gh-100320: Fix path calculations on Windows when python.exe is moved outside of the normal location (GH-100947)

1 files changed, 30 insertions, 18 deletions

diff --git a/Modules/getpath.py b/Modules/getpath.py
index fc533a8..e847141 100644
--- a/Modules/getpath.py
+++ b/Modules/getpath.py
@@ -597,25 +597,27 @@ else:
 
     # Detect exec_prefix by searching from executable for the platstdlib_dir
     if PLATSTDLIB_LANDMARK and not exec_prefix:
-        if executable_dir:
-            if os_name == 'nt':
-                # QUIRK: For compatibility and security, do not search for DLLs
-                # directory. The fallback below will cover it
-                exec_prefix = executable_dir
-            else:
-                exec_prefix = search_up(executable_dir, PLATSTDLIB_LANDMARK, test=isdir)
+        if os_name == 'nt':
+            # QUIRK: Windows always assumed these were the same
+            # gh-100320: Our PYDs are assumed to be relative to the Lib directory
+            # (that is, prefix) rather than the executable (that is, executable_dir)
+            exec_prefix = prefix
+        if not exec_prefix and executable_dir:
+            exec_prefix = search_up(executable_dir, PLATSTDLIB_LANDMARK, test=isdir)
         if not exec_prefix and EXEC_PREFIX:
             exec_prefix = EXEC_PREFIX
         if not exec_prefix or not isdir(joinpath(exec_prefix, PLATSTDLIB_LANDMARK)):
             if os_name == 'nt':
                 # QUIRK: If DLLs is missing on Windows, don't warn, just assume
-                # that it's all the same as prefix.
-                # gh-98790: We set platstdlib_dir here to avoid adding "DLLs" into
-                # sys.path when it doesn't exist, which would give site-packages
-                # precedence over executable_dir, which is *probably* where our PYDs
-                # live. Ideally, whoever changes our layout will tell us what the
-                # layout is, but in the past this worked, so it should keep working.
-                platstdlib_dir = exec_prefix = prefix
+                # that they're in exec_prefix
+                if not platstdlib_dir:
+                    # gh-98790: We set platstdlib_dir here to avoid adding "DLLs" into
+                    # sys.path when it doesn't exist in the platstdlib place, which
+                    # would give Lib packages precedence over executable_dir where our
+                    # PYDs *probably* live. Ideally, whoever changes our layout will tell
+                    # us what the layout is, but in the past this worked, so it should
+                    # keep working.
+                    platstdlib_dir = exec_prefix
             else:
                 warn('Could not find platform dependent libraries <exec_prefix>')
 
@@ -701,8 +703,14 @@ elif not pythonpath_was_set:
                         except OSError:
                             break
                         if isinstance(v, str):
-                            pythonpath.append(v)
+                            pythonpath.extend(v.split(DELIM))
                         i += 1
+                    # Paths from the core key get appended last, but only
+                    # when home was not set and we aren't in a build dir
+                    if not home_was_set and not venv_prefix and not build_prefix:
+                        v = winreg.QueryValue(key, None)
+                        if isinstance(v, str):
+                            pythonpath.extend(v.split(DELIM))
                 finally:
                     winreg.CloseKey(key)
             except OSError:
@@ -714,13 +722,17 @@ elif not pythonpath_was_set:
             pythonpath.append(joinpath(prefix, p))
 
     # Then add stdlib_dir and platstdlib_dir
-    if os_name == 'nt' and venv_prefix:
-        # QUIRK: Windows generates paths differently in a venv
+    if os_name == 'nt':
+        # QUIRK: Windows generates paths differently
         if platstdlib_dir:
             pythonpath.append(platstdlib_dir)
         if stdlib_dir:
             pythonpath.append(stdlib_dir)
-        if executable_dir not in pythonpath:
+        if executable_dir and executable_dir not in pythonpath:
+            # QUIRK: the executable directory is on sys.path
+            # We keep it low priority, so that properly installed modules are
+            # found first. It may be earlier in the order if we found some
+            # reason to put it there.
             pythonpath.append(executable_dir)
     else:
         if stdlib_dir: