GH-128520: Make pathlib._abc.WritablePath a sibling of ReadablePath
#129014
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…blePath` In the private pathlib ABCs, support write-only virtual filesystems by making `WritablePath` inherit directly from `JoinablePath`, rather than subclassing `ReadablePath`. There are two complications: - `ReadablePath.open()` applies to both reading and writing - `ReadablePath.copy` is secretly an object that supports the *read* side of copying, whereas `WritablePath.copy` is a different kind of object supporting the *write* side We untangle these as follow: - A new `pathlib._abc.magic_open()` function replaces the `open()` method, which is dropped from the ABCs but remains in `pathlib.Path`. The function works like `io.open()`, but additionally accepts objects with `__open_rb__()` or `__open_wb__()` methods as appropriate for the mode. These new dunders are made abstract methods of `ReadablePath` and `WritablePath` respectively. If the pathlib ABCs are made public, we could consider blessing an "openable" protocol and supporting it in `io.open()`, removing the need for `pathlib._abc.magic_open()`. - `ReadablePath.copy` becomes a true method, whereas `WritablePath.copy` is deleted. A new `ReadablePath._copy_reader` property provides a `CopyReader` object, and similarly `WritablePath._copy_writer` is a `CopyWriter` object. Once pythonGH-125413 is resolved, we'll be able to move the `CopyReader` functionality into `ReadablePath.info` and eliminate `ReadablePath._copy_reader`.
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In the private pathlib ABCs, support write-only virtual filesystems by making
WritablePathinherit directly fromJoinablePath, rather than subclassingReadablePath. There are two complications:Problem 1:
ReadablePath.open()applies to both reading and writingSolution: a new
pathlib._abc.magic_open()function replaces theopen()method, which is dropped from the ABCs but remains inpathlib.Path. The function works likeio.open(), but additionally accepts objects with__open_rb__()or__open_wb__()methods as appropriate for the mode. These new dunders are made abstract methods ofReadablePathandWritablePathrespectively. If the pathlib ABCs are made public, we could consider blessing an "openable" protocol and supporting it inio.open(), removing the need forpathlib._abc.magic_open().Problem 2:
ReadablePath.copyis secretly an object that supports the read side of copying, whereasWritablePath.copyis a subclass additionally supporting the write sideSolution:
ReadablePath.copybecomes a true method, whereasWritablePath.copyis deleted. A newReadablePath._copy_readerproperty provides aCopyReaderobject, and similarlyWritablePath._copy_writeris aCopyWriterobject. Once GH-125413 is resolved, we'll be able to move theCopyReaderfunctionality intoReadablePath.infoand eliminateReadablePath._copy_reader.