gh-154904: Speed up import of shutil by probing compression extensions

shutil imported bz2, lzma and compression.zstd at module scope only to set
the _*_SUPPORTED flags, then discarded the bindings.  Executing those pure
Python wrappers also pulled in compression._common[._streams] and
compression.zstd._zstdfile, a cost paid by every program that imports
shutil even if it never touches an archive.

Their only importable dependency that may be missing is the extension
module each one wraps, so probe _bz2, _lzma and _zstd directly instead.
This keeps the exact ImportError semantics of the previous code -- unlike
a find_spec() check, which resolves the always-present wrapper and would
report success on builds where the extension fails to load.  The wrappers
are still imported by tarfile/zipfile when an archive is actually created
or extracted.

Cuts the net cost of "import shutil" by ~2.5 ms (-19.6%) and of
"import urllib.request" by ~2.3 ms (-6.4%).

Author: maxday

Lib/shutil.py

@@ -18,23 +18,28 @@
 except ImportError:
     _ZLIB_SUPPORTED = False
 
+# bz2, lzma and compression.zstd are pure Python wrappers whose only
+# importable dependency that may be missing is the extension module they
+# wrap.  Probe those extensions directly instead: it gives the same answer
+# without executing the wrappers, which shutil only needs when an archive
+# is actually created or extracted.
 try:
-    import bz2
-    del bz2
+    import _bz2
+    del _bz2
     _BZ2_SUPPORTED = True
 except ImportError:
     _BZ2_SUPPORTED = False
 
 try:
-    import lzma
-    del lzma
+    import _lzma
+    del _lzma
     _LZMA_SUPPORTED = True
 except ImportError:
     _LZMA_SUPPORTED = False
 
 try:
-    from compression import zstd
-    del zstd
+    import _zstd
+    del _zstd
     _ZSTD_SUPPORTED = True
 except ImportError:
     _ZSTD_SUPPORTED = False

Lib/test/test_shutil.py

@@ -10,6 +10,7 @@
 import os.path
 import errno
 import functools
+import importlib
 import socket
 import subprocess
 import random
@@ -32,6 +33,7 @@
 from test import support
 from test.support import os_helper, socket_helper
 from test.support.os_helper import TESTFN, FakePath
+from test.support.script_helper import assert_python_ok
 
 TESTFN2 = TESTFN + "2"
 TESTFN_SRC = TESTFN + "_SRC"
@@ -2321,6 +2323,40 @@ def _boo(filename, extract_dir, extra):
         unregister_unpack_format('Boo2')
         self.assertEqual(get_unpack_formats(), formats)
 
+    def test_compression_supported_flags(self):
+        # shutil determines compression support by probing the extension
+        # modules (_bz2, _lzma, _zstd) rather than importing the pure Python
+        # wrappers.  The answer must match what importing the wrapper does,
+        # including on builds where the extension is missing or fails to load.
+        for wrapper, supported in (
+            ('zlib', shutil._ZLIB_SUPPORTED),
+            ('bz2', shutil._BZ2_SUPPORTED),
+            ('lzma', shutil._LZMA_SUPPORTED),
+            ('compression.zstd', shutil._ZSTD_SUPPORTED),
+        ):
+            with self.subTest(wrapper=wrapper):
+                try:
+                    importlib.import_module(wrapper)
+                except ImportError:
+                    importable = False
+                else:
+                    importable = True
+                self.assertEqual(supported, importable)
+
+    def test_compression_wrappers_not_imported_by_shutil(self):
+        # Importing shutil must not pull in the compression wrappers: they are
+        # only needed once an archive is actually created or extracted, and
+        # importing them measurably slows down every process that uses shutil.
+        wrappers = ('bz2', 'lzma', 'compression', 'compression.zstd')
+        script = (
+            'import sys, shutil; '
+            f'print([m for m in {wrappers!r} if m in sys.modules])'
+        )
+        # -I so that a sitecustomize/usercustomize importing one of these
+        # cannot make the test fail spuriously.
+        rc, stdout, stderr = assert_python_ok('-I', '-c', script)
+        self.assertEqual(stdout.decode().strip(), '[]', stderr)
+
 
 class TestMisc(BaseTest, unittest.TestCase):
 

Misc/NEWS.d/next/Library/2026-07-30-02-15-00.gh-issue-154904.Kx9mAq.rst

@@ -0,0 +1,4 @@
+Speed up :mod:`shutil` import by probing the ``_bz2``, ``_lzma`` and
+``_zstd`` extension modules instead of importing the :mod:`bz2`,
+:mod:`lzma` and :mod:`compression.zstd` wrappers, which are now only
+imported when an archive is actually created or extracted.