[Python-ideas] allow overriding files used for the input builtin
Amit Green
amit.mixie at gmail.com
Fri Sep 29 12:17:39 EDT 2017
Hmm, very good point.
Flushing standard error is essential:
- This is because output of standard output & standard error are often
redirected to the same file -- frequently the terminal -- and its important
to flush one of them before output to the other (since these are typically
line buffered you only see this if you output partial lines).
- Here is an example of not flushing & the mess it causes:
>>> import sys
>>> sys.stdout.write('hi'); sys.stderr.write(' HMM ');
sys.stdout.write('there\n')
HMM hithere
- Here is the same example with flushing:
>>> sys.stdout.write('hi'); sys.stdout.flush(); sys.stderr.write(' HMM ');
sys.stderr.flush(); sys.stdout.write('there\n')
hi HMM there
In fact, I think you need to improve your interface and documentation:
- If either of the parameters 'outfile', or 'errfile' are passed in &
not the 'none' value then the following happens:
- Both sys.stdout & sys.stderr are flushed (in that order) before
being redirected. (NOTE: Both are flushed, even if only one is
redirected).
- Before restoring sys.stdout & sys.stderr; then outfile & errfile
are flushed (if both have been used then they are flushed in that order).
You would of course need to write the documentation clearer than I did
above (writing documentation well is not my skill) -- I wrote it to convey
exactly what has to happen.
On Fri, Sep 29, 2017 at 12:01 PM, Wren Turkal <w00t at fb.com> wrote:
> Steven and Amit,
>
>
> I originally configured the mailing list for digest delivery and can't
> reply directly to his message. However, I now seen it, I will update the PR
> with the suggested name changes as soon as my "make test" finishes. FWIW, I've
> changed to direct message delivery.
>
>
> With regard to ferr (the corresponding variable in the builtin_input c
> function before the change), I saw this code:
>
>
> /* First of all, flush stderr */
> tmp = _PyObject_CallMethodId(ferr, &PyId_flush, NULL);
> if (tmp == NULL)
> PyErr_Clear();
> else
> Py_DECREF(tmp);
>
> And I assumed that it was important to be able to override a stderr as a
> result.
>
>
> I think there are few options here to resolve that:
>
>
> 1. Remove the errfile parameter and just do what happened before.
> 2. Remove the errfile and the above code (FWIW, I am not sure I
> understand the importance of flushing stderr before taking input).
> 3. Document the explicit purpose of the errfile. Amit, you'd responded
> with something about this (again digest reply, sorry). I am not sure how to
> concisely do that given Amit's descriptions. 😊
>
>
> I am honestly leaning toward 2 unless I can figure out why the flushing of
> stderr is actually needed.
>
>
> wt
>
>
> ------------------------------
> *From:* Amit Green <amit.mixie at gmail.com>
> *Sent:* Friday, September 29, 2017 8:13:21 AM
>
> *To:* Wren Turkal
> *Cc:* python-ideas at python.org
> *Subject:* Re: [Python-ideas] allow overriding files used for the input
> builtin
>
> Yes, infile, outfile & errfile would be consistent with python naming
> convention (also mentioned by Steven D'Aprano above)
>
> One of python's greatest strength is its library, the consistency of the
> library, and how well documented the library is (in fact, I think the
> library is a greater strength than even the very nice syntax of python in
> general).
>
> By "consistency of the library" I mean: functions do pretty much what you
> expect, they use consistent error mechanism & the documentation pretty much
> accurately documents what the function does -- especially as to showing its
> results & how it handles errors.
>
> Regarding this though, this then brings up the question (above from Steven
> D'Aprano) -- what would the the "errfile" parameter do?
>
> - As a general principle of consistency python library functions, and
> python itself, do not output to errfile, but instead throw errors.
> - (There are very minor exceptions such as exceptions thrown in
> __del__ functions; which are caught by python & then printed to standard
> error).
>
> I would thus think you don't want the errfile parameter -- unless it would
> be for catching these __del__ method that get triggered by input failing
> (for example your 'infile' parameter when called, allocated an object,
> which gets deallocated & throws an exception inside of its __del__ method).
>
> If this is the purpose, then (back to 'how well documented the library
> is') -- it should be documented this is the purpose of the "errfile"
> parameter ;-)
>
> [A secondary reason you might want to redirect "errfile" is that the
> passed in input or output file's, themselves do output to standard error
> ...]
>
>
>
> On Fri, Sep 29, 2017 at 10:50 AM, Wren Turkal <w00t at fb.com> wrote:
>
>> I am happy to rename the args. What do you think about infile, outfile,
>> and errfile?
>>
>>
>> FWIW, I did consider "in", "out", and "err", but "in" is a keyword, and I
>> didn't think those quite captured the full meaning.
>>
>>
>> wt
>> ------------------------------
>> *From:* Amit Green <amit.mixie at gmail.com>
>> *Sent:* Thursday, September 28, 2017 11:18:18 PM
>> *To:* Wren Turkal
>> *Cc:* python-ideas at python.org
>> *Subject:* Re: [Python-ideas] allow overriding files used for the input
>> builtin
>>
>> I'm fine with the idea in general of extra keyword parameters to the
>> input function.
>>
>> A few points:
>>
>> Your example code, needs try/catch to match what the input with
>> parameters does -- and yes, its way nicer to be able to use it the example
>> you have shown than play games with try/catch (Personally I also refuse to
>> ever change sys.stdin, or sys.stdout, as I consider that a bad coding
>> style).
>>
>> Mostly though I would like to ask, please do not name keyword arguments
>> with names like 'fin' & 'fout'. This is almost unreadable and make's code
>> almost indecipherable to others the first time they see the function & its
>> keyword arguments (First impressions are very important).
>>
>> Both a function name & its keyword parameters need to be as
>> understandable as possible when a user encounters them for the first time.
>>
>> On Fri, Sep 29, 2017 at 1:53 AM, Wren Turkal <w00t at fb.com> wrote:
>>
>>> Hi there,
>>>
>>>
>>> I have posted an idea for improvement with a PR of an implementation to
>>> https://bugs.python.org/issue31603
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.python.org_issue31603&d=DwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=OAN5uLR4JWXbIgcvx315Zw&m=K1-OD0dhslOiAqYQdEyr0Oppl8TaroBPvXr8h_Z8XxM&s=sjDmBcI00MbWPPjuLMPlBnZHoFZOHoTxaCo2KCYlEd4&e=>
>>> .
>>>
>>>
>>> The basic idea is to add fin, fout, and ferr file object parameters and
>>> default to using what is used today when the args are not specified. I
>>> believe this would be useful to allow captures input and send output to
>>> specific files when using input. The input builtin has some logic to use
>>> readline if it's available. It would be nice to be able to use this same
>>> logic no matter what files are being used for input/output.
>>>
>>>
>>> This is meant to turn code like the following:
>>>
>>> orig_stdin = sys.stdin
>>>
>>> orig_stdout = sys.stdout
>>>
>>> with open('/dev/tty', 'r+') as f:
>>>
>>> sys.stdin = f
>>>
>>> sys.stdout = f
>>>
>>> name = input('Name? ')
>>>
>>> sys.stdin = orig_stdin
>>>
>>> sys.stdout = orig_stdout
>>>
>>> print(name)
>>>
>>>
>>> into something more like this:
>>>
>>> with open('/dev/tty', 'r+') as f:
>>>
>>> name = input('Name? ', fin=f, fout=f)
>>>
>>> print(name)
>>>
>>>
>>> It's nice that it makes the assignment to a global variable to change
>>> the file used for input/output to no longer be needed.
>>>
>>>
>>> I had this idea the other day, and I realized that it would be super
>>> easy to implement, so I went ahead the threw up a PR also.
>>>
>>>
>>> Would love to see if anyone else is interested in this. I think it's
>>> pretty cool that the core logic really didn't need to be changed other than
>>> plumbing in the new args.
>>>
>>>
>>> FWIW, this change introduces no regressions and adds a few more tests to
>>> test the new functionality. Honestly, I think this functionality could
>>> probably be used to simplify some of the other tests as well, but I wanted
>>> to gauge what folks thought of the change before going farther.
>>>
>>>
>>> Wren Turkal
>>>
>>> Existential Production Engineer of the Ages
>>>
>>> Facebook, Inc.
>>>
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>>>
>>>
>>
>
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