gh-75723: Idempotent .pth execution in site.addsitedir

Lib/site.py

@@ -390,36 +390,36 @@ def addsitedir(sitedir, known_paths=None, *, defer_processing_start_files=False)
  if not sitedircase in known_paths:
  sys.path.append(sitedir) # Add path component
  known_paths.add(sitedircase)
- try:
- names = os.listdir(sitedir)
- except OSError:
- return
+try:
+names = os.listdir(sitedir)
+except OSError:
+return
 
- # The following phases are defined by PEP 829.
- # Phases 1-3: Read .pth files, accumulating paths and import lines.
- pth_names = sorted(
- name for name in names
- if name.endswith(".pth") and not name.startswith(".")
- )
- for name in pth_names:
- _read_pth_file(sitedir, name, known_paths)
-
- # Phases 6-7: Discover .start files and accumulate their entry points.
- # Import lines from .pth files with a matching .start file are discarded
- # at flush time by _exec_imports().
- start_names = sorted(
- name for name in names
- if name.endswith(".start") and not name.startswith(".")
- )
- for name in start_names:
- _read_start_file(sitedir, name)
-
- # Generally, when addsitedir() is called explicitly, we'll want to process
- # all the startup file data immediately. However, when called through
- # main(), we'll want to batch up all the startup file processing. main()
- # will set this flag to True to defer processing.
- if not defer_processing_start_files:
- process_startup_files()
+# The following phases are defined by PEP 829.
+# Phases 1-3: Read .pth files, accumulating paths and import lines.
+pth_names = sorted(
+name for name in names
+if name.endswith(".pth") and not name.startswith(".")
+)
+for name in pth_names:
+_read_pth_file(sitedir, name, known_paths)
+
+# Phases 6-7: Discover .start files and accumulate their entry points.
+# Import lines from .pth files with a matching .start file are discarded
+# at flush time by _exec_imports().
+start_names = sorted(
+name for name in names
+if name.endswith(".start") and not name.startswith(".")
+)
+for name in start_names:
+_read_start_file(sitedir, name)
+
+# Generally, when addsitedir() is called explicitly, we'll want to process
+# all the startup file data immediately. However, when called through
+# main(), we'll want to batch up all the startup file processing. main()
+# will set this flag to True to defer processing.
+if not defer_processing_start_files:
+process_startup_files()
 
  if reset:
  known_paths = None

Lib/test/test_site.py

@@ -118,6 +118,7 @@ def pth_file_tests(self, pth_file):
  "%s not in sys.modules" % pth_file.imported)
  self.assertIn(site.makepath(pth_file.good_dir_path)[0], sys.path)
  self.assertFalse(os.path.exists(pth_file.bad_dir_path))
+ self.assertFalse(os.path.exists(pth_file.idempotent_fail_path))
 
  def test_addpackage(self):
  # Make sure addpackage() imports if the line starts with 'import',
@@ -202,6 +203,16 @@ def test_addsitedir_explicit_flush(self):
  site.process_startup_files()
  self.pth_file_tests(pth_file)
 
+ def test_addsitedir_idempotent(self):
+ pth_file = PthFile()
+ pth_file.cleanup(prep=True)
+
+ with pth_file.create():
+ dirs = set()
+ dirs = site.addsitedir(pth_file.base_dir, dirs)
+ dirs = site.addsitedir(pth_file.base_dir, dirs)
+ self.pth_file_tests(pth_file)
+
  def test_addsitedir_dotfile(self):
  pth_file = PthFile('.dotfile')
  # Ensure we have a clean slate.
@@ -411,6 +422,7 @@ def __init__(self, filename_base=TESTFN, imported="time",
  self.bad_dirname = bad_dirname
  self.good_dir_path = os.path.join(self.base_dir, self.good_dirname)
  self.bad_dir_path = os.path.join(self.base_dir, self.bad_dirname)
+ self.idempotent_fail_path = os.path.join(self.base_dir, 'idempotent')
 
  @contextlib.contextmanager
  def create(self):
@@ -427,6 +439,13 @@ def create(self):
  try:
  print("#import @bad module name", file=FILE)
  print("\n", file=FILE)
+
+ PROG = f'''\
+if {self.imported!r} in sys.modules:
+ open({self.idempotent_fail_path!r}, 'a+').close()
+'''
+ print(f"import sys; exec({PROG!r})", file=FILE)
+
  print("import %s" % self.imported, file=FILE)
  print(self.good_dirname, file=FILE)
  print(self.bad_dirname, file=FILE)
@@ -455,6 +474,8 @@ def cleanup(self, prep=False):
  os.rmdir(self.good_dir_path)
  if os.path.exists(self.bad_dir_path):
  os.rmdir(self.bad_dir_path)
+ if os.path.exists(self.idempotent_fail_path):
+ os.remove(self.idempotent_fail_path)
 
 class ImportSideEffectTests(unittest.TestCase):
  """Test side-effects from importing 'site'."""

Misc/NEWS.d/next/Core_and_Builtins/2026-03-31-16-15-15.gh-issue-75723.BZ4Rsn.rst

@@ -0,0 +1 @@
+Avoid re-executing ``.pth`` files when :func:`site.addsitedir` is called for a known directory.