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Creating a tuple is faster than creating a list. Creating a list is slower because two memory blocks need to be accessed. An element in a tuple cannot be removed or replaced.
We can create a list of tuples i.e. the elements of the tuple can be enclosed in a list and thus will follow the characteristics in a similar manner as of a Python list. Since, Python Tuples utilize less amount of space, creating a list of tuples would be more useful in every aspect.
list( myBigList[i] for i in [87, 342, 217, 998, 500] )
I compared the answers with python 2.5.2:
19.7 usec: [ myBigList[i] for i in [87, 342, 217, 998, 500] ]
20.6 usec: map(myBigList.__getitem__, (87, 342, 217, 998, 500))
22.7 usec: itemgetter(87, 342, 217, 998, 500)(myBigList)
24.6 usec: list( myBigList[i] for i in [87, 342, 217, 998, 500] )
Note that in Python 3, the 1st was changed to be the same as the 4th.
Another option would be to start out with a numpy.array which allows indexing via a list or a numpy.array:
>>> import numpy
>>> myBigList = numpy.array(range(1000))
>>> myBigList[(87, 342, 217, 998, 500)]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: invalid index
>>> myBigList[[87, 342, 217, 998, 500]]
array([ 87, 342, 217, 998, 500])
>>> myBigList[numpy.array([87, 342, 217, 998, 500])]
array([ 87, 342, 217, 998, 500])
The tuple doesn't work the same way as those are slices.
What about this:
from operator import itemgetter
itemgetter(0,2,3)(myList)
('foo', 'baz', 'quux')
Maybe a list comprehension is in order:
L = ['a', 'b', 'c', 'd', 'e', 'f']
print [ L[index] for index in [1,3,5] ]
Produces:
['b', 'd', 'f']
Is that what you are looking for?
It isn't built-in, but you can make a subclass of list that takes tuples as "indexes" if you'd like:
class MyList(list):
def __getitem__(self, index):
if isinstance(index, tuple):
return [self[i] for i in index]
return super(MyList, self).__getitem__(index)
seq = MyList("foo bar baaz quux mumble".split())
print seq[0]
print seq[2,4]
print seq[1::2]
printing
foo
['baaz', 'mumble']
['bar', 'quux']
>>> map(myList.__getitem__, (2,2,1,3))
('baz', 'baz', 'bar', 'quux')
You can also create your own List class which supports tuples as arguments to __getitem__ if you want to be able to do myList[(2,2,1,3)].
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