Optimize `pathlib.Path.glob()` by avoiding repeated calls to `os.path.normcase()`

[barneygale]

As part of removing "flavour" classes in #31691, I changed pathlib's glob() implementation: previously it used re.IGNORECASE to implement case-insensitive matches, whereas after it called os.path.normcase() on the pattern and the paths. The new behaviour is a little slower, and I think we should restore the previous implementation.

Linked PRs

• GH-104104: Optimize pathlib.Path.glob() by avoiding repeated calls to os.path.normcase() #104105

[eryksun]

Using re.IGNORECASE is wrong on Windows. Correct case-insensitive filename comparisons on Windows have to use os.path.normcase(). The implementation calls WinAPI LCMapStringEx() with the invariant locale to get a simple lowercase conversion of characters in the basic multilingual plane (BMP). This case mapping is always a one-to-one character mapping, and no mapping is implemented for characters beyond the BMP. This basically matches how filesystems on Windows implement case-insensitive name comparisons, except they use an uppercase conversion, which doesn't matter for checking equality. (It matters a bit for the sort order of filenames.)

For example, Python's lowercase mapping of "İ" (U+0130) is a two-character string:

>>> print(ascii('İ'.lower()))
'i\u0307'

To a Windows filesystem, "İ" and "i\u0307" are different filenames.

>>> open('i\u0307', 'w').close()
>>> open('\u0130', 'w').close()
>>> names = os.listdir()
>>> names
['i̇', 'İ']
>>> os.path.normcase(names[0])
'i̇'
>>> os.path.normcase(names[1])
'İ'
>>> os.path.normcase(names[0]) == os.path.normcase(names[1])
False

Using re.IGNORECASE yields a false positive match:

>>> re.match(names[1], names[0], re.IGNORECASE)
<re.Match object; span=(0, 1), match='i'>

[barneygale]

That bug existed in all previous versions of pathlib; I didn't intentionally fix it IIRC. Personally I consider it pretty minor. I'm hoping to add a case_sensitive argument to glob() in #102710, and so I'd rather keep the case normalization rules reasonably generic.

[eryksun]

Okay, I didn't review the code in detail. I just wanted you to be aware that Windows paths should not be case-insensitively compared for equality using Python's lowercase mapping. The system's locale-invariant, non-linguistic case mapping has to be used when comparing paths.

[eryksun]

This case mapping is always a one-to-one character mapping, and no mapping is implemented for characters beyond the BMP.

Of course when I actually checked this just now I discovered a bug. When os.path.normcase() was modified to use WinAPI LCMapStringEx() instead of str.lower(), I only verified that it worked correctly for characters in the BMP and that using str.lower() was wrong for 200 characters in the BMP. Unfortunately the documented behavior of LCMapStringEx() is wrong for non-BMP characters. It's supposed to default to "file system" rules, for which the surrogate pair representation of a non-BMP character should be handled as two UCS-2 ordinals. But LCMapStringEx() actually implements a case mapping for 40 non-BMP characters:

>>> os.listdir()
[]
>>> chars = [c for i in range(65536, sys.maxunicode) if normcase(c:=chr(i)) != c]
>>> len(chars)
40
>>> [(c, normcase(c)) for c in chars]
[('𐐀', '𐐨'), ('𐐁', '𐐩'), ('𐐂', '𐐪'), ('𐐃', '𐐫'), ('𐐄', '𐐬'), ('𐐅', '𐐭'), ('𐐆', '𐐮'), ('𐐇', '𐐯'), ('𐐈',
 '𐐰'), ('𐐉', '𐐱'), ('𐐊', '𐐲'), ('𐐋', '𐐳'), ('𐐌', '𐐴'), ('𐐍', '𐐵'), ('𐐎', '𐐶'), ('𐐏', '𐐷'), ('𐐐', '𐐸'),
 ('𐐑', '𐐹'), ('𐐒', '𐐺'), ('𐐓', '𐐻'), ('𐐔', '𐐼'), ('𐐕', '𐐽'), ('𐐖', '𐐾'), ('𐐗', '𐐿'), ('𐐘', '𐑀'), ('𐐙',
 '𐑁'), ('𐐚', '𐑂'), ('𐐛', '𐑃'), ('𐐜', '𐑄'), ('𐐝', '𐑅'), ('𐐞', '𐑆'), ('𐐟', '𐑇'), ('𐐠', '𐑈'), ('𐐡', '𐑉'),
 ('𐐢', '𐑊'), ('𐐣', '𐑋'), ('𐐤', '𐑌'), ('𐐥', '𐑍'), ('𐐦', '𐑎'), ('𐐧', '𐑏')]

To the filesystem, each of these normalized names is unique:

>>> len(os.listdir())
0
>>> for c in chars: open(c, 'w').close(); open(normcase(c), 'w').close()
...
>>> len(os.listdir())
80

I don't know how to efficiently resolve this bug, short of calling a lower-level NTAPI function instead of LCMapStringEx(). I think we need a new path comparison function in os.path. On Windows, it can call CompareStringOrdinal() and default to a case-insensitive comparison. CompareStringOrdinal() uses the same uppercase mapping as the filesystem.