default value in a list
Jeff Shannon
jeff at ccvcorp.com
Fri Jan 21 20:04:11 EST 2005
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)
>>>
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.
Jeff Shannon
Technician/Programmer
Credit International
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