
 
+.. _sys-path-init-_pth-files:
+
 _pth files
 ----------
 

Misc/NEWS.d/next/Core_and_Builtins/2026-08-18-14-19-51.gh-issue-155254.75TMlb.rst

@@ -0,0 +1 @@
+Avoid warning about missing standard library when using :ref:`sys-path-init-_pth-files`.

Modules/getpath.py

@@ -766,10 +766,10 @@ def search_up(prefix, *landmarks, test=isfile):
 # SANITY CHECKS
 # ******************************************************************************
 
-# Warn if the standard library is missing, unless pythonpath_was_set was set, as
-# that skips parts of the stdlib directories calculation — assume the provided
-# pythonpath is correct. This is how subinterpreters initialize the path for eg.
-if not py_setpath and not pythonpath_was_set:
+# Warn if we did a search for the standard library and couldn't find it. Never
+# show a warning if paths were provided explicitly, since we trust the caller
+# knows what they're doing even if the layout doesn't look "normal".
+if not (py_setpath or pythonpath_was_set or pth):
     home_hint = f"The Python 'home' directory was set to {home!r}, is this correct?"
     if (not stdlib_zip or not isfile(stdlib_zip)) and (not stdlib_dir or not isdir(stdlib_dir)):
         hint = home_hint if home else f'sys.prefix is set to {prefix}, is this correct?'