Eryk Sun added the comment:
For COM[n] and LPT[n], only ASCII 1-9 and superscript 1-3 (U+00b9, U+00b2, and
U+00b3) are handled as decimal digits. For example:
>>> print(*(ascii(chr(c)) for c in range(1, 65536)
... if _getfullpathname('COM%s' % chr(c))[0] == '\\'), sep=', ')
'1', '2', '3', '4', '5', '6', '7', '8', '9', '\xb2', '\xb3', '\xb9'
The implementation uses iswdigit in ntdll.dll. (ntdll.dll is the system DLL
that has the user-mode runtime library and syscall stubs -- except the Win32k
syscall stubs are in win32u.dll.) ntdll's private CRT uses the C locale
(Latin-1, not just ASCII), and it classifies these superscript digits as
decimal digits:
>>> ntdll = ctypes.WinDLL('ntdll')
>>> print(*(chr(c) for c in range(1, 65536) if ntdll.iswdigit(c)))
0 1 2 3 4 5 6 7 8 9 ² ³ ¹
Unicode, and thus Python, does not classify these superscript digits as decimal
digits, so I just hard-coded the list.
Here's an example with an attached debugger to show the runtime library calling
iswdigit:
>>> name = 'COM\u2074'
>>> _getfullpathname(name)
Breakpoint 0 hit
ntdll!iswdigit:
00007ffe`9ad89d90 ba04000000 mov edx,4
0:000> kc 6
Call Site
ntdll!iswdigit
ntdll!RtlpIsDosDeviceName_Ustr
ntdll!RtlGetFullPathName_Ustr
ntdll!RtlGetFullPathName_UEx
KERNELBASE!GetFullPathNameW
python36_d!os__getfullpathname_impl
The argument is in register rcx:
0:000> r rcx
rcx=0000000000002074
Skip to the ret instruction, and check the result in register rax:
0:000> pt
ntdll!iswctype+0x20:
00007ffe`9ad89e40 c3 ret
0:000> r rax
rax=0000000000000000
0:000> g
Since U+2074 isn't considered a decimal digit, 'COM⁴' is not a reserved DOS
device name. The system handles it as a regular filename:
'C:\\Temp\\COM⁴'
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