MRAB wrote:
gburde...@gmail.com wrote:
Is there a built-in method in python that lets you specify a "default"
value that will be returned whenever you try to access a list item
that is out of bounds? Basically, it would be a function like this:
def item(x,index,default):
try:
return x[index]
except IndexError:
return default
So if a=[0,1,2,3], then item(a,0,44)=0, item(a,1,44)=1, and item(a,
1000,44)=44, item(a,-1000,44)=44
What I want to know is whether there is a built-in method or notation
for this.
>
There's no built-in method or notation for that.
> What if, for example, we could do something like a [1000,44] ?
That's actually using a tuple (1000, 44) as the index. Such tuples can
be used as keys in a dict and are used in numpy for indexing
multi-dimensional arrays, so it's definitely a bad idea.
If such a method were added to the 'list' class then the best option (ie
most consistent with other classes) would be get(index, default=None).
But there's nothing stopping you from creating your own subclass
of "list" that allows for defaults:
class DefaultList(list):
def __init__(self, default, *args, **kwargs):
list.__init__(self, *args, **kwargs)
self.default = default
def __getitem__(self, index):
try:
return list.__getitem__(self, index)
except IndexError:
return self.default
ml = DefaultList(42, [1,2,3,4])
for i in range(-5,5):
print i, ml[i]
(yeah, there's likely a "proper" way of using super() to do the
above instead of "list.__<whatever>__" but the above worked for a
quick 3-minute example composed in "ed"). The output of the
above is
-5 42
-4 1
-3 2
-2 3
-1 4
0 1
1 2
2 3
3 4
4 42
One might want to add other methods for __add__/__radd__ so that
a DefaultList is returned instead of a "list", but this is
python...the sky's the limit.
-tkc
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