Description

azchohfi

opened on Nov 6, 2026

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Feature or enhancement

On Windows, python -c pass imports two modules that Linux doesn't, and neither is needed on most starts:

• encodings/__init__.py imports create_win32_code_page_codec from encodings._win_cp_codecs at module level (since 3.14). It is only called from win32_code_page_search_function, for a cpNNN name with no codec module.
• zipimport imports time at module level for time.mktime in _parse_dostime. On Windows, time module init runs _tzset, which calls GetTimeZoneInformation. The first call in a process costs about 1 ms here.

From -X importtime (3.14.4, self time, median of 15): _win_cp_codecs 0.5 ms, time 0.96 ms on Windows (0.07 ms on Linux).

Moving both imports into the functions that use them (patch below) made python -c pass 3.6 ms faster (38.4 -> 34.4 ms; 95% CI 3.1-4.2 ms over 300 interleaved runs, release build of 3.14.4 without PGO). Each change alone is worth about 2 ms (_win_cp_codecs 2.1, time 2.4).

test_codecs, test_zipimport, test_zipimport_support, test_time, test_signal and test_sys pass with the change (3.14.4 build).

Both imports are unchanged on main (2781ffe). The change, against main:

patch

diff --git a/Lib/encodings/__init__.py b/Lib/encodings/__init__.py
index 04594ea..bbccb93 100644
--- a/Lib/encodings/__init__.py
+++ b/Lib/encodings/__init__.py
@@ -157,8 +157,6 @@ def search_function(encoding):
 codecs.register(search_function)
 
 if sys.platform == 'win32':
-    from ._win_cp_codecs import create_win32_code_page_codec
-
     def win32_code_page_search_function(encoding):
         encoding = encoding.lower()
         if not encoding.startswith('cp'):
@@ -173,6 +171,7 @@ if sys.platform == 'win32':
         except (OverflowError, OSError):
             return None
 
+        from ._win_cp_codecs import create_win32_code_page_codec
         return create_win32_code_page_codec(cp)
 
     codecs.register(win32_code_page_search_function)
diff --git a/Lib/zipimport.py b/Lib/zipimport.py
index 09a85f6..aae77df 100644
--- a/Lib/zipimport.py
+++ b/Lib/zipimport.py
@@ -16,7 +16,6 @@ import _frozen_importlib as _bootstrap  # for _verbose_message
 import _imp  # for check_hash_based_pycs
 import _io  # for open
 import marshal  # for loads
-import time  # for mktime
 
 __all__ = ['ZipImportError', 'zipimporter']
 
@@ -767,6 +766,8 @@ def _compile_source(pathname, source, module):
 # Convert the date/time values found in the Zip archive to a value
 # that's compatible with the time stamp stored in .pyc files.
 def _parse_dostime(d, t):
+    # Imported here: importing time at start-up costs a time zone lookup.
+    import time
     return time.mktime((
         (d >> 9) + 1980,    # bits 9..15: year
         (d >> 5) & 0xF,     # bits 5..8: month

I used GitHub Copilot (Claude Opus 5.5 model) to trace this, build and time the prototype, and draft this issue. The measurements were run on my machine.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-161510: Lazily import time and encodings._win_cp_codecs at startup #161732

Activity

1. NekoAsakura

   added

   type-featureA feature request or enhancementA feature request or enhancement

   performancePerformance or resource usagePerformance or resource usage

   OS-windows

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Nov 6, 2026
2. aisk commented on Nov 7, 2026

   

   aisk

   on Nov 7, 2026

   Member

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   Since main now supports lazy imports (PEP 810), using lazy import here should be a simpler and better way to resolve this.
3. azchohfi commented on Nov 9, 2026

   

   azchohfi

   on Nov 9, 2026

   Author

   More actions

   Agreed, this works. I tried it on a build of main (dcf9087) with just two changes:

   -    from ._win_cp_codecs import create_win32_code_page_codec
   +    lazy from ._win_cp_codecs import create_win32_code_page_codec
   
   -import time  # for mktime
   +lazy import time  # for mktime
   

   I verified that:

   • python -c pass no longer imports either module, including with the frozen zipimport.
   • Both modules still load when needed. codecs.lookup('cp20127') loads _win_cp_codecs, and importing from a zip containing a .pyc alongside its source loads time.
   • Over 200 alternating runs of python -c pass, lazy imports saved 1.7 ms, compared to 2.0 ms with the imports moved into the functions. The difference is within measurement noise.
   • All 9 test suites pass: test_codecs, test_zipimport, test_zipimport_support, test_time, test_sys, test_cmd_line, test_lazy_import, test_embed, and test_frozen.

   The improvement on main is smaller than the 3.6 ms I measured on 3.14.4, but both approaches give essentially the same result.

   Thanks for the suggestion! Happy to open a PR with the lazy imports.
4. aisk

   added a parent issue on Nov 9, 2026

   • Improve import time of various stdlib modules #154675
5. aisk commented on Nov 9, 2026

   

   aisk

   on Nov 9, 2026

   Member

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   Actually, time is widely used by other modules, including repl. So the optimization may have limited benefits, except for simple commands like python -c 'print(123 + 456)'.

   But the change is very small, so I personally think we can go ahead with it.
6. aisk

   added

   topic-lazy-imports

   on Nov 9, 2026

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