pathlib._WindowsFlavour.is_reserved assumes Windows uses an exact match up to the file extension for reserved DOS device names. However, this misses cases involving trailing spaces and colons, such as the following examples:

Trailing colon:

    >>> pathlib.Path('C:/foo/NUL:').is_reserved()
    False
    >>> print(os.path._getfullpathname('C:/foo/NUL:'))
    \\.\NUL

Trailing spaces:

    >>> pathlib.Path('C:/foo/NUL  ').is_reserved()
    False
    >>> print(os.path._getfullpathname('C:/foo/NUL  '))
    \\.\NUL

Trailing spaces followed by a file extension:

    >>> pathlib.Path('C:/foo/NUL  .txt').is_reserved()
    False
    >>> print(os.path._getfullpathname('C:/foo/NUL  .txt'))
    \\.\NUL

Windows calls RtlIsDosDeviceName_Ustr to check whether a path represents a DOS device name. Here's a link to the reverse-engineered implementation of this function in ReactOS 4.1:

http://code.reactos.org/browse/reactos/branches/ros-branch-0_4_1/reactos/sdk/lib/rtl/path.c?r=71210#to85

The ReactOS implementation performs the following steps:

    * Return false for a UNC or unknown path type or an empty path.
    * Strip a final ":" if present. Return false if it was the
      only character.
    * Strip trailing dot and space characters.
    * Iterate over the path in reverse. If the current character is
      a "\\" or "/" or a ":" drive letter separator (at index 1),
      then if the next character matches the first letter of a DOS
      device name, splice out the base name as a potential match.
      Else return false.
    * Return false if the first character at this point does not 
      match the first letter of a DOS device name.
    * Remove the file extension, starting at the first dot or colon.
    * Remove trailing spaces.
    * Return the name offset and length if it equals
        * "COM" or "LPT" plus a digit
        * "PRN", "AUX", "NUL, or "CON"
    * Else return false.

It seems that ":" and "." are effectively equivalent for the purposes of is_reserved. Given this is the case, it could return whether parts[-1].partition('.')[0].partition(':')[0].rstrip(' ').upper() is in self.reserved_names. Or maybe use a regex for the entire check.

If a script is running on Windows, I think the best approach is to call os.path.abspath, which calls _getfullpathname. This lets Windows itself determine if the path maps to the \\.\ device namespace. However, I realize that is_reserved is intended to be cross-platform.

By the way, the comment for this method says that r"foo\NUL" isn't reserved, but it is. Maybe the author checked by trying to open NUL in a non-existing foo directory. DOS device names are only reserved in practice when opening and creating files in existing directories (as opposed to reserved in principle with GetFullPathName, which doesn't check for a valid path). NT can thus return an error that's consistent with how DOS behaved in the 1980s -- because that's really important, you know.

Also, "CONIN$" and "CONOUT$" need to be added to the list of reserved names. Prior to Windows 8 these two names are reserved only for the current directory, which for the most part also applies to "CON". 

For Windows 8+, the redesign to use a real console device means that these three console devices are handled in exactly the same way as the other reserved DOS device names. For example:

Windows 10

    >>> print(os.path.abspath('C:/Temp/conout$  : spam . eggs'))
    \\.\conout$

Windows 7

    >>> print(os.path.abspath('C:/Temp/conout$  : spam . eggs'))
    C:\Temp\conout$  : spam . eggs

The attached patch adds tests and the suggested enhancement to _WindowsFlavour.is_reserved. 

Shouldn't it also return True if the name contains ASCII control characters? They're only valid in NTFS stream names. Also, I think a name containing a colon that's not part of a DOS drive letter spec should be considered reserved. Otherwise it could designate an NTFS named stream (e.g. "path\filename:streamname:$DATA"), which is rarely desired and not universally supported, e.g. FAT32 doesn't support file streams. I'm thinking of a program that calls this method to ensure that a path is reasonably 'safe' for use on Windows -- i.e. isn't inherently invalid and won't do something surprising like open NUL or write to a named stream.

Are 'COM\u0661' or 'COM\u2074' reserved names?

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'

Thanks for the estimation Eryk. Can you create a pull request for your patch?

See also bpo-36534 "tarfile: handling Windows (path) illegal characters in archive member names".

See also bpo-37517 "Improve error messages for Windows reserved file names".

I've put Eryk's patch up as a PR: https://github.com/python/cpython/pull/26698

New changeset 56c1f6d7edad454f382d3ecb8cdcff24ac898a50 by Barney Gale in branch 'main':
bpo-27827: identify a greater range of reserved filename on Windows. (GH-26698)
https://github.com/python/cpython/commit/56c1f6d7edad454f382d3ecb8cdcff24ac898a50

New changeset 8789add99164177f29a8cd319a834187c65ab16c by Miss Islington (bot) in branch '3.10':
bpo-27827: identify a greater range of reserved filename on Windows. (GH-26698) (GH-27421)
https://github.com/python/cpython/commit/8789add99164177f29a8cd319a834187c65ab16c

New changeset debb751f11f5221eafcdf07a14c7e9408350ad9a by Miss Islington (bot) in branch '3.9':
bpo-27827: identify a greater range of reserved filename on Windows. (GH-26698) (#27422)
https://github.com/python/cpython/commit/debb751f11f5221eafcdf07a14c7e9408350ad9a

Barney, thanks for pushing this across the finish line! ✨ 🍰 ✨  

And of course, Eryk for the report and original patch.