Import fewer modules in igzip and igzip_threaded

CHANGELOG.rst

@@ -10,6 +10,9 @@ Changelog
 
 version 1.8.1-dev
 -----------------
++ Speed up ``import isal.igzip`` and ``import isal.igzip_threaded`` by
+  importing ``argparse`` and ``shutil`` only when the command line interface
+  runs, and by dropping the ``multiprocessing`` import.
 + Updated ISA-L to 2.32.1.
 + Python 3.10 and PyPy 3.10 are no longer supported. Added support for PyPy
   3.12 and Python 3.15.

src/isal/igzip.py

@@ -24,12 +24,10 @@
 """Similar to the stdlib gzip module. But using the Intel Storage Accelaration
 Library to speed up its methods."""
 
-import argparse
 import builtins
 import gzip
 import io
 import os
-import shutil
 import struct
 import sys
 import time
@@ -255,6 +253,7 @@ def decompress(data):
 
 
 def _argument_parser():
+    import argparse  # Only the command line interface needs it
     parser = argparse.ArgumentParser()
     parser.description = (
         "A simple command line interface for the igzip module. "
@@ -302,6 +301,7 @@ def _argument_parser():
 
 
 def main():
+    import shutil  # Only the command line interface needs it
     args = _argument_parser().parse_args()
 
     compresslevel = args.compresslevel or _COMPRESS_LEVEL_TRADEOFF

src/isal/igzip_threaded.py

@@ -7,7 +7,6 @@
 
 import builtins
 import io
-import multiprocessing
 import os
 import queue
 import struct
@@ -52,10 +51,7 @@ def open(filename, mode="rb", compresslevel=igzip._COMPRESS_LEVEL_TRADEOFF,
         try:
             threads = len(os.sched_getaffinity(0))
         except:  # noqa: E722
-            try:
-                threads = multiprocessing.cpu_count()
-            except:  # noqa: E722
-                threads = 1
+            threads = os.cpu_count() or 1
     if "r" in mode:
         gzip_file = io.BufferedReader(
             _ThreadedGzipReader(filename, block_size=block_size))

tests/test_igzip.py

@@ -15,6 +15,7 @@
 import re
 import shutil
 import struct
+import subprocess
 import sys
 import tempfile
 import zlib
@@ -261,6 +262,18 @@ def test_compress_infile_out_file_no_name(tmp_path, capsysbinary):
     assert output[4:8] == b"\x00\x00\x00\x00"  # No timestamp set.
 
 
+def test_import_does_not_load_cli_modules():
+    # Each of these modules takes milliseconds to import
+    code = ("import sys; before = set(sys.modules); "
+            "import isal.igzip, isal.igzip_threaded; "
+            "print(*(set(sys.modules) - before))")
+    result = subprocess.run([sys.executable, "-c", code], check=True,
+                            capture_output=True, text=True)
+    new_modules = set(result.stdout.split())
+    assert "isal.igzip_threaded" in new_modules
+    assert not new_modules & {"argparse", "shutil", "multiprocessing"}
+
+
 def test_decompress():
     assert igzip.decompress(COMPRESSED_DATA) == DATA