Slicing Strings#

So we already saw how to index into a string to get a value, but what if you wanted to index into a string to get more than one character (e.g. get the first 10 characters of a string)? Python provides a very powerful syntax for accessing multiple characters in a string called slicing.

The syntax slicing looks very similar to simple indexing but lets you specify a start and a stop index separated by a colon s[<start>:<stop>] . The start index is inclusive and the stop is exclusive. For example, you could type the following:

s = 'hello world!'
print(s[2:7])

The slice shown above means “all characters starting at index 2 and up to (not including) 7” (because stop is exclusive, just like with range ). If you take this with the picture below, it can clarify why the cell above prints llo w .

hello world string indexing

Below, we show some more examples. Feel free to try modifying this to test out your own examples! Notice, you can use more complex start/stop indices like an expression based on the length of the string.

s = 'hello world!'
print(s[2:5])
print(s[0:len(s)])
print(s[1:len(s)-3])

It’s very common that you either want to start from the beginning of the string or go to the end in a slice. Python provides a short-hand syntax to infer the start/stop point of a slice by omitting the start/stop value by omitting them before/after a : .

For example, if you were to write the following:

s = 'hello world!'
print(s[:3])

Python infers that the start should be 0 since the start was omitted in the start:stop slice.

Additionally, you can omit the stop point and it will go to the end

s = 'hello world!'
print(s[3:])

Step Size#

The last thing that comes up with specifying slices is the fact that you can also specify a step size for a slice. For example, you might want every other character between 2 (inclusive) and 8 (exclusive). To do this, you can write the following

s = 'hello world!'
print(s[2:8:2])

Notice this behaves exactly the same as the range function! Your intuition there for how the step size works applies here.

Recap#

  • Python lets you ask for multiple characters in a sequence using this slice syntax

  • You can specify a start/stop/step size just like you could with range

  • You can also leave out a start/stop/step and it infers it for you