On Fri, 21 Jan 2005 17:04:11 -0800, Jeff Shannon <[EMAIL PROTECTED]> wrote:
>TB wrote:
>
>> Hi,
>>
>> Is there an elegant way to assign to a list from a list of unknown
>> size? For example, how could you do something like:
>>
>>
>>>>> a, b, c = (line.split(':'))
>>
>> if line could have less than three fields?
>
>(Note that you're actually assigning to a group of local variables,
>via tuple unpacking, not assigning to a list...)
>
>One could also do something like this:
>
> >>> l = "a:b:c".split(':')
> >>> a, b, c, d, e = l + ([None] * (5 - len(l)))
> >>> print (a, b, c, d, e)
>('a', 'b', 'c', None, None)
> >>>
Or
>>> a, b, c, d, e = ('a:b:c'.split(':')+[None]*4)[:5]
>>> print (a, b, c, d, e)
('a', 'b', 'c', None, None)
You could even be profligate and use *5 in place of that *4,
if that makes an easier idiom ;-)
Works if there's too many too:
>>> a, b = ('a:b:c'.split(':')+[None]*2)[:2]
>>> print (a, b)
('a', 'b')
>
>Personally, though, I can't help but think that, if you're not certain
>how many fields are in a string, then splitting it into independent
>variables (rather than, say, a list or dict) *cannot* be an elegant
>solution. If the fields deserve independent names, then they must
>have a definite (and distinct) meaning; if they have a distinct
>meaning (as opposed to being a series of similar items, in which case
>you should keep them in a list), then which field is it that's
>missing? Are you sure it's *always* the last fields? This feels to
>me like the wrong solution to any problem.
>
>Hm, speaking of fields makes me think of classes.
>
> >>> class LineObj:
>... def __init__(self, a=None, b=None, c=None, d=None, e=None):
>... self.a = a
>... self.b = b
>... self.c = c
>... self.d = d
>... self.e = e
>...
> >>> l = "a:b:c".split(':')
> >>> o = LineObj(*l)
> >>> o.__dict__
>{'a': 'a', 'c': 'c', 'b': 'b', 'e': None, 'd': None}
> >>>
>
>This is a bit more likely to be meaningful, in that there's almost
>certainly some logical connection between the fields of the line
>you're splitting and keeping them as a class demonstrates that
>connection, but it still seems a bit smelly to me.
>
That gives me an idea:
>>> def foo(a=None, b=None, c=None, d=None, e=None, *ignore):
... return a, b, c, d, e
...
>>> a, b, c, d, e = foo(*'a:b:c'.split(':'))
>>> print (a, b, c, d, e)
('a', 'b', 'c', None, None)
But then, might as well do:
>>> def bar(nreq, *args):
... if nreq <= len(args): return args[:nreq]
... return args+ (nreq-len(args))*(None,)
...
>>> a, b, c, d, e = bar(5, *'a:b:c'.split(':'))
>>> print (a, b, c, d, e)
('a', 'b', 'c', None, None)
>>> a, b = bar(2, *'a:b:c'.split(':'))
>>> print (a, b)
('a', 'b')
>>> a, b, c = bar(3, *'a:b:c'.split(':'))
>>> print (a, b, c)
('a', 'b', 'c')
But usually, I would like n + tail, where I know n is a safe bet, e.g.,
>>> def ntail(n, *args):
... return args[:n]+(args[n:],)
...
>>> a, b, t = ntail(2, *'a:b:c:d:e'.split(':'))
>>> print (a, b, t)
('a', 'b', ('c', 'd', 'e'))
>>> a, b, t = ntail(2, *'a:b'.split(':'))
>>> print (a, b, t)
('a', 'b', ())
People have asked to be able to spell that as
a, b, *t = 'a:b:c:d:e'.split(':')
Regards,
Bengt Richter
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