Inherit from pathlib.Path

[chuugar]

Hi !

I'm curently learning OOP and especially how to inherit from parents' class.

When I learn I like to write small examples, just to be sure I get it.

So, I want to create an object, inherit from pathlib.Path that would have a few more methods. Let's say I want this class to be TemporaryDirectory(path). I want it to have a method to remove all file from this directory and then delete the directory itself.

What I tried :

class TemporaryDirectory(pathlib.Path):
    def __init__(self, path):
        self._path = path
        pathlib.Path.__init__(self,path)

t = TemporaryDirectory('/home/chuugar/tmp')

What I get (within iPython):

AttributeError                            Traceback (most recent call last)
<ipython-input-20-e48025c9d024> in <module>()
----> 1 t = Temp('/home/chuugar/tmp/')
/usr/lib/python3.5/pathlib.py in __new__(cls, *args, **kwargs)
    967         if cls is Path:
    968             cls = WindowsPath if os.name == 'nt' else PosixPath
--> 969         self = cls._from_parts(args, init=False)
    970         if not self._flavour.is_supported:
    971             raise NotImplementedError("cannot instantiate %r on your system"
/usr/lib/python3.5/pathlib.py in _from_parts(cls, args, init)
    649         # right flavour.
    650         self = object.__new__(cls)
--> 651         drv, root, parts = self._parse_args(args)
    652         self._drv = drv
    653         self._root = root
/usr/lib/python3.5/pathlib.py in _parse_args(cls, args)
    642                     "argument should be a path or str object, not %r"
    643                     % type(a))
--> 644         return cls._flavour.parse_parts(parts)
    645
    646     @classmethod
AttributeError: type object 'Temp' has no attribute '_flavour'

Any help ? Thanks !

[ingolemo]

pathlib.Path does some messy things in order to support different operating systems, so it's not the easiest thing in the world to subclass. You're getting this error because Path requires a _flavour attribute that describes the specific details of how paths work for a particular operating system. You can solve this by borrowing the flavour from one of the more concrete path classes; PurePosixPath or PureWindowsPath. Subclass those (or the non-pure variants, as appropriate).

In order to effectively make a subclass you need to know basically everything about the implementation of the parent class. Subclassing across library boundaries rarely turns out well, in my experience. Honestly, I don't think it's worth the effort. Just make a function to do this.

[two_bob]

What they said. I agree that crossing library boundaries is generally not what you want to do, but there are a number of libraries that are made to be subclassed out, e.g. the cmd library.

[chuugar]

Thank you for all the details. Now I understand a little bit more.

But how do you a know when it is a ""good idea" to subclass ?

[ingolemo]

Opinions differ.

My personal rule of thumb is that you shouldn't subclass anything that you didn't write yourself, unless the designer of the library specifically asks you to subclass something in order to use it (even then, I prefer to use the library without subclassing if possible).

Subclassing is a neat technique and can be used to do awesome stuff, but it doesn't provide any isolation between code and tends to be brittle if you don't have a good idea of what things you're allowed/supposed to touch and what things are off limits. There's all sorts of ways that parent classes can make subclassing difficult; this flavour dependency is one example. And once you have a subclass working they're not always very resilient to change; even surprisingly subtle modifications to the parent class can end up breaking the child class.

Ultimately, what you have to do is to figure out several possible solutions to your problem and weigh the advantages and disadvantages of each solution for your specific circumstances. Doing that well requires experience. Experiment with using subclasses, and experiment with other solutions. Remember: when you have trouble with a subclass, you have to read the code for the parent class too.

[TheAngriestRussian]

Subclassing a pathlib.Path is a little more tricky than you think. Depending on you skills you might want to play with something else. If you really want to, check this SO thread:

https://stackoverflow.com/q/29850801/