Issue 21539: pathlib's Path.mkdir() should allow for "mkdir -p" functionality

As of Python 3.2, `os.makedirs()` takes `exist_ok` as a keyword argument.  If set to true, the function does not raise an error if the directory already exists.  This makes the function's behavior similar to `mkdir -p` on the UNIX commandline.  (See https://docs.python.org/3/library/os.html#os.makedirs)

However, this functionality is missing in Python's new pathlib. In fact, the documentation for `Path.mkdir()` even mentions that if `parent=True` is passed, then it should behave like `mkdir -p`.  This is accurate in that it indeed recursively makes any parent directories necessary, but it is inaccurate in that it raises an error if the directory already exists.

I propose that either `parents=True` should imply that there is no error if the directory already exists (which would be a backwards-incompatible change); or, there could be a new `exist_ok=True`, which could suppress the error for an existing directory.  Either way, it ought to be possible to get the `mkdir -p` functionality easily from pathlib.  And the documentation for `Path.mkdir()` should be updated to explain how to get this `mkdir -p` functionality, with the existing (inaccurate) `mkdir -p` mention removed.

I think we can a new `exist_ok` parameter indeed. It should probably be False by default, though.

I've just been writing some new code to use pathlib and ran into this one myself.  An exist_ok=False is fine, although it's a slight shame that for backward compatibility we can't adopt os.makedirs() signature exactly.

Le 27/05/2014 20:32, Barry A. Warsaw a écrit :
>
> I've just been writing some new code to use pathlib and ran into
> this
one myself. An exist_ok=False is fine, although it's a slight shame that
for backward compatibility we can't adopt os.makedirs() signature exactly.

What do you mean by that? The os.makedirs() signature is 
os.makedirs(name, mode=0o777, exist_ok=False)

On May 27, 2014, at 08:48 PM, Antoine Pitrou wrote:

>What do you mean by that? The os.makedirs() signature is 
>os.makedirs(name, mode=0o777, exist_ok=False)

Right, but this is Path.mkdir's signature:

Path.mkdir(mode=0o777, parents=False)

so it's too late to make exist_ok=False the second argument (i.e. the one
after `mode`).  Oh well.

Updated patch. Thank you Barry and Antoine.

I like it.  Thanks for the contribution!  Assigning to myself since I plan on working to land this in 3.5.

New changeset 8af80d68bcdc by Barry Warsaw in branch 'default':
- Issue #21539: Add a *exists_ok* argument to `Pathlib.mkdir()` to mimic
http://hg.python.org/cpython/rev/8af80d68bcdc

messages: + msg219468
stage: patch review