[issue20341] Argument Clinic: add "nullable ints"

[Brett Cannon]

Change by Brett Cannon :


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[issue20341] Argument Clinic: add "nullable ints"

[Tal Einat]

Change by Tal Einat :


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[issue20341] Argument Clinic: add nullable ints

[Serhiy Storchaka]

Changes by Serhiy Storchaka storch...@gmail.com:


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[issue20341] Argument Clinic: add nullable ints

[Josh Rosenberg]

Changes by Josh Rosenberg shadowranger+pyt...@gmail.com:


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[issue20341] Argument Clinic: add nullable ints

[Martin v . Löwis]

Martin v. Löwis added the comment:

If this feature is contentious, then a PEP is need to decide on it. As it turns 
out, PEP 436 already discusses the nullable feature. But, as it also turns out, 
PEP 436 hasn't been accepted yet (at least I could not find any records on that 
having happened, and the PEP says that it is in draft state).

So what I think needs to happen next is:
- opposition to the PEP or certain features should be recorded in the PEP
- a BDFL delegate needs to be nominated, or Guido needs to declare that he 
wants to rule on the PEP himself
- a decision on the PEP needs to be made.

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[issue20341] Argument Clinic: add nullable ints

[Raymond Hettinger]

Raymond Hettinger added the comment:

FWIW, I agree with Antoine that Nullable Int is a misnomer.

Also, I don't buy into this mission to change signatures for optional ints into 
int-or-none.   In particular, itertools.repeat() is fine as-is.  Optional int 
APIs have been around for a long time, they work fine, and they really don't 
need to be changed.  AFAICT, there is no real issue here that warrants changes 
to existing APIs.


 s = [10, 20, 30]
 s.pop(None)
Traceback (most recent call last):
  File pyshell#5, line 1, in module
s.pop(None)
TypeError: 'NoneType' object cannot be interpreted as an integer


 int('42', None)
Traceback (most recent call last):
  File pyshell#7, line 1, in module
int('42', None)
TypeError: 'NoneType' object cannot be interpreted as an integer

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

FWIW, Martin agreed with me at EuroPython that nullable int is the proper 
name.  So does Wikipedia:

http://en.wikipedia.org/wiki/Nullable_type


As for repeat_new: like I said, I don't intend to check in this change when 
checking in nullable ints.  (I really just used it because I was doing the work 
on a plane and just didn't feel like finding a different example.)  I'm simply 
not interested in picking a fight about it.

However, this is using the general approach endorsed by Guido:

https://mail.python.org/pipermail/python-dev/2014-January/131673.html

And indeed, see thread for further specific discussion of repeat_new.


I feel the position of these have been in Python for years, they're fine 
misses the point.  Builtins never had signatures before.  Now that they can 
have signatures, the design decision of writing builtins with non-Pythonic 
signatures has come home to roost.  How can the signature for 
itertools.repeat() convey that times is an optional parameter without it 
having a default value?

I view giving all builtins in Python valid signatures as a worthwhile goal unto 
itself.  So ISTM that allowing times to accept None to mean repeat forever 
is reasonable: it makes the function more Pythonic, and it means it can have a 
meaningful signature.  But if this goal doesn't interest you, I can see why 
you'd think this was a waste of time.

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[issue20341] Argument Clinic: add nullable ints

[Raymond Hettinger]

Raymond Hettinger added the comment:

Larry, ISTM that you are bulldozing your way through something that isn't an 
actual problem to be solved.

  I can see why you'd think this was a waste of time.

I don't think it is just a waste of time;  I think it is a bad idea.  You have 
a very strong notion of how function signatures should look (i.e. the number of 
arguments being irrelevant) and you want to impose your ideas on existing, 
stable APIs for zero benefit.   

 I view giving all builtins in Python valid signatures
 as a worthwhile goal unto itself.

I can already model the behavior of repeat() using *args and **kwds, just like 
I can for int(), list.pop(), range(), and slice().

You don't seem to get that those tools already work, that people understand 
them, that they've been stable for a long time, and that they don't need to 
change for any reason other than that you've worked yourself into a snit about 
it.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

Here's a fresh patch.  After discussing with Martin at EuroPython, I moved the 
implementation into Python/getargs.c, and the prototypes into modsupport.h.  
I'm still using the example of repeat.new to show what it looks like and how 
it works.  However I don't plan on checking the changes to repeat.new in when I 
check in the patch--I'd want to discuss it with Raymond first.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

Whoops, here's the patch.

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Added file: http://bugs.python.org/file36165/larry.nullable.ints.4.txt

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

  Yes, that is a different name that seems to mean much the same
  thing.
 and which is much more understandable by a Python developer.

Thanks for your opinion.


  Changing error to an char and moving it to the end would
  save exactly zero bytes, because the compiler *will* align
  stack variables to 4 byte boundaries.
 
 Except if other stack variables happen to be shorter than an int,
 perhaps.

Do we use a lot of those?


 But regardless, it doesn't cost anything to do so, so why not do it?

I thought I just got through explaining how it is slightly slower.

There is no benefit in changing them to char, so why do it?

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

I insist that nullable is a very bad name. It also suggests that
somehow we'll get a NULL pointer (in C) when the parameter is absent or
None, which is not the case.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

I think it's a fine name, otherwise I would not have chosen it in the first 
place.  Your high-spirited bikeshedding has been noted and not acted upon.  I 
have no plans to change the name.  Please drop it.

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

The only place where nullable appears in the doc tree is... clinic.rst (in 
reference to places where a C pointer can be NULL, which *is* a reasonable use 
of the term).

There's another problem in the patch: if you define nullable_int_t and 
nullable_Py_ssize_t in a public header (longobject.h), then you probably need 
to prefix them with _Py (so: _Py_nullable_int_t and _Py_nullable_ssize_t, 
for example). Same for the #define's.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

How many existing occurrences of or_none did you find in the CPython tree?  
Perhaps we can turn our attention to other languages like SQL and C# and borrow 
their term for value that is allowed to be of a specific type (or types) as 
well as the null value.

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

 How many existing occurrences of or_none did you find in the CPython
 tree?

If I split that into the individual words or and None, quite a lot.
Looking for or None turns quite a bunch of matches too.

or_none simply applies the PEP 8 convention for variable names made of
multiple words.

Of course, you can propose another name that involves none, too.

 Perhaps we can turn our attention to other languages like SQL and C#
 and borrow their term for value that is allowed to be of a specific
 type (or types) as well as the null value.

It is quite well-known and obvious that Python was inspired by C# and
SQL, so why not indeed?
Perhaps you should start by asking Guido if he wants to rename None to
NULL.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

Drop.  It.

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

 Drop.  It.

Seriously? Why exactly did you ask for comments if your only response is
to be snarky and dismissive?

This is not your toy project. It is perfectly legitimate for us to
question API and naming decisions.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

I don't think it's fair to call my responses snarky.  On the other hand, your 
suggestion that I view Argument Clinic as my toy project is obviously meant 
as an insult.

It is fair to call my responses dismissive, because I find nothing compelling 
about your arguments.

I would color your final sentence slightly.  Yes, there are plenty of 
legitimate discussions about API semantics and nomenclature.  But there are 
infinitely many pointless and time-wasting discussions.  Therefore not every 
question about API and naming decisions is de facto legitimate.

I don't mean to say that your bringing it up originally was unreasonable, that 
was entirely appropriate.  But surely I've made it clear by now: I find no 
strength to your arguments, and I'm not changing the name based on them.  At 
this point all you're accomplishing is making me angry.

The name nullable has been in continuous in Argument Clinic since very early 
on.  It has always been in the PEP, and was in the original prototype published 
Dec 3 2012.  Nobody has said anything about it, until you, just now, on this 
issue.  I suggest the reason nobody has said anything before is because the 
name is fine.


A clarification over something you said previously: you suggested that the name 
would confuse Python developers.  Just to be crystal clear: the name is not 
exposed to Python *users*.  Right now it's solely visible to CPython core 
developers.  If at some point Argument Clinic is  deemed a public tool, it 
would also be visible to people writing third-party extension modules.  I am 
not proposing we use the term nullable in user-facing documentation or APIs.  
(I happen to think it would be fine for users too, but there's utterly no point 
in having that debate now.)  For what it's worth, I like to think CPython core 
developers are not so easily confused as you suggest.

Also: Python draws inspiration from lots of places.  The syntax for decorators 
was borrowed from Java autodoc comments (and Java annotations, though I think 
it actually slightly predates those).  I'm not aware of any existing Python 
precedent we can draw on, so drawing inspiration from C# and SQL strikes me as 
perfectly appropriate.  On the other hand, I see no precedent bolstering your 
suggestion.

You yourself told someone in IRC, when they were pestering me about some 
Argument Clinic bikeshedding, let's leave Larry alone.  I appreciated it at 
the time.  I'd appreciate it if you'd follow that advice now too.

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

 I would color your final sentence slightly.  Yes, there are plenty of
 legitimate discussions about API semantics and nomenclature.  But
 there are infinitely many pointless and time-wasting discussions.
 Therefore not every question about API and naming decisions is de
 facto legitimate.

This is a fair position, thanks.

 At this point all you're accomplishing is making me angry.

Argument Clinic discussions and activity seem to annoy me more than they
bring me anything, so I'll avoid involving myself in it any further.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

The bug you saw was an easy fix; the converter function needs to return 1 
(success) from the == Py_None branch.

And yes, more unit tests would be good, fixing the documentation would be good 
too.  I did say the patch was nowhere near ready.

I will assume unless I hear otherwise that I should proceed with the patch and 
flesh out all the different and float types.

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[issue20341] Argument Clinic: add nullable ints

[Tal Einat]

Tal Einat added the comment:

You look very handsome today Larry :)

Regardless, I do think this is a good idea. I've converted several functions 
that have -1 as a default value for ints where they ideally should have None. 
Having None as the default will be much clearer and less error prone for users.

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[issue20341] Argument Clinic: add nullable ints

[Tal Einat]

Tal Einat added the comment:

My only comment from reviewing the patch is that you seem to have used 
NULLABLE_PY_SSIZE_T_INITIALIZE by accident one time in int_converter (in 
clinic.py).

Other than that, as you already know, make sure to return 1 in both converters 
in the if (arg == Py_None) branches.

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[issue20341] Argument Clinic: add nullable ints

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 I don't remember who specifically needed the nullable ints, so I just added a 
 bunch of Derby contestants.

There are no much need in nullable ints, if they are needed, different use 
cases require different semantic. I don't think we can now write general 
converters which will cover different use cases. May be in 3.5, when behavior 
changes will be possible.

I know only one use case for nullable Py_ssize_t converter (and 
mmap_convert_ssize_t is much simpler). itertolls.repeat doesn't need it, 
because it shouldn't accept None.

Let defer this issue until most of code will be converted. Then we can 
accumulate statistic about nullable in converters.

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[issue20341] Argument Clinic: add nullable ints

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

And thank you for the fix for the __init__ methods. Could you please commit it? 
The I'll update patches for bz2 and lzma modules.

Looks as Vajrasky Kok has encountered with yet one bug in generated __init__ 
wrapper(issue20185).

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

Then you disagree with Guido, who says that optional integer arguments without 
a viable publishable default value should accept None to mean use the default 
value:

https://mail.python.org/pipermail/python-dev/2014-January/131673.html
https://mail.python.org/pipermail/python-dev/2014-January/131711.html

There are a fair number of places where people are abusing positional
parameters because they can't specify a good default.  I plan to put a stop to 
that, but first I need to give them a viable alternative.

In the specific case of itertools.repeat, I'm going to talk to the maintainer 
about what he wants to do.  The current behavior is too magic for Python and 
iiuc Guido regards it as a bug.  My guess is a default of None, as the author 
was deliberately preventing you from passing in -1.  But we shall see.

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[issue20341] Argument Clinic: add nullable ints

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I thought it was too late for 3.4, and the conversion should not change the 
behavior (we can change the behavior later in 3.5).

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

I guess we should ask Guido for clarification.  But I got the impression that 
we could *gently* tweak the signatures of functions if they were not 
representable in Python syntax.  He thought that giving the parameter of 
_sha1.sha1() a default value of b'' was fine for example, and in the case of 
itertools.repeat he considered the behavior so odd he was guessing it was a bug.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

Actually, the documented behavior of itertools.repeat() is that the times 
argument takes a default of None.

Equivalent to:

def repeat(object, times=None):
...

http://docs.python.org/3/library/itertools.html#itertools.repeat

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[issue20341] Argument Clinic: add nullable ints

[Jeremy Kloth]

Changes by Jeremy Kloth jeremy.kloth+python-trac...@gmail.com:


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[issue20341] Argument Clinic: add nullable ints

[Tal Einat]

Tal Einat added the comment:

Found two bugs in the draft patch in Objects/longobject.c:

1) In _PyLong_NullableIntConverter, there is no index variable, so the 
Py_DECREF and Py_XDECREF need to be removed from the end of the function.

2) In _PyLong_NullablePy_ssize_tConverter, the index variable will point to a 
Python object only when both if blocks are skipped. Therefore, the Py_XDECREF 
after fail: at the end of the function needs to be removed.

With these two changes, I successfully used this patch to convert deque_init in 
Modules/_collectionsmodule.c, simplifying the input validation code 
considerably.

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

Some things:
- the concept of a nullable thing in Python doesn't exist; why not optional?
- why is there a error field in the new structs?
- the fact that the structs are defined in longobject.h looks bonkers
- boolean fields can be char instead of int (and moved at the end to pack 
the structure more efficiently)
- PyLong_AsSsize_t can't fail?
- does this change have a performance impact?

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

- the concept of a nullable thing in Python doesn't exist; why not optional?

That's not what it means. Python parameters are optional if they
have a default value.  These parameters are nullable, in the sense
that they can be either of a specific type or None.  (But Noneable
seemed like a bad name).  They are not necessarily optional.

My inspiration for the name was Nullable in C#:

http://msdn.microsoft.com/en-us/library/1t3y8s4s.aspx

Argument Clinic already has converters that support nullable:
str, UNICODE, and Py_buffer.  These map to the semantics of 'z', 'S*', etc.  I 
guess we could change it if it were a terrible name, but it isn't.


 - why is there a error field in the new structs?

It seemed like a good idea at the time.  Maybe it's redundant.  Can I get back 
to you?


 - the fact that the structs are defined in longobject.h looks bonkers

Okay.  What would be better?  modsupport.h?


 - boolean fields can be char instead of int (and moved at the
   end to pack the structure more efficiently)

Is Python really compiled with packed structures?  I assumed -O3 turned on 
dword alignment in structures.

Aligned accesses are faster, and the additional memory use would be negligible. 
 These will only ever be declared as stack variables in parsing functions.


 - PyLong_AsSsize_t can't fail?

Not on an object returned by PyNumber_Index().  And by the way I literally 
copied and pasted the code that implements 'n'--it does that too.


 - does this change have a performance impact?

Compared to just using i, it adds the converter call and the == Py_None 
check.  The performance impact should be so small as to be difficult to measure.

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

 That's not what it means. Python parameters are optional if they
 have a default value.  These parameters are nullable, in the sense
 that they can be either of a specific type or None.  (But Noneable
 seemed like a bad name).  They are not necessarily optional.

int(or_none=True) ?

  - the fact that the structs are defined in longobject.h looks bonkers
 
 Okay.  What would be better?  modsupport.h?

Hmm, do we have a getargs.h ?

  - boolean fields can be char instead of int (and moved at the
end to pack the structure more efficiently)
 
 Is Python really compiled with packed structures?

You don't understand me. If you write:
  struct X { int A; char B; char C; }
the structure will be packed *by definition* (IIRC).

  - PyLong_AsSsize_t can't fail?
 
 Not on an object returned by PyNumber_Index().

And what if the long is too long to fit in a Py_ssize_t?

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[issue20341] Argument Clinic: add nullable ints

[Tal Einat]

Tal Einat added the comment:

Another bug in the patch:

In int_converter.__init__ in clinic.py, the patch adds:

if not isinstance(self.default, int):
fail(Illegal default value for int_converter)

This fails if no default is specified, e.g. for positional only argument. The 
condition should be:

if not (self.default is unspecified or isinstance(self.default, int)):

Ditto for Py_ssize_t_converter.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

 int(or_none=True) ?

Yes, that is a different name that seems to mean much the same thing.


 Hmm, do we have a getargs.h ?

No.  Would it help if I attached the output of ls Include
to this issue?


   - boolean fields can be char instead of int (and moved at the
 end to pack the structure more efficiently)
  
  Is Python really compiled with packed structures?
 
 You don't understand me. If you write:
   struct X { int A; char B; char C; }
 the structure will be packed *by definition* (IIRC).

I understand you fine.  That's not guaranteed by the C standard,
and I've used compilers that didn't pack structures by default.
More to the point, unaligned accesses on Intel are more expensive
than aligned accesses, and on other architectures I've written
C code using packed structures with unaligned accesses that
produced a bus error.

http://en.wikipedia.org/wiki/Data_structure_alignment#Data_structure_padding

And finally: I removed error--you were right, it was unnecessary
for the nullable ints.  So now it's just

  { int error; whatever i; }

Changing error to an char and moving it to the end would
save exactly zero bytes, because the compiler *will* align
stack variables to 4 byte boundaries.  I can give you a sample
C program if you want proof.


   - PyLong_AsSsize_t can't fail?
  Not on an object returned by PyNumber_Index().
 And what if the long is too long to fit in a Py_ssize_t?

Ah.  I thought PyNumber_Index guaranteed it would fit, but
I was mistaken.  Code is fixed.


New patch posted.  I believe this fixes all the bugs cited so far.


Also, I don't know what the right thing to do about itertools.repeat
is.  The current signature is a bug, it should not behave differently
between repeat(o, -1) and repeat(o, times=-1).  If we go by the
documentation, repeat(o, None) and repeat(o, times=None) are what we
want.  But I bet code out there relies on repeat(o, times=-1), so we
may be stuck supporting that.  You should consider my signature for
itertools.repeat in these draft patches as a proof of concept, not
as code that should be checked in.

The issue is being discussed on #19145.

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Added file: http://bugs.python.org/file33635/larry.nullable.ints.draft.2

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[issue20341] Argument Clinic: add nullable ints

[Antoine Pitrou]

Antoine Pitrou added the comment:

  int(or_none=True) ?
 
 Yes, that is a different name that seems to mean much the same thing.

and which is much more understandable by a Python developer.

 Changing error to an char and moving it to the end would
 save exactly zero bytes, because the compiler *will* align
 stack variables to 4 byte boundaries.

Except if other stack variables happen to be shorter than an int,
perhaps. But regardless, it doesn't cost anything to do so, so why not
do it?

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[issue20341] Argument Clinic: add nullable ints

[Tal Einat]

Tal Einat added the comment:

The new patch still uses NULLABLE_PY_SSIZE_T_INITIALIZE instead of 
NULLABLE_INT_T_INITIALIZE in one place in int_converter.converter_init.

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

Fix attached.  Might as well.

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Added file: http://bugs.python.org/file33639/larry.nullable.ints.draft.3

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

New submission from Larry Hastings:

Attached is a *preliminary* patch for Argument Clinic that adds nullable ints.  
If you use the converter int(nullable=True), then:

* The type you get back is not int but nullable_int_t, a structure
  containing three fields: error, is_null, and i.  error is 1 if
  there was an error in parsing, else 0.  is_null is 1 if None was
  passed in, else 0.  i is the integer value if an integer was
  passed in, else undefined.
* You may now use a default value of None.  If you do, is_null is 1.
* You may still use a default integer.  If you do, i will be set
  to that.
* If you use neither then obviously it's a required argument.


I'd appreciate just a preliminary review, saying something like
this is a good idea, keep going, or
you should change your approach, or
this is a bad idea and should not be committed, or
you look very handsome today Larry.

I don't remember who specifically needed the nullable ints, so I just added a 
bunch of Derby contestants.

Serhiy: I also added a fix for the bug you mentioned in #20294 after it was 
closed: the docstring for __new__ and __init__ methods now uses the class name 
instead of the name of the method.  Note that this fix doesn't matter much; 
once I commit a fix for #20189, that signature will always be hidden.

--
assignee: larry
components: Extension Modules
files: larry.nullable.ints.draft
messages: 208740
nosy: Yury.Selivanov, brett.cannon, georg.brandl, larry, serhiy.storchaka, 
vajrasky, zach.ware
priority: normal
severity: normal
stage: patch review
status: open
title: Argument Clinic: add nullable ints
type: enhancement
versions: Python 3.4
Added file: http://bugs.python.org/file33605/larry.nullable.ints.draft

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[issue20341] Argument Clinic: add nullable ints

[Larry Hastings]

Larry Hastings added the comment:

I should have mentioned--a sample using nullable ints is in itertoolsmodule.c.  
longobject.{ch} changed to add the converters, and obviously clinic.py changed 
to add support for the converters.

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[issue20341] Argument Clinic: add nullable ints

[Vajrasky Kok]

Vajrasky Kok added the comment:

 itertools.repeat.__doc__
'repeat(object, times=None)\nReturns an iterator which returns the object the 
specified number of times.\n\nIf times is None, returns the object endlessly.'
 itertools.repeat('a', times=None)
Traceback (most recent call last):
  File stdin, line 1, in module
TypeError: __new__() argument 2 must be (unspecified), not None

Also, maybe it's a good idea to add case for None and negative times in 
itertools.repeat unit test. You can get inspiration from 
http://bugs.python.org/issue19145

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