# Filling in a tuple from unknown size list

Discussion in 'Python' started by boblatest, Nov 27, 2009.

1. ### boblatestGuest

Hello all,

normal nntp account.)

Here's my question: Given a list of onknown length, I'd like to be
able to do the following:

(a, b, c, d, e, f) = list

If the list has fewer items than the tuple, I'd like the remaining
tuple elements to be set to "None". If the list is longer, I'd like
the excess elements to be ignored.

The code snippet below does what I want, I was just wondering if there
was an interesting "Pythonic" way of expressing the same thing.

Thanks,
robert

def iter_inf(li, n):
for i in range(n):
if i < len(li):
r = li
else:
r = None
i += 1
yield r

li = ['a', 'b', 'c']
(a, b, c, d, e) = iter_inf(li, 5)
print a, b, c, d, e
boblatest, Nov 27, 2009

2. ### Peter OttenGuest

boblatest wrote:

> Hello all,
>
> normal nntp account.)
>
> Here's my question: Given a list of onknown length, I'd like to be
> able to do the following:
>
> (a, b, c, d, e, f) = list
>
> If the list has fewer items than the tuple, I'd like the remaining
> tuple elements to be set to "None". If the list is longer, I'd like
> the excess elements to be ignored.
>
> The code snippet below does what I want, I was just wondering if there
> was an interesting "Pythonic" way of expressing the same thing.
>
> Thanks,
> robert
>
> def iter_inf(li, n):
> for i in range(n):
> if i < len(li):
> r = li
> else:
> r = None
> i += 1
> yield r
>
>
> li = ['a', 'b', 'c']
> (a, b, c, d, e) = iter_inf(li, 5)
> print a, b, c, d, e

Here's an alternative implementation that works with arbitrary iterables:

>>> from itertools import chain, repeat, islice
>>> a, b, c = islice(chain("a", repeat(None)), 3)
>>> a, b, c

('a', None, None)
>>> a, b, c = islice(chain("abcde", repeat(None)), 3)
>>> a, b, c

('a', 'b', 'c')

Peter
Peter Otten, Nov 27, 2009

3. ### Stefan BehnelGuest

boblatest, 27.11.2009 13:18:
> Here's my question: Given a list of onknown length, I'd like to be
> able to do the following:
>
> (a, b, c, d, e, f) = list
>
> If the list has fewer items than the tuple, I'd like the remaining
> tuple elements to be set to "None". If the list is longer, I'd like
> the excess elements to be ignored.

fillUp = [None] * 6
(a, b, c, d, e, f) = (list + fillUp)[:6]

Stefan
Stefan Behnel, Nov 27, 2009
4. ### Dave AngelGuest

Peter Otten wrote:
> boblatest wrote:
>
>
>> Hello all,
>>
>> normal nntp account.)
>>
>> Here's my question: Given a list of onknown length, I'd like to be
>> able to do the following:
>>
>> (a, b, c, d, e, f) = list
>>
>> If the list has fewer items than the tuple, I'd like the remaining
>> tuple elements to be set to "None". If the list is longer, I'd like
>> the excess elements to be ignored.
>>
>> The code snippet below does what I want, I was just wondering if there
>> was an interesting "Pythonic" way of expressing the same thing.
>>
>> Thanks,
>> robert
>>
>> def iter_inf(li, n):
>> for i in range(n):
>> if i < len(li):
>> r = li
>> else:
>> r = None
>> i += 1
>> yield r
>>
>>
>> li = ['a', 'b', 'c']
>> (a, b, c, d, e) = iter_inf(li, 5)
>> print a, b, c, d, e
>>

>
> Here's an alternative implementation that works with arbitrary iterables:
>
>
>>>> from itertools import chain, repeat, islice
>>>> a, b, c = islice(chain("a", repeat(None)), 3)
>>>> a, b, c
>>>>

> ('a', None, None)
>
>>>> a, b, c = islice(chain("abcde", repeat(None)), 3)
>>>> a, b, c
>>>>

> ('a', 'b', 'c')
>
> Peter
>
>
>

Python 3.x has some extension to the way tuple unpacking works, and may
solve this problem (or half of it). I'm too lazy this morning to look
it up.

In Python 2.x I can't see any better way than Peter's elegant solution.
I would originally have done the chain and repeat, but followed it by
[:3] slice notation. I suspect that building an actual list would be
cheap enough, but I like the islice better.

DaveA
Dave Angel, Nov 27, 2009
5. ### Jon ClementsGuest

On 27 Nov, 12:18, boblatest <> wrote:
> Hello all,
>
> normal nntp account.)
>
> Here's my question: Given a list of onknown length, I'd like to be
> able to do the following:
>
> (a, b, c, d, e, f) = list
>
> If the list has fewer items than the tuple, I'd like the remaining
> tuple elements to be set to "None". If the list is longer, I'd like
> the excess elements to be ignored.
>
> The code snippet below does what I want, I was just wondering if there
> was an interesting "Pythonic" way of expressing the same thing.
>
> Thanks,
> robert
>
> def iter_inf(li, n):
>     for i in range(n):
>         if i < len(li):
>             r = li
>         else:
>             r = None
>         i += 1
>         yield r
>
> li = ['a', 'b', 'c']
> (a, b, c, d, e) =  iter_inf(li, 5)
> print a, b, c, d, e

An alternative to Peter's itertools implementation is this
monstrosity...

import re

class name_elements:
def __init__(self, iterable, *names):
self.__names = set(names)
if len(self.__names) != len(names):
raise ValueError('names must be unique')
for it, name in zip(iterable, names):
if not re.match('[a-zA-Z][a-zA-Z0-9_]*', name):
raise ValueError("name '%s' is not valid" % name)
setattr(self, name, it)
def __getattr__(self, item):
if item not in self.__names:
raise ValueError("name '%s' not present" % item)
return self.__dict__.get(item, None)

>>> res = name_elements(['a', 'b', 'c'], 'a', 'b', 'c', 'd', 'e')
>>> print res.a, res.b, res.c, res.d, res.e

a b c None None
>>> print res.f

Traceback (most recent call last):
File "<pyshell#23>", line 1, in <module>
print res.f
File "/home/jon/rubbish.py", line 10, in __getattr__
raise AttributeError("name '%s' not present" % item)
AttributeError: name 'f' not present

It removes the need to know the number being unpacked (I can see this
being overlooked) and the variable names could come from a list
instead of being named in code. Asking for a name that doesn't exist
is an exception, while all other values default to None.

However, it's not totally identical to unpacking... (and most likely
slower)

I s'pose namedtuple could also be abused in a similar fashion.

Or, if you don't mind []'ing:
>>> dd = defaultdict(lambda: None)
>>> dd.update(zip(['a', 'b', 'c'], range(5)))
>>> print dd['a'], dd['c'], dd['blah']

0 2 None

Is it obvious I'm trying to avoid doing proper work!?

Cheers,
Jon.
Jon Clements, Nov 27, 2009
6. ### Steven D'ApranoGuest

On Fri, 27 Nov 2009 04:18:08 -0800, boblatest wrote:

> Here's my question: Given a list of onknown length, I'd like to be able
> to do the following:
>
> (a, b, c, d, e, f) = list
>
> If the list has fewer items than the tuple, I'd like the remaining tuple
> elements to be set to "None". If the list is longer, I'd like the excess
> elements to be ignored.

I'd call that a code-smell. If I saw that in code, I'd think long and
hard about why it was there and if I could eliminate the names a...f and
just work directly with the list.

But if you really do need it, I think the simplest and best way is to use
the technique Stefan suggested:

a, b, c, d, e, f = (list + [None]*6)[:6]

provided list is short. If you fear that list might be huge and copying
it will be prohibitively expensive:

a, b, c, d, e, f = (list[:6] + [None]*6)[:6]

--
Steven
Steven D'Aprano, Nov 27, 2009
7. ### MelGuest

Steven D'Aprano wrote:
> On Fri, 27 Nov 2009 04:18:08 -0800, boblatest wrote:
>> Here's my question: Given a list of onknown length, I'd like to be able
>> to do the following:
>>
>> (a, b, c, d, e, f) = list
>>
>> If the list has fewer items than the tuple, I'd like the remaining tuple
>> elements to be set to "None". If the list is longer, I'd like the excess
>> elements to be ignored.

> I'd call that a code-smell. If I saw that in code, I'd think long and
> hard about why it was there and if I could eliminate the names a...f and
> just work directly with the list.

It's a common enough thing at the boundaries of your program, letting user
input in through the gates, as it were. Deeper in, I agree; that stuff
should have been dealt with at the gates.

Mel.
Mel, Nov 27, 2009
8. ### Stefan BehnelGuest

Mel, 27.11.2009 18:47:
> Steven D'Aprano wrote:
>> On Fri, 27 Nov 2009 04:18:08 -0800, boblatest wrote:
>>> Here's my question: Given a list of onknown length, I'd like to be able
>>> to do the following:
>>>
>>> (a, b, c, d, e, f) = list
>>>
>>> If the list has fewer items than the tuple, I'd like the remaining tuple
>>> elements to be set to "None". If the list is longer, I'd like the excess
>>> elements to be ignored.

>
>> I'd call that a code-smell. If I saw that in code, I'd think long and
>> hard about why it was there and if I could eliminate the names a...f and
>> just work directly with the list.

>
> It's a common enough thing at the boundaries of your program, letting user
> input in through the gates, as it were. Deeper in, I agree; that stuff
> should have been dealt with at the gates.

But that may have a code smell on it, too. In most cases, when users
provide excessive arguments that the program would ignore, that's best
treated as an error.

Stefan
Stefan Behnel, Nov 27, 2009
9. ### John MachinGuest

On Nov 27, 11:18 pm, boblatest <> wrote:
> Hello all,
>
> normal nntp account.)
>
> Here's my question: Given a list of onknown length, I'd like to be
> able to do the following:
>
> (a, b, c, d, e, f) = list
>
> If the list has fewer items than the tuple, I'd like the remaining
> tuple elements to be set to "None". If the list is longer, I'd like
> the excess elements to be ignored.

WRONG -- sweeping excess input under the carpet is a nasssssty perlish
trick.
John Machin, Nov 28, 2009
10. ### Ned DeilyGuest

In article
<>,
inhahe <> wrote:
> maybe that thing in python 3 that someone mentioned is the answer, but
> otherwise i always think Python should admit something like this:
>
> a, b, c, *d = list
>
> i.e. if list were [1,2,3,4,5], you'd get a=1, b=2, c=3, d=[4, 5]

Extended iterable unpacking (http://www.python.org/dev/peps/pep-3132/)
is implemented in python 3.

\$ python3
Python 3.1.1 (r311:74543, Aug 24 2009, 18:44:04)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
>>> a, b, c, *d = [1,2,3,4,5]
>>> d

[4, 5]

--
Ned Deily,
Ned Deily, Nov 29, 2009