Clash Royale CLAN TAG#URR8PPP

Subclass `pathlib.Path` fails

I would like to enhance the class pathlib.Path but the simple example above dose not work.

pathlib.Path

from pathlib import Path

class PPath(Path):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)

test = PPath("dir", "test.txt")


Here is the error message I have.

Traceback (most recent call last):
File "/Users/projetmbc/test.py", line 14, in <module>
test = PPath("dir", "test.txt")
File "/anaconda/lib/python3.4/pathlib.py", line 907, in __new__
self = cls._from_parts(args, init=False)
File "/anaconda/lib/python3.4/pathlib.py", line 589, in _from_parts
drv, root, parts = self._parse_args(args)
File "/anaconda/lib/python3.4/pathlib.py", line 582, in _parse_args
return cls._flavour.parse_parts(parts)
AttributeError: type object 'PPath' has no attribute '_flavour'


What I am doing wrong ?

5 Answers
5

You can subclass the concrete implementation, so this works:

class Path(type(pathlib.Path())):


Here's what I did with this:

import pathlib

class Path(type(pathlib.Path())):
def open(self, mode='r', buffering=-1, encoding=None, errors=None, newline=None):
if encoding is None and 'b' not in mode:
encoding = 'utf-8'
return super().open(mode, buffering, encoding, errors, newline)

Path('/tmp/a.txt').write_text("я")


Here is the definition of the Path class. It does something rather clever. Rather than directly returning an instance of Path from its __new__(), it returns an instance of a subclass, but only if it's been invoked directly as Path() (and not as a subclass).

Path

Path

__new__()

Path()

Otherwise, it expects to have been invoked via either WindowsPath() or PosixPath(), which both provide a _flavour class attribute via multiple inheritance. You must also provide this attribute when subclassing. You'll probably need to instantiate and/or subclass the _Flavour class to do this. This is not a supported part of the API, so your code might break in a future version of Python.

WindowsPath()

PosixPath()

_flavour

_Flavour

TL;DR: This idea is fraught with peril, and I fear that my answers to your questions will be interpreted as approval rather than reluctant assistance.

Path()

I have opened a bug track here after a little discussion on the Python dev. list.

Sorry for this double answer but here is a way to achieve what I want. Thanks to Kevin that points me to the source of pathlib and the fact we have here constructors.

pathlib

import pathlib
import os

def _extramethod(cls, n):
print("=== "*n)

class PathPlus(pathlib.Path):
def __new__(cls, *args):
if cls is PathPlus:
cls = pathlib.WindowsPath if os.name == 'nt' else pathlib.PosixPath

setattr(cls, "extramethod", _extramethod)

return cls._from_parts(args)

test = PathPlus("C:", "Users", "projetmbc", "onefile.ext")

print("File ?", test.is_file())
print("Dir ?", test.is_dir())
print("New name:", test.with_name("new.name"))
print("Drive ?", test.drive)

test.extramethod(4)


This prints the following lines.

File ? False
Dir ? False
New name: C:/Users/projetmbc/new.name
Drive ?
=== === === ===


WindowsPath

PosixPath

PathPlus

Here is a simple way to do things regarding to the observation made by Kevin.

class PPath():
def __init__(self, *args, **kwargs):
self.path = Path(*args, **kwargs)


Then I will need to use a trick so as to automatically bind all the Path's methods to my PPpath class. I think that will be funny to do.

__getattr__

__div__

__rdiv__

foo / bar

__div__

__truediv__

__rtruediv__

div

Path

is_file

__getattr__

Path

It's work too.

from pathlib import Path

class SystemConfigPath(type(Path())):
def __new__(cls, **kwargs):
path = cls._std_etc()
return super().__new__(cls, path, **kwargs)

@staticmethod
def _std_etc():
return '/etc'

name = SystemConfigPath()
name = name / 'apt'
print(name)


Printed:

/etc/apt


@staticmethod can be replaced by @classmethod

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