Accessing non-consecutive elements of a list or string in python

Question

As far as I can tell, this is not officially not possible, but is there a "trick" to access arbitrary non-sequential elements of a list by slicing?

For example:

>>> L = range(0,101,10) >>> L [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100] 

Now I want to be able to do

a,b = L[2,5] 

so that a == 20 and b == 50

One way besides two statements would be something silly like:

a,b = L[2:6:3][:2] 

But that doesn't scale at all to irregular intervals.

Maybe with list comprehension using the indices I want?

[L[x] for x in [2,5]] 

I would love to know what is recommended for this common problem.

Answer 1

Probably the closest to what you are looking for is itemgetter (or look here for Python 2 docs):

>>> L = list(range(0, 101, 10))  # works in Python 2 or 3 >>> L [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100] >>> from operator import itemgetter >>> itemgetter(2, 5)(L) (20, 50) 

Answer 2

If you can use numpy, you can do just that:

>>> import numpy >>> the_list = numpy.array(range(0,101,10)) >>> the_indices = [2,5,7] >>> the_subset = the_list[the_indices] >>> print the_subset, type(the_subset) [20 50 70] <type 'numpy.ndarray'> >>> print list(the_subset) [20, 50, 70] 

numpy.array is very similar to list, just that it supports more operation, such as mathematical operations and also arbitrary index selection like we see here.