Remove pathlib flavours

[barneygale]

BPO	44136
Nosy	@pitrou, @barneygale
PRs	bpo-44136: remove pathlib._Flavour #26141 bpo-44412: add os.path.fileuri() function. #26708 bpo-44136: pathlib: merge _Flavour.make_uri() into PurePath.as_uri() #30320 bpo-44136: pathlib: merge _Flavour.is_reserved() into PurePath.is_reserved() #30321 gh-68320, gh-88302 - Allow for pathlib.Path subclassing #31691

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/pitrou'
closed_at = None
created_at = <Date 2021-05-14.18:28:58.393>
labels = ['type-bug', 'library', '3.11']
title = 'Remove pathlib flavours'
updated_at = <Date 2022-03-05.02:18:42.694>
user = 'https://github.com/barneygale'

bugs.python.org fields:

activity = <Date 2022-03-05.02:18:42.694>
actor = 'barneygale'
assignee = 'pitrou'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-05-14.18:28:58.393>
creator = 'barneygale'
dependencies = []
files = []
hgrepos = []
issue_num = 44136
keywords = ['patch']
message_count = 3.0
messages = ['393685', '395759', '413857']
nosy_count = 2.0
nosy_names = ['pitrou', 'barneygale']
pr_nums = ['26141', '26708', '30320', '30321', '31691']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44136'
versions = ['Python 3.11']

[barneygale]

Following bpo-39899, bpo-43757 and bpo-43012, pathlib._Flavour and its subclasses are looking a bit pointless.

The implementations of is_reserved() and make_uri() (~as_uri()) can be readily moved to into PurePosixPath and PureWindowsPath, which removes some indirection. This follows the pattern of OS-specific stuff in PosixPath and WindowsPath.

The remaining methods, such as splitroot(), can be pulled into Pure*Path with an underscore prefix.

I'm generally a believer in composition over inheritance, but in this case _Flavour seems too small and too similar to PurePath to separate out into 3 extra classes.

There should be no impact on public APIs or performance.

[barneygale]

Depends: bpo-39899, bpo-43757, bpo-44316, bpo-44403, bpo-44412

[barneygale]

^ just to bring my previous comment up-to-date:

I'm no longer pursuing adding os.path.isreserved() and os.path.fileuri() functions, or modifying normpath(). At least, not for now!

Instead, my plan is to move flavour functionality as follows (as classmethods with underscore prefixes):

_Flavour --> PurePath
_PosixFlavour --> PurePath
_WindowsFlavour --> PureWindowsPath

As a result, PurePath will use POSIX syntax by default. This is fully backwards-compatible, as users can't create PurePath objects! PurePath.__new__() instantiates PurePosixPath or PureWindowsPath. But it will matter for future user subclasses of PurePath/Path, where we usually want POSIX syntax.

I think there will be three PRs involved. PR 30320 and PR 30321 move _Flavour.make_uri() and _Flavour.is_reserved() respectively; these are reasonably standalone. If/when they land, I'll make a larger PR that moves the remaining methods into PurePath and PureWindowsPath.