islice

Learn slicing with islice.

islice

islice(iterable, stop) or islice(iterable, start, stop, step) takes a portion of an iterable. It is similar to list slicing but also works on iterators; it does not create an intermediate list.

Example: list(islice(range(10), 3)) returns [0, 1, 2]. list(islice(range(10), 2, 8, 2)) returns [2, 4, 6]. Negative indices are not supported; this is a restriction compared to list slicing.

Memory advantage

islice gives a memory advantage in large data streams. For example, while reading a file line by line, islice(file, 100) takes the first 100 lines without loading the whole file into memory.

Similarly, on infinite iterators islice(count(1), 10) safely takes the first 10 items. When wrapped with list() the result becomes a list; but using it directly in a loop keeps memory usage minimal.

Exercise Take the first three with islice

Start with from itertools import islice. Take the first three items of range with result = list(islice(range(10), 3)). Then print with print(result).

Show example solution

# Take the first three of range with islice
from itertools import islice

result = list(islice(range(10), 3))
print(result)

Test yourself

When is islice preferred?
- To slice iterators
- When negative indices are needed
- Only on lists
- To create a new list
Show answer

To slice iterators — islice slices iterators; it does not produce a list.

islice call

from itertools import islice

result = list(____(range(10), 3))
print(result)

Show answer

from itertools import islice

result = list(islice(range(10), 3))
print(result)

Match the islice parts

islice

Slicing function
range(10)

Iterable
stop

End index
step

Step

Next lesson: combinations and permutations

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