[New-bugs-announce] [issue32665] pathlib.Path._from_parsed_parts should call cls.__new__(cls)

qb-cea report at bugs.python.org
Thu Jan 25 07:27:25 EST 2018

New submission from qb-cea <quentin.bouget at cea.fr>:


I tried subclassing pathlib.Path and provide it with a new attribute (basically an accessor to an extended attribute). I am rather new to the concept of __slots__ and __new__() but here is how I pictured it should look:

from errno import ENODATA
from os import getxattr, setxattr
from pathlib import Path

class Path_(type(Path())):

    __slots__ = ("new_attr",)

    def __new__(cls, *args, new_attr=None, **kwargs):
        self = super().__new__(cls, *args, **kwargs)
        self._new_attr = new_attr
        return self

    def new_attr(self):
        if self._new_attr:
            return self._new_attr

            new_attr = getxattr(self, "user.new_attr")
        except OSError as exc:
            if exc.errno != ENODATA:
                raise exc
            self._new_attr = new_attr
            return new_attr

        new_attr = b"something_dynamic" # for example uuid4().bytes
        setxattr(self, "user.new_attr", new_attr)
        self._new_attr = new_attr
        return new_attr

The issue I have is that although my class defines its own __new__() method, it is not always called by the methods of pathlib.Path. For example:

>>> Path_("/etc").parent.new_attr
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/reproducer.py", line 19, in new_attr
    if self._new_attr:
AttributeError: _new_attr

The current workaround I use consists in redefining pathlib.Path's _from_parsed_parts() method in my class: instead of creating a new object using:
my implementation uses:

This is the first time I play with the __new__() special method, so it is possible I missed something, if so, sorry for the noise.

components: Library (Lib)
files: reproducer.py
messages: 310671
nosy: pitrou, qb-cea
priority: normal
severity: normal
status: open
title: pathlib.Path._from_parsed_parts should call cls.__new__(cls)
versions: Python 3.6
Added file: https://bugs.python.org/file47409/reproducer.py

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