Explicitly select items from a list or tuple

Question

list( myBigList[i] for i in [87, 342, 217, 998, 500] )

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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.

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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)].