René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Support for sharing state between pathlib subclasses","articleBody":"# Feature or enhancement\r\n\r\nThis enhancement proposes that we allow state to be shared between related instances of subclasses of `pathlib.PurePath` and `pathlib.Path`.\r\n\r\n# Pitch\r\n\r\nNow that #68320 is resolved, users can subclass `pathlib.PurePath` and `pathlib.Path` directly:\r\n\r\n```python\r\nimport pathlib\r\n\r\nclass MyPath(pathlib.Path):\r\n    def my_custom_method(self):\r\n        pass\r\n\r\netc = MyPath('/etc')\r\netc_hosts = etc / 'hosts'\r\netc_hosts.my_custom_method()\r\n```\r\n\r\nHowever, some user implementations of classes - such as `TarPath` or `S3Path` - would require underlying `tarfile.TarFile` or `botocore.Resource` objects to be shared between path objects (*etc* and *etc_hosts* in the example above). Such sharing of resources is presently rather difficult, as there's no single instance method used to derive new path objects.\r\n\r\nThis feature request proposes that we add a new `PurePath.makepath()` method, which is called whenever one path object is derived from another, such as in `joinpath()`, `iterdir()`, etc. The default implementation of this method looks something like:\r\n\r\n```python\r\ndef makepath(self, *args):\r\n    return type(self)(*args)\r\n```\r\n\r\nUsers may redefine this method in a subclass, in conjunction with a customized initializer:\r\n\r\n```python\r\nclass SessionPath(pathlib.Path):\r\n    def __init__(self, *args, session_id):\r\n        super().__init__(*args)\r\n        self.session_id = session_id\r\n\r\n    def makepath(self, *args):\r\n        return type(self)(*args, session_id=self.session_id)\r\n\r\netc = SessionPath('/etc', session_id=42)\r\netc_hosts = etc / 'hosts'\r\nprint(etc_hosts.session_id)  # 42\r\n```\r\n\r\nI propose the name \"makepath\" for this method due to its close relationship with the existing \"joinpath\" method: `a.joinpath(b) == a.makepath(a, b)`.\r\n\r\n# Performance\r\n\r\nedit: this change has been taken care of elsewhere, and so implementing this feature request should no longer have much affect on performance.\r\n\r\n~~This change will affect the performance of some pathlib operations, because it requires us to remove the `_from_parsed_parts()` constructor, which is an internal optimization used in cases where path parsing and normalization can be skipped (for example, in the `parents` sequence). I suggest that, within the standard library, pathlib is not a particularly performance-sensitive module -- few folks reach to pathlib for reason of speed. Within pathlib itself, the savings from optimizing these \"pure\" methods are usually drowned out by the I/O costs of \"impure\" methods. With the appeal of this feature in mind, I believe the performance cost is justified.~~\r\n\r\n~~However, if the performance degradation is considered unacceptable, there's a possible  alternative: add a *normalize* keyword argument to the path initializer and to `makepath()`.  This would require some serious internal surgery to make work, and might be difficult to communicate to users, so at this stage it's not my preferred route forward.~~\r\n\r\n# Previous discussion\r\n\r\nhttps://discuss.python.org/t/make-pathlib-extensible/3428/47 (and replies)\r\n\r\n\u003c!-- gh-linked-prs --\u003e\r\n### Linked PRs\r\n* gh-100481\r\n* gh-103975\n* gh-104215\n\u003c!-- /gh-linked-prs --\u003e\r\n","author":{"url":"https://github.com/barneygale","@type":"Person","name":"barneygale"},"datePublished":"2022-12-23T23:07:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":10},"url":"https://github.com/100479/cpython/issues/100479"}