Arithmetic sequences in Python

[Paul Rubin]

Re: Arithmetic sequences in Python

aleax@mail.comc ast.net (Alex Martelli) writes:[color=blue]
> I much prefer the current arrangement where dict(a=b,c=d) means {'a':b,
> 'c':d} -- it's much more congruent to how named arguments work for every
> other case. Would you force us to quote argument names in EVERY
> functioncall... ?![/color]

Ehh, ok. There could be some special marker to evaluate the lhs, but
the present method is fine too.

[Paul Rubin]

Re: Arithmetic sequences in Python

aleax@mail.comc ast.net (Alex Martelli) writes:[color=blue][color=green]
> > How would you make a one-element list, which we'd currently write as [3]?
> > Would you have to say list((3,))?[/color]
>
> Yep. I don't particularly like the "mandatory trailing comma" in the
> tuple's display form, mind you, but, if it's good enough for tuples, and
> good enough for sets (how else would you make a one-element set?),[/color]

If you really want to get rid of container literals, maybe the best
way is with constructor functions whose interfaces are slightly
different from the existing type-coercion functions:

listx(1,2,3) => [1, 2, 3]
listx(3) => [3]
listx(listx(3)) => [[3]]
dictx((a,b), (c,d)) => {a:b, c:d}
setx(a,b,c) => Set((a,b,c))

listx/dictx/setx would be the display forms as well as the constructor forms.

[Tom Anderson]

Re: Arithmetic sequences in Python

On Fri, 20 Jan 2006, it was written:
[color=blue]
> aleax@mail.comc ast.net (Alex Martelli) writes:
>[color=green][color=darkred]
>>> How would you make a one-element list, which we'd currently write as
>>> [3]? Would you have to say list((3,))?[/color]
>>
>> Yep. I don't particularly like the "mandatory trailing comma" in the
>> tuple's display form, mind you, but, if it's good enough for tuples,
>> and good enough for sets (how else would you make a one-element set?),[/color]
>
> If you really want to get rid of container literals, maybe the best way
> is with constructor functions whose interfaces are slightly different
> from the existing type-coercion functions:
>
> listx(1,2,3) => [1, 2, 3]
> listx(3) => [3]
> listx(listx(3)) => [[3]]
> dictx((a,b), (c,d)) => {a:b, c:d}
> setx(a,b,c) => Set((a,b,c))
>
> listx/dictx/setx would be the display forms as well as the constructor forms.[/color]

Could these even replace the current forms? If you want the equivalent of
list(sometuple) , write list(*sometuple ). With a bit of cleverness down in
the worky bits, this could be implemented to avoid the apparent overhead
of unpacking and then repacking the tuple. In fact, in general, it would
be nice if code like:

def f(*args):
fondle(args)

foo = (1, 2, 3)
f(*foo)

Would avoid the unpack/repack.

The problem is that you then can't easily do something like:

mytable = ((1, 2, 3), ("a", "b", "c"), (Tone.do, Tone.re, Tone.mi))
mysecondtable = map(list, mytable)

Although that's moderately easy to work around with possibly the most
abstract higher-order-function i've ever written:

def star(f):
def starred_f(args) :
return f(*args)
return starred_f

Which lets us write:

mysecondtable = map(star(list), mytable)

While we're here, we should also have the natural complement of star, its
evil mirror universe twin:

def bearded_star(f) :
def bearded_starred _f(*args):
return f(args)
return bearded_starred _f

Better names (eg "unpacking" and "packing") would obviously be needed.

tom

--
I might feel irresponsible if you couldn't go almost anywhere and see
naked, aggressive political maneuvers in iteration, marinating in your
ideology of choice. That's simply not the case. -- Tycho Brahae

[Paul Rubin]

Re: Arithmetic sequences in Python

Tom Anderson <twic@urchin.ea rth.li> writes:[color=blue][color=green]
> > listx/dictx/setx would be the display forms as well as the constructor forms.[/color]
>
> Could these even replace the current forms? If you want the equivalent
> of list(sometuple) , write list(*sometuple ).[/color]

The current list function is supposed to be something like a typecast:

list() = []
xlist() = [] # ok

list(list()) = [] # casting a list to a list does nothing
xlist(xlist()) = [[]] # make a new list, not the same

list(xrange(4)) = [0,1,2,3]
xlist(xrange(4) ) = [xrange(4)] # not the same

list((1,2)) = [1,2]
xlist((1,2)) = [(1,2)]

etc.

[Tom Anderson]

Re: Arithmetic sequences in Python

On Sat, 21 Jan 2006, it was written:
[color=blue]
> Tom Anderson <twic@urchin.ea rth.li> writes:
>[color=green][color=darkred]
>>> listx/dictx/setx would be the display forms as well as the constructor
>>> forms.[/color]
>>
>> Could these even replace the current forms? If you want the equivalent
>> of list(sometuple) , write list(*sometuple ).[/color]
>
> The current list function is supposed to be something like a typecast:[/color]

A what?

;-|
[color=blue]
> list() = []
> xlist() = [] # ok
>
> list(list()) = [] # casting a list to a list does nothing
> xlist(xlist()) = [[]] # make a new list, not the same
>
> list(xrange(4)) = [0,1,2,3]
> xlist(xrange(4) ) = [xrange(4)] # not the same
>
> list((1,2)) = [1,2]
> xlist((1,2)) = [(1,2)][/color]

True, but so what? Is it that it has to be that way, or is it just that it
happens to be that way now?

tom

--
It's the 21st century, man - we rue _minutes_. -- Benjamin Rosenbaum

[Steve Holden]

Re: Arithmetic sequences in Python

Paul Rubin wrote:[color=blue]
> Tom Anderson <twic@urchin.ea rth.li> writes:
>[color=green][color=darkred]
>>>listx/dictx/setx would be the display forms as well as the constructor forms.[/color]
>>
>>Could these even replace the current forms? If you want the equivalent
>>of list(sometuple) , write list(*sometuple ).[/color]
>
>
> The current list function is supposed to be something like a typecast:
>[/color]
list() isn't a function, it's a type.
[color=blue][color=green][color=darkred]
>>> type(list)[/color][/color][/color]
<type 'type'>

I'm not happy about the way the documentation represents types as
functions, as this obscures the whole essence of Python's object
orientation.

[color=blue]
> list() = []
> xlist() = [] # ok
>
> list(list()) = [] # casting a list to a list does nothing
> xlist(xlist()) = [[]] # make a new list, not the same
>
> list(xrange(4)) = [0,1,2,3]
> xlist(xrange(4) ) = [xrange(4)] # not the same
>
> list((1,2)) = [1,2]
> xlist((1,2)) = [(1,2)]
>
> etc.[/color]

I presume that here "=" means "evaluates to"?

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

[Christoph Zwerschke]

Re: Arithmetic sequences in Python

Alex Martelli wrote:[color=blue][color=green][color=darkred]
>>>> print set([1,2,3])[/color][/color]
> set([1, 2, 3])
>
> input and output could be identical. Do YOU have any good reason why
> sets should print out as set(...) and lists should NOT print out as
> list(...)? Is 'list' somehow "deeper" than 'set', to deserve a special
> display-form syntax which 'set' doesn't get? Or are you enshrining a
> historical accident to the level of an erroneously assumed principle?[/color]

These are valid points, but they lead me to the opposite conclusion: Why
not let {a,b,c} stand for set([a,b,c])? That would be very intuitive
since it is the mathematical notation already and since it resembles the
notation of dictionaries which are similar to sets.

(This has been probably discussed already. One problem I'm already
seeing is that {} would be ambiguous.)

Anyway, I think the fact that the notation for a set is clumsy is no
good reason to make the notation for a list clumsy as well.

-- Christoph

[Paul Rubin]

Re: Arithmetic sequences in Python

Steve Holden <steve@holdenwe b.com> writes:[color=blue][color=green]
> > The current list function is supposed to be something like a
> > typecast:
> >[/color]
> list() isn't a function, it's a type.[/color]

I'm not sure what the distinction is supposed to be. "list" is anyway
callable, and lambda a:list(a) is certainly a function.
[color=blue][color=green]
> > xlist((1,2)) = [(1,2)]
> > etc.[/color]
>
> I presume that here "=" means "evaluates to"?[/color]

Yeah, although I meant something more informal, like mathematical
equivalence.

Maybe the preferred spellings for the constructors would use capital
letters: List, Dict, Set, instead of listx or xlist or whatever. That
would break the current meaning of Set but I hope not much depends on
that yet.

[Alex Martelli]

Re: Arithmetic sequences in Python

Paul Rubin <http://phr.cx@NOSPAM.i nvalid> wrote:
[color=blue]
> Steve Holden <steve@holdenwe b.com> writes:[color=green][color=darkred]
> > > The current list function is supposed to be something like a
> > > typecast:
> > >[/color]
> > list() isn't a function, it's a type.[/color]
>
> I'm not sure what the distinction is supposed to be. "list" is anyway[/color]

You can subclass a type, you can check for it with isinstance, etc, all
things you couldn't do if list was still a factory function as in 2.1
and back.


Alex

[Alex Martelli]

Re: Arithmetic sequences in Python

Christoph Zwerschke <cito@online.de > wrote:
...[color=blue]
> These are valid points, but they lead me to the opposite conclusion: Why
> not let {a,b,c} stand for set([a,b,c])? That would be very intuitive[/color]

As syntax sugar goes, that would be on a par with the current "dict
display" notation, at least.
[color=blue]
> (This has been probably discussed already. One problem I'm already
> seeing is that {} would be ambiguous.)[/color]

Yep, using {} for both sets and dicts wouldn't be a good idea. I
suspect most core Python developers think of dicts as more fundamental
than sets, so... (I may disagree, but I just don't care enough about
such syntax sugar to consider even starting a debate about it on
python-dev, particularly knowing it would fail anyway).
[color=blue]
> Anyway, I think the fact that the notation for a set is clumsy is no
> good reason to make the notation for a list clumsy as well.[/color]

I don't agree that <typename>(<arg uments>) is a clumsy notation, in
general; rather, I consider "clumsy" much of the syntax sugar that is
traditional in Python. For example, making a shallow copy of a list L
with L[:] is what strikes me as clumsy -- list(L) is SO much better.
And I vastly prefer dict(a=1,b=2) over the clumsy {'a':1, 'b':2}.

I suspect I'm unusual in that being deeply familiar with some notation
and perfectly used to it does NOT necessarily make me LIKE that
notation, nor does it make me any less disposed to critical reappraisal
of it -- the brains of most people do appear to equate habit with
appreciation. In the light of my continuous and unceasing critical
reappraisal of Python's syntax choices, I am quite convinced that many
of them are really brilliant -- with the "display forms" of some
built-in types being one area where I find an unusually high density of
non-brilliance, AKA clumsiness. But, that's just me.


Alex

[Christoph Zwerschke]

Re: Arithmetic sequences in Python

Alex Martelli wrote:[color=blue]
> Yep, using {} for both sets and dicts wouldn't be a good idea. I
> suspect most core Python developers think of dicts as more fundamental
> than sets, so... (I may disagree, but I just don't care enough about
> such syntax sugar to consider even starting a debate about it on
> python-dev, particularly knowing it would fail anyway).[/color]

I'm still not convinced. At least I'd prefer {a,b,c} over any other
proposed solutions (http://wiki.python.org/moin/Python3%2e0Suggestions)
such as <a,b,c> or |a,b,c|.

You can argue that the notation for sets can be clumsy because they
aren't used so much as lists or dicts, but you can also argue the other
way around that sets aren't used much because the notation is clumsy
(and because they didn't exist from the beginning).

For instance, if sets had a simple notation, they could be used in more
cases, e.g. replace integer masks (see again

http://wiki.python.org/moin/Python3%2e0Suggestions):

pat = re.compile("som e pattern", re.I|re.S|re.X)
would become
pat = re.compile("som e pattern", {re.I, re.S, re.X})
[color=blue]
> I don't agree that <typename>(<arg uments>) is a clumsy notation, in
> general; rather, I consider "clumsy" much of the syntax sugar that is
> traditional in Python.[/color]

If you really could write list(a,b,c) instead of list((a,b,c)) I do
somewhat agree.
[color=blue]
> For example, making a shallow copy of a list L
> with L[:] is what strikes me as clumsy -- list(L) is SO much better.[/color]

Certainly.
[color=blue]
> And I vastly prefer dict(a=1,b=2) over the clumsy {'a':1, 'b':2}.[/color]

Ok, but only as long as you have decent keys...
[color=blue]
> I suspect I'm unusual in that being deeply familiar with some notation
> and perfectly used to it does NOT necessarily make me LIKE that
> notation, nor does it make me any less disposed to critical reappraisal
> of it -- the brains of most people do appear to equate habit with
> appreciation. In the light of my continuous and unceasing critical
> reappraisal of Python's syntax choices, I am quite convinced that many
> of them are really brilliant -- with the "display forms" of some
> built-in types being one area where I find an unusually high density of
> non-brilliance, AKA clumsiness. But, that's just me.[/color]

Ordinary people are lazy. If we have learned something and got
accustomed to it, we don't want to relearn. It is inconvenient. And
there is this attitude: "If I had so much trouble learning a clumsy
notation, why should future generations have it easier."

And in programming languages, you also have the downward compatibility
problem. Having to change all your old programs makes people even more
dislike any thoughts about changes...

-- Christoph

[Steven D'Aprano]

Re: Arithmetic sequences in Python

On Sun, 22 Jan 2006 16:40:48 -0800, Paul Rubin wrote:
[color=blue]
> Steve Holden <steve@holdenwe b.com> writes:[color=green][color=darkred]
>> > The current list function is supposed to be something like a
>> > typecast:
>> >[/color]
>> list() isn't a function, it's a type.[/color]
>
> I'm not sure what the distinction is supposed to be. "list" is anyway
> callable, and lambda a:list(a) is certainly a function.[/color]


class Parrot:
def __init__(self):
pass

Parrot is callable. Is it a function?


Types are types, classes are classes, functions are functions.

Admittedly I still confused between the various flavours of functions
(function, bound method, unbound method, class method, static method...)
*wink* but the difference between types and functions is fairly clear.

Just don't ask about the difference between type and class... *wink*



--
Steven.

[Bengt Richter]

Re: Arithmetic sequences in Python

On Mon, 23 Jan 2006 21:43:16 +1100, Steven D'Aprano <steve@REMOVETH IScyber.com.au> wrote:
[color=blue]
>On Sun, 22 Jan 2006 16:40:48 -0800, Paul Rubin wrote:
>[color=green]
>> Steve Holden <steve@holdenwe b.com> writes:[color=darkred]
>>> > The current list function is supposed to be something like a
>>> > typecast:
>>> >
>>> list() isn't a function, it's a type.[/color]
>>
>> I'm not sure what the distinction is supposed to be. "list" is anyway
>> callable, and lambda a:list(a) is certainly a function.[/color]
>
>
>class Parrot:
> def __init__(self):
> pass
>
>Parrot is callable. Is it a function?
>[/color]
No. It is an object that inherits a __call__ method that makes it callable with a (<args>) source syntax trailer
in a similar way to an object created with a def (which is a function in python convention, and which
also inherits a __call__ method), but the similarity does not make Parrot a function:
[color=blue][color=green][color=darkred]
>>> class Parrot:[/color][/color][/color]
... def __init__(self):
... pass
...
(BTW you could have inherited a do-nothing __init__ ;-)
[color=blue][color=green][color=darkred]
>>> type(Parrot).mr o()[/color][/color][/color]
[<type 'classobj'>, <type 'object'>][color=blue][color=green][color=darkred]
>>> type(Parrot).mr o()[0].__call__[/color][/color][/color]
<slot wrapper '__call__' of 'classobj' objects>
Or, showing more explicitly where it comes from:[color=blue][color=green][color=darkred]
>>> type(Parrot).mr o()[0].__dict__['__call__'][/color][/color][/color]
<slot wrapper '__call__' of 'classobj' objects>

Invoked:[color=blue][color=green][color=darkred]
>>> type(Parrot).mr o()[0].__dict__['__call__'](Parrot)[/color][/color][/color]
<__main__.Parro t instance at 0x02EF340C>

Hm, actually that is like calling the im_func of the unbound method of a newstyle class ...
I think maybe actually Parrot() causes
[color=blue][color=green][color=darkred]
>>> type(Parrot).mr o()[0].__dict__['__call__'].__get__(Parrot , type(Parrot))()[/color][/color][/color]
<__main__.Parro t instance at 0x02EF398C>

A function is also an object with a __call__ method. E.g., compare above the line with
calling foo (after definition just below) the long way:
[color=blue][color=green][color=darkred]
>>> def foo(): return 'returned by foo'[/color][/color][/color]
...[color=blue][color=green][color=darkred]
>>> type(foo).mro()[0].__dict__['__call__'].__get__(foo, type(foo))()[/color][/color][/color]
'returned by foo'

Playing with that a little:[color=blue][color=green][color=darkred]
>>> type(foo).mro()[/color][/color][/color]
[<type 'function'>, <type 'object'>][color=blue][color=green][color=darkred]
>>> type(foo).mro()[0][/color][/color][/color]
<type 'function'>[color=blue][color=green][color=darkred]
>>> type(foo).mro()[0].__call__[/color][/color][/color]
<slot wrapper '__call__' of 'function' objects>[color=blue][color=green][color=darkred]
>>> type(foo).mro()[0].__call__(foo)[/color][/color][/color]
'returned by foo'[color=blue][color=green][color=darkred]
>>> foo.__call__[/color][/color][/color]
<method-wrapper object at 0x02EF340C>[color=blue][color=green][color=darkred]
>>> foo.__call__()[/color][/color][/color]
'returned by foo'[color=blue][color=green][color=darkred]
>>> type(foo).mro()[0].__call__.__get __[/color][/color][/color]
<method-wrapper object at 0x02EF340C>[color=blue][color=green][color=darkred]
>>> type(foo).mro()[0].__call__.__get __(foo, type(foo))[/color][/color][/color]
<method-wrapper object at 0x02EF39AC>[color=blue][color=green][color=darkred]
>>> type(foo).mro()[0].__call__.__get __(foo, type(foo))()[/color][/color][/color]
'returned by foo'
or[color=blue][color=green][color=darkred]
>>> foo()[/color][/color][/color]
'returned by foo'
[color=blue]
>
>Types are types, classes are classes, functions are functions.[/color]
classes seem to be classobjs, designed to implement classic class behavior
but using the new machinery to achieve compatible integration.
[color=blue]
>
>Admittedly I still confused between the various flavours of functions
>(function, bound method, unbound method, class method, static method...)
>*wink* but the difference between types and functions is fairly clear.
>
>Just don't ask about the difference between type and class... *wink*
>[/color]
Why not? ;-)

Regards,
Bengt Richter