[Python-ideas] chdir context manager

[MRAB]

On 2013-01-19 18:07, Terry Reedy wrote:
> On 1/19/2013 10:06 AM, Daniel Shahaf wrote:
>> Terry Reedy wrote on Sat, Jan 19, 2013 at 08:37:17 -0500:
>>> On 1/19/2013 5:10 AM, Daniel Shahaf wrote:
>>>> The following is a common pattern (used by, for example,
>>>> shutil.make_archive):
>>>>
>>>>       save_cwd = os.getcwd()
>>>>       try:
>>>>           foo()
>>>>       finally:
>>>>           os.chdir(save_cwd)
>>>>
>>>> I suggest this deserves a context manager:
>>>>
>>>>       with saved_cwd():
>>>>           foo()
>>>
>>> So to me, your proposal is only 1/2 or 2/3 of a context manager. (And
>>> 'returns an open file descriptor for the saved directory' seems backward
>>> or wrong for a context manager.) It does not actually make a new
>>
>> What should __enter__ return, then?
>>
>> It could return None, the to-be-restored directory's file descriptor, or
>> the newly-changed-to directory (once a "directory to chdir to" optional
>> argument is added).  The latter could be either a pathname (string) or
>> a file descriptor (since it's just passed through to os.chdir).
>>
>> It seems to me returning the old dir's fd would be the most useful of
>> the three option, since the other two are things callers already have
>> --- None, which is global, and the argument to the context manager.
>
> make_archive would prefer the old dir pathname, as it wants that for the
> logging call. But I do not think that that should drive design.
>
>>> context. A proper temp_cwd context manager should have one parameter,
>>> the new working directory, with chdir(new_cwd) in the enter method. To
>>> allow for conditional switching, the two chdir system calls could be
>>> conditional on new_cwd (either None or '' would mean no chdir calls).
>>>
>>
>> I think making the new_cwd argument optional would be useful if the
>> context manager body does multiple chdir() calls:
>>
>>      with saved_cwd():
>>          os.chdir('/foo')
>>          do_something()
>>          os.chdir('/bar')
>>          do_something()
>>
>> I'm not sure if that's exactly what you suggest --- you seem to be
>> suggesting that saved_cwd(None) will avoid calling fchdir() from
>> __exit__()?
>
> I was, but that is a non-essential optimization. My idea is basically
> similar to Bueno's except for parameter absent versus None (and the two
> cases could be handled differently).
>
> I think this proposal suffers a bit from being both too specific and too
> general. Eli explained the 'too specific' part: there are many things
> that might be changed and changed back. The 'too general' part is that
> specific applications need different specific details. There are various
> possibilities of what to do in and return from __enter__.
>
> However, given the strong -1 from at least three core developers and
> one other person, the detail seem moot.
>
FWIW, -1 from me too because, as has been said already, you shouldn't 
really
be using os.chdir; use absolute paths instead.