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[issue14705] Add 'bool' format character to PyArg_ParseTuple*

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May 2, 2012, 2:29 AM

Post #1 of 36 (124 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple*

New submission from Larry Hastings <larry [at] hastings>:

Currently functions that parse their arguments with the PyArg_ParseTuple family which want to take a boolean-ish parameter face two choices:
* take an "int", then test whether or not the int is 0, or
* take an "object", then call PyObject_IsTrue themselves.

The former is foolish; though it works with ints and bools, it doesn't work with any other type (float, str, list, etc) which strictly speaking are valid for boolean fields. And this is common enough that the latter should not be necessary.

I propose to add support for a new format character to the PyArg_ParseTuple family: 'b', which specifies 'boolean'. Its
implementation would be much the same as that of 'd' except:
* the output type would be "int" instead of "double",
* it would check for a -1 instead of calling PyErr_Occured, and
* it would call PyObject_IsTrue instead of PyFloat_AsDouble.

If we cared, I could also add 'B', which would only accept
either Py_True or Py_False. But I see no reason why we'd ever want
to strictly enforce the type... do you? (I can see MvL's argument now:
"We've lived this long without it. YAGNI.")

If there's interest I'll knock out a patch. I expect it to be less than ten lines not counting documentation and test. (Less than twenty if
folks actually want 'B'.)

----------
assignee: larry
components: Interpreter Core
messages: 159781
nosy: larry
priority: normal
severity: normal
status: open
title: Add 'bool' format character to PyArg_ParseTuple*
type: enhancement
versions: Python 3.3

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May 2, 2012, 3:39 AM

Post #2 of 36 (103 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Serhiy Storchaka <storchaka [at] gmail> added the comment:

Yes, I too have encountered this in the process of working on issue 14626. For this purpose it is appropriate to use a special converter (with 'O&'). I suppose it must be a strict сonverter; there is no point in specifying followlinks=[1, 2, 3]. If it is somewhere need a nonstrict ones -- use 'O' and then check the how to in this particular case.

Would be good to have a special letter for this format, but note that 'b' and 'B' are already used. It is better first to use the сonverter, and expand the format only when the enough number of uses.

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nosy: +storchaka

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May 2, 2012, 4:23 AM

Post #3 of 36 (106 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Larry Hastings <larry [at] hastings> added the comment:

> For this purpose it is appropriate to use a special converter
> (with 'O&').

The converter works--but, then, a similar converter would also work
for double, and float, and long long and many others. If long long
is special enough to merit explicit support in PyArg_ParseTuple,
then bool definitely is, as I strongly suspect bool is used much more
frequently.

> I suppose it must be a strict сonverter; there is no point
> in specifying followlinks=[1, 2, 3].

I don't see a need for that either, but--where would you draw the line? What is the principle that says "ints are okay, floats are... okay, lists are not"?

Python has a long-established concept of the "truthiness" of an expression. I *strongly* suggest we stick with that unless we have a *very* good reason why not.

> note that 'b' and 'B' are already used.

I'm a moron! I must have looked right at it and it didn't register.

't' (truth) and 'f' (false) are also taken. As is 'c' (conditional).
The only thing I can come up with that's remotely related and
isn't already taken is 'p' for predicate.

> It is better first to use the сonverter, and expand the format
> only when the enough number of uses.

I suspect there are already loads of places in the standard library
that should be using this rather than abusing 'i'.

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May 3, 2012, 1:16 AM

Post #4 of 36 (95 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Larry Hastings <larry [at] hastings> added the comment:

My first patch. Adds 'p' and 'P', along with documentation and unit tests.

----------
keywords: +patch
stage: -> patch review
Added file: http://bugs.python.org/file25441/larry.parse.tuple.p.and.P.1.diff

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May 3, 2012, 3:10 AM

Post #5 of 36 (98 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Serhiy Storchaka <storchaka [at] gmail> added the comment:

Patch looks good to me.

I however prefer to use 'P'.

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May 4, 2012, 3:22 AM

Post #6 of 36 (96 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Eric V. Smith <eric [at] trueblade> added the comment:

The patch looks good to me.

Are there any places in the current code base that would use "P"? "p" seems the more useful case.

Are you planning on changing existing code to use P or p, or just use it going forward?

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nosy: +eric.smith

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May 4, 2012, 3:38 AM

Post #7 of 36 (96 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Mark Dickinson <dickinsm [at] gmail> added the comment:

I also think that 'P' looks too strict to be really useful. I can't think of too many cases where I'd really want to insist on a boolean argument (and reject values of 0 or 1).

Maybe implement just 'p' for now and consider 'P' later?

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nosy: +mark.dickinson

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May 4, 2012, 4:13 AM

Post #8 of 36 (97 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Larry Hastings <larry [at] hastings> added the comment:

I looked through the Python sources and couldn't find any instances of a function or method with an argument that only allowed you to pass in either True or False.

Serily already said he would use 'P' over 'p', although I too am unconvinced that's a good idea. Serily: why would you unhesitatingly prefer 'P' to 'p'?

Certainly I see loads of uses for 'p'. For example, when converting code from Python to C that already relied on Python's standard definition of truthiness.

I did find some spots that took an object and converted to bool with PyObject_IsTrue, like _json.Encoder(allow_nan) and pickle._Pickler(fix_imports). These too would be well-served by 'p'.

I also found some funny in-between cases. For example, stat_float_times and the three-argument form of os.symlink both claim to take a boolean but actually take 'i' (integer). This is relying on bool.__int__(). We certainly couldn't use 'P' here. We could consider switching these to 'p', though in all likelyhood we'll just leave 'em alone.

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May 4, 2012, 4:44 AM

Post #9 of 36 (93 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Martin v. Löwis <martin [at] v> added the comment:

I think there should be a test case also where PyObject_IsTrue gives an exception (which I think can happen if __bool__ raises an exception).

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nosy: +loewis

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May 4, 2012, 4:45 AM

Post #10 of 36 (95 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Larry Hastings <larry [at] hastings> added the comment:

> I think there should be a test case also where PyObject_IsTrue gives an
> exception (which I think can happen if __bool__ raises an exception).

I'd be happy to add such a test, but I don't know of any types like that. Can anyone suggest one?

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May 4, 2012, 4:48 AM

Post #11 of 36 (94 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Martin v. Löwis <martin [at] v> added the comment:

>> I think there should be a test case also where PyObject_IsTrue gives an
>> exception (which I think can happen if __bool__ raises an exception).
>
> I'd be happy to add such a test, but I don't know of any types like
> that. Can anyone suggest one?

class NotTrue:
def __bool__(self):
raise NotImplementedError

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May 4, 2012, 5:17 AM

Post #12 of 36 (94 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Larry Hastings <larry [at] hastings> added the comment:

Added test forcing a failure for 'p' (and 'P'). This made me have to handle errors a little differently, so it was definitely good to test it. Thanks for the suggestion, Martin!

Also changed wording in documentation ever-so-slightly.

----------
Added file: http://bugs.python.org/file25453/larry.parse.tuple.p.and.P.2.diff

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May 4, 2012, 5:28 AM

Post #13 of 36 (94 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Eric V. Smith <eric [at] trueblade> added the comment:

Now that I think about this some more, I think I'd structure the 'p' tests as:

for expr in [False, None, True, 1, 0]: # add the rest
self.assertEqual(bool(expr), getargs_p(expr))

Since the salient point is that 'p' returns the same value as bool(), right?

And for the one that raises an exception, you'll have to check that bool and getargs_p both raise the same type of exception.

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May 4, 2012, 5:48 AM

Post #14 of 36 (93 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Serhiy Storchaka <storchaka [at] gmail> added the comment:

> Serily: why would you unhesitatingly prefer 'P' to 'p'?

My name is Serhiy. :)

'P' has the advantage that you can safely backward-compatibly remove the
restriction by replacing 'P' on 'p'. :)

> I also found some funny in-between cases.

This is historical legacy, some still use 1/0 instead True/False. In the
end, bool subclasses int. But we have no middlecase for latin 'P'.

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May 4, 2012, 5:56 AM

Post #15 of 36 (93 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Serhiy Storchaka <storchaka [at] gmail> added the comment:

> Since the salient point is that 'p' returns the same value as bool(), right?

Yes, and bool_new() is a candidate number one for using new feature.

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May 4, 2012, 6:00 AM

Post #16 of 36 (94 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Eric V. Smith <eric [at] trueblade> added the comment:

If bool_new() is going to use 'p', then my suggestion shouldn't be the only test of 'p'.

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May 4, 2012, 6:43 AM

Post #17 of 36 (92 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Mark Dickinson <dickinsm [at] gmail> added the comment:

In this line in the patch (Python/getargs.c):

+ if (val == -1 || PyErr_Occurred()) {

Isn't that call to PyErr_Occurred() redundant?

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May 5, 2012, 9:20 AM

Post #18 of 36 (91 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Larry Hastings <larry [at] hastings> added the comment:

Attached is rev 3 of my patch, incorporating mainly backing out of dumb ideas. Thanks for the feedback, Serhiy and Mark!

> My name is Serhiy. :)

My genuine apologies! In my defense it was rather late.

> 'P' has the advantage that you can safely backward-compatibly
> remove the restriction by replacing 'P' on 'p'. :)

That's not a reason to use 'P'. Why you should use 'P' in the first place?

> In this line in the patch (Python/getargs.c):
> + if (val == -1 || PyErr_Occurred()) {
> Isn't that call to PyErr_Occurred() redundant?

Certainly one of the two expressions is!

My thinking was: if the call fails, then the val == -1 will be an early-exit and we can save the call to PyErr_Occurred. This was always a dumb idea, as the 99.999999% case is that PyObject_IsTrue succeeds, in which case we would have called PyErr_Occured anyway. Some savings!

I can't find any documentation on permitted return values from nb_bool. However, PyObject_IsTrue itself says about its own return value:

/* if it is negative, it should be either -1 or -2 */

So I definitely shouldn't check specifically for -1.

Having meditated on it, I think either I should either just call PyErr_Occured, check for explicit failure (val < 0), or explicit success (val >= 0). I've opted for the last of those.

I considered briefly trying to make 'P' handle subclasses of bool. But then I hit the problem of: okay, what now? Call nb_bool? I note that bool itself doesn't define nb_bool. Anyway, what lunatic would subclass bool?

I'm really on the fence about 'P'. Serhiy is definitely pro-, everyone else seems to think it shouldn't be used. However nobody has argued against its inclusion. At the moment I'm -0 on it myself, but since the code is written...

Do we have an anti-champion?

----------
Added file: http://bugs.python.org/file25465/larry.parse.tuple.p.and.P.3.diff

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May 5, 2012, 9:24 AM

Post #19 of 36 (92 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Benjamin Peterson <benjamin [at] python> added the comment:

Since no one has produced a good example needing "P", I say drop it. At any rate, it can be almost trivially imitated with "O!".

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nosy: +benjamin.peterson

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May 5, 2012, 9:37 AM

Post #20 of 36 (94 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Georg Brandl <georg [at] python> added the comment:

Indeed, "because the code is written" is not a good argument if even you yourself are -0.

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nosy: +georg.brandl

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May 5, 2012, 9:45 AM

Post #21 of 36 (95 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Mark Dickinson <dickinsm [at] gmail> added the comment:

> Having meditated on it, I think either I should either just call
> PyErr_Occured, check for explicit failure (val < 0), or explicit success
> (val >= 0). I've opted for the last of those.

Yes, I think that works; it avoids a relatively expensive PyErr_Occurred() call in the non-failure case. The new code looks fine to me!

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May 5, 2012, 9:50 AM

Post #22 of 36 (92 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Mark Dickinson <dickinsm [at] gmail> added the comment:

> I considered briefly trying to make 'P' handle subclasses of bool.

Not an issue: bool can't be subclassed. :-)

Python 3.2.3 (default, Apr 13 2012, 00:15:25)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> class MyBool(bool):
... pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: type 'bool' is not an acceptable base type

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May 5, 2012, 10:12 AM

Post #23 of 36 (92 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Serhiy Storchaka <storchaka [at] gmail> added the comment:

> That's not a reason to use 'P'. Why you should use 'P' in the first place?

I just guided by the principle "Explicit is better than implicit".
Statements 'if' and 'while' I consider explicit and expect cast to bool.
Implicit cast to bool in the transfer of parameter causes discomfort (no
one (no one Pythonist) expects the implicit cast to str). Unfortunately,
for historical reasons, there is a lot of code, where the parameters are
casted to bool or int is used instead of bool. Therefore, we cannot
simply enter the restrictions.

Well, I was certainly wrong. Don't let me mislead you.

> Anyway, what lunatic would subclass bool?

Class bool cannot be subclassed further.

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May 5, 2012, 10:19 AM

Post #24 of 36 (94 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Larry Hastings <larry [at] hastings> added the comment:

Serhiy, I'm having a little trouble with your English. (But I'm sure your English is far better than my... uh, anything besides English.) So let me ask a question with a clear yes/no answer:

Do you still think 'P' is better than 'p'?

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May 5, 2012, 10:55 AM

Post #25 of 36 (93 views)
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[issue14705] Add 'bool' format character to PyArg_ParseTuple* [In reply to]

Serhiy Storchaka <storchaka [at] gmail> added the comment:

> Do you still think 'P' is better than 'p'?

No.

I hope I haven't made a lot of mistakes in the previous sentence.

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