Trying to use Pathlib and Path.replace on Windows if drive are different leads to an issue:

  File "D:\\myscript.py", line 184, in update
    client_generated_path.replace(destination_folder)
  File "c:\\program files (x86)\\python35-32\\Lib\\pathlib.py", line 1273, in replace
    self.\_accessor.replace(self, target)
  File "c:\\program files (x86)\\python35-32\\Lib\\pathlib.py", line 377, in wrapped
    return strfunc(str(pathobjA), str(pathobjB), \*args)
OSError: [WinError 17] The system cannot move the file to a different disk drive: 'C:\\\\MyFolder' -\> 'D:\\\\MyFolderNewName'

This is a known situation of os.rename, and workaround I found is to use shutil or to copy/delete manually in two steps (e.g. http://stackoverflow.com/questions/21116510/python-oserror-winerror-17-the-system-cannot-move-the-file-to-a-different-d)

When using Pathlib, it's not that easy to workaround using shutil (even if thanks to Brett Cannon now shutil accepts Path in Py3.6, not everybody has Py3.6). At least this should be documented with a recommendation for that situation. I love Pathlib and it's too bad my code becomes complicated when it was so simple :(

Just to confirm, I was able to workaround it with Py3.6:

    # client_generated_path.replace(destination_folder)
    shutil.move(client_generated_path, destination_folder)

It is reasonable to think about adding a similar feature to pathlib if it doesn't support it and just special-case the drive-to-drive scenario for Windows

Moving a file across volumes isn't atomic. Getting an exception in this case can be useful. There could be a "strict" keyword-only parameter that defaults to False. If it's true, then replace() won't try to move the file.

I agree this needs to be different from replace(), due to not being atomic. That makes it an enhancement, so I'm removing 3.5.

I'm +1 on Path supporting something like shutil.move().

I also support the idea of getting something like shutil.move() into pathlib.

I should also mention that rename() (https://docs.python.org/3/library/pathlib.html#pathlib.Path.rename) and replace() (https://docs.python.org/3/library/pathlib.html#pathlib.Path.replace) already do exist, so it might be best to add a keyword-only flag to one of those for this use-case.

I'm also +1 on adding pathlib.Path.move(). However I'd prefer we move the shutil implementation into pathlib, as proposed here. The implementation in shutil would become a stub that calls into pathlib, roughly:

def move(src, dst, copy_function=copy2):
    return pathlib.Path(src).move(dst, copy_function=copy2)

I'm interested in this approach because I'm hoping to add a pathlib.AbstractPath class in future. Users would be able to subclass AbstractPath, and by implementing a small handful of low-level abstract methods like stat(), open() and iterdir(), benefit from high-level methods like move() in their objects. This would work even if the source and destination are different kinds of path (e.g. a local filesystem path and a path in a tar archive).

To achieve this we'd need to make pathlib accept bytes paths, as shutil does. Personally I'm fine with this - it's not much work and perfectly reasonable.

We'd also need to move other shutil functions that move() relies on to pathlib, notably the copy*() functions, excluding copyfileobj() and friends.

On rename() / replace(): I'd like these to call through to move(atomic=True). IIRC these methods are identical on POSIX and only differ slightly on Windows. That's a wrinkle we should smooth out in pathlib IMO.

I'd prefer we move the shutil implementation into pathlib

Sounds reasonable to me, though would we want to include copy_function in the API? It seems like an opportunity to say that Path is opinionated about the "most move-like way to copy", and if you need to override that then handle it yourself or stick with shutil.

I think @barneygale 's plan is sound.

I agree with @zooba, exposing the copy_function in the pathlib API seems incompatible with the general style of pathlib. Otherwise the plan looks good.

I agree about copy_function. Perhaps we could do something like this?

    def copy(self, dst, *, follow_symlinks=True, copy_mode=True, copy_stat=False):
        ...

    def move(self, dst, *, atomic=False, **kwargs):
        if atomic:
            return os.rename(self, dst)
        self.copy(dst, **kwargs)
        self.unlink()

I don't like the atomic argument, and especially its default. Implementations should get to implement it as efficiently as possible without needing to respect that kind of argument, and callers shouldn't expect that level of control over semantics.

For example, on Windows we explicitly disallow os.rename from moving files to other drives, because it wouldn't match our documented behaviour. But a new WindowsPath.move function should be able to allow that, and take advantage of the OS optimisations (yes, they exist!) for moving a file rather than copying it across drives.

If we were to define semantics for atomic, the only way to allow this would be "atomic=True will try to move, and atomic=False won't bother trying to move", at which point why would anybody ever pass False? Especially since there's no back-compat here, and so code on older versions will have to stick with try: ...rename ... except: ... copy.

We should make these functions an improvement in semantics, not just discoverability, and ideally more appropriate for whatever platform they're being used on than the POSIX semantics inherent to posixmodule.

So I think the only option we want on move is follow_symlinks=True,¹ and the method "does whatever is necessary to move the file or directory as efficiently as possible into the new location while matching the semantics of an in-place rename" (roughly, copy_mode=True, copy_stat=True)

Similarly, I think we should simplify copy down to only have follow_symlinks=True,² and the method "does whatever is necessary to copy the file or directory as efficiently as possible into the new location while matching the semantics of a newly created file" (roughly, copy_mode=some bits, copy_stat=some bits - and this one is why the vague definition is important 😉 )

My example was a bit rubbish - here's a corrected version:

    def copy(self, dst, *, follow_symlinks=True, copy_mode=True, copy_stat=True):
        ...

    def move(self, dst, *, atomic=False, **kwargs):
        try:
            return os.rename(self, dst)
        except OSError:
            if atomic:
                raise  # Move can't be achieved atomically
        self.copy(dst, **kwargs)
        self.unlink()

So move(atomic=True) would call os.rename() only and allow exceptions to propagate, whereas atomic=False will try a rename, and failing that use a copy/delete. I think there's common use cases for both.

I think I agree with you on removing copy_mode and copy_stat.

I suppose one could argue that folks should just use os.rename() if they want an atomic rename! :)

I suppose one could argue that folks should just use os.rename() if they want an atomic rename! :)

Yeah, I'll argue that one ;)

We'll have to explain that move may result in both old and new names existing simultaneously while the move takes place, depending on the operating system and file systems in use. That's a perfectly fine place to say that if you'd rather that not happen, and are willing to accept exceptions if it's unavoidable, then you should use rename.

Should we deprecate pathlib.Path.rename() and replace() when we add move()?

I've been closely following the shutil API so far, resulting in this:

(new methods shown in italics)

Having given it some thought, I have the following concerns:

1. copy() and move() are the shortest/most obvious methods, but the former accepts only files, whereas the latter accepts both files and directories
2. move() can be used to move any kind of file, but there are no corresponding methods for copying and deletion.

And so I'd like to propose that:

1. We rename copy() to copyfile()
2. We rename rmtree() to delete(), and copytree() to copy(), and add support for files to both methods.

This would result in:

Thoughts?

(Also, if we wanted to add support for moving/copying into an existing directory via new methods, it would be better to have move_into() and copy_into(), rather than move_into(), copyfile_into() and copytree_into() IMO.)

+1 from me on the revised names and the _into forms.

Do we even need a copyfile() function, if copy() will do the same?

Do we even need a copyfile() function, if copy() will do the same?

Good point - perhaps not!