Kit Kit - 1 month ago 4
Python Question

Explicitly select items from a Python list or tuple

I have the following Python list (can also be a tuple):

myList = ['foo', 'bar', 'baz', 'quux']


I can say

>>> myList[0:3]
['foo', 'bar', 'baz']
>>> myList[::2]
['foo', 'baz']
>>> myList[1::2]
['bar', 'quux']


How do I explicitly pick out items whose indices have no specific patterns? For example, I want to select
[0,2,3]
. Or from a very big list of 1000 items, I want to select
[87, 342, 217, 998, 500]
. Is there some Python syntax that does that? Something that looks like:

>>> myBigList[87, 342, 217, 998, 500]

Answer
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.