[R-pkg-devel] How to write example results to a tempdir()?

[William Dunlap]

The withr package has a bunch of simple functions executing code in various
contexts, e.g., in a given directory or with given options() settings:

> withr::with_dir
function (new, code)
{
    old <- setwd(dir = new)
    on.exit(setwd(old))
    force(code)
}
<environment: namespace:withr>
> withr::with_dir(tempdir(), getwd())
[1] "/tmp/RtmpvClVBM"

Perhaps some could be moved into core R since they are good for running
examples
and tests.



Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Thu, Apr 26, 2018 at 10:20 AM, Duncan Murdoch <murdoch.duncan at gmail.com>
wrote:

> On 26/04/2018 1:04 PM, Henrik Bengtsson wrote:
>
>> On Thu, Apr 26, 2018 at 6:28 AM, Spencer Graves
>> <spencer.graves at effectivedefense.org> wrote:
>>
>>>
>>>
>>> On 2018-04-26 07:11, Jose A Guijarro wrote:
>>>
>>>>
>>>> El 25/04/18 a las 20:21, Duncan Murdoch escribió:
>>>>
>>>>>
>>>>> On 25/04/2018 1:32 PM, Sarah Goslee wrote:
>>>>>
>>>>>>
>>>>>> Don't change the working directory! That has all kinds of unpleasant
>>>>>> side effects for the unsuspecting user, possibly even more so than
>>>>>> writing to a file.
>>>>>>
>>>>>> Instead, write the file to the temp directory, and read it from there,
>>>>>> with e.g.
>>>>>>
>>>>>> wd <- tempdir()
>>>>>> write(dat, file.path(wd, 'Ttest_1981-2000.dat'))
>>>>>>
>>>>>> Using file.path() means that the appropriate path delimiter for that
>>>>>> OS will be used.
>>>>>>
>>>>>
>>>>>
>>>>> That's one good way to do it.  But it is possible to change directory
>>>>> and
>>>>> change back at the end of the example.  For example,
>>>>>
>>>>> wd <- tempdir()
>>>>> savedir <- setwd(wd)
>>>>>
>>>>> ... # the original code that writes and reads in the current dir
>>>>>
>>>>> setwd(savedir)
>>>>>
>>>>> There's a worry that an error in the middle of the code will leave the
>>>>> user in the wrong place.  If that's really unlikely to happen, then
>>>>> this
>>>>> code is a little simpler than Sarah's suggestion.
>>>>>
>>>>> If it is likely, you can use tryCatch(..., finally = setwd(savedir)),
>>>>> but
>>>>> I think Sarah's solution would be preferable in most cases:  many
>>>>> users will
>>>>> not understand what tryCatch() does.
>>>>>
>>>>
>>>
>>>
>>> Hi, Duncan, et al.:
>>>
>>>
>>>        Under what circumstances should one also use "on.exit":
>>>
>>>
>>> wd <- tempdir()
>>> savedir <- setwd(wd)
>>> on.exit(setwd(savedir))?
>>>
>>>
>>>        Even senior R programmers may miss a failure mode.  In a function,
>>> this works well:  I don't need to remember to "setwd(savedir)" later in
>>> the
>>> code, which could be a problem if I have multiple exit points.  I don't
>>> know
>>> how it would work in a vignette or the examples section of a *.Rd file.
>>> tryCatch(..., finally = setwd(savedir)) condenses this into one line ...
>>> and
>>> is too terse for me in many cases.
>>>
>>
>> FWIW, it can also be used in a local() call, e.g.
>>
>> local({
>>    wd <- tempdir()
>>    savedir <- setwd(wd)
>>    on.exit(setwd(savedir))
>>    [...]
>> })
>>
>
> Thanks for pointing that out.  That also has the advantage that it doesn't
> wipe out variables named "wd" and "savedir".
>
> On the other hand, it has the disadvantage that it doesn't leave wd and
> savedir behind for the user to examine, and cut and paste of the central
> executable lines won't work, you need to cut the whole local() call.
>
> Duncan Murdoch
>
>
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> https://stat.ethz.ch/mailman/listinfo/r-package-devel
>

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