platform: import _wmi on first use

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调研方向

阅读 Lib/platform.py，尤其是模块级的 _wmi 导入和 _wmi_query()；issue 中包含一项拟议更改和测得的行为。检查 test_platform 和 test_wmi 测试，然后确认导入 platform 时不会加载 _wmi，并且所述 Windows 辅助函数仍返回相同的值。

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描述

OS-windows performance stdlib type-feature
Feature or enhancement

Lib/platform.py imports _wmi at module level on Windows. _wmi.pyd loads oleaut32.dll and propsys.dll (with _wmi.pyd itself and their dependencies, 7 more images in the process), but WMI is only queried by win32_ver(), version(), release(), uname() and the processor/machine helpers. Code that only imports platform, or calls python_version() or system(), pays for it anyway. pytest is one example: it imports platform on every run.

Moving the import into _wmi_query() (keeping the "unavailable" state so a failed import or query is not retried):

60 runs each, perf_counter around the import before after difference
import platform 18.6 ms 12.9 ms −5.5 ms [−6.0, −5.1]
import pytest 206 ms 196 ms −9.5 ms [−13.6, −4.7]

(python-build-standalone 3.14.4, Windows 11 26100.) win32_ver(), uname(), machine(), version() and processor() return the same values; test_platform and test_wmi pass.

The change, against main (2781ffe1f2):

diff --git a/Lib/platform.py b/Lib/platform.py
index c87bf2f..375acc2 100644
--- a/Lib/platform.py
+++ b/Lib/platform.py
@@ -114,10 +114,10 @@ import sys
 import functools
 import itertools
 lazy import warnings
-try:
-    import _wmi
-except ImportError:
-    _wmi = None
+# _wmi is imported on first use (see _wmi_query): loading it maps the OLE
+# and property-system DLLs, which most users of this module never need.
+# None means not imported yet, False means unavailable.
+_wmi = None
 
 ### Globals & Constants
 
@@ -332,6 +332,12 @@ def _syscmd_ver(system='', release='', version='',
 
 def _wmi_query(table, *keys):
     global _wmi
+    if _wmi is None:
+        try:
+            import _wmi as wmi
+        except ImportError:
+            wmi = False
+        _wmi = wmi
     if not _wmi:
         raise OSError("not supported")
     table = {
@@ -344,7 +350,7 @@ def _wmi_query(table, *keys):
             table,
         )).split("\0")
     except OSError:
-        _wmi = None
+        _wmi = False
         raise OSError("not supported")
     split_data = (i.partition("=") for i in data)
     dict_data = {i[0]: i[2] for i in split_data}

(PEP 810's lazy import doesn't fit here because the import sits in try/except ImportError.)

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

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