Slicing is a easy way to create sub-lists from larger lists.
If you remember back to our exercise on data types, we can use a slice to obtain a subset of items from a list. Remember that a string is just a list of characters.
>>> my_string = "Hello, world!"
>>> my_string[7:12] # from 7 to 12
'world'
You can also leave out one of the numbers in the slice. Leaving out the first number is equivalent to using a zero - you can think of this as “from the beginning.” Leaving out the last number is equivalent to using the length of the list you’re slicing - you can think of this as “until the end.”
>>> my_string = "Hello, world!"
>>> my_string[:5] # from zero to 5
'Hello'
>>> my_string[7:] # from 7 to the end
'world!'
You can also leave out both sides of the slice! You can think of this as “from the beginning, until the end.” Why? This is an easy way to copy a list!
>>> my_new_string = my_string[:]
>>> my_new_string
'Hello, world!'
You aren’t limited to positive numbers for your slicing, either. A negative number on the left side will wrap around to the other side of your list. A negative number on the right side is equivalent to the length of the list minus your number. For example:
>>> my_string = "Hello, world!"
>>> my_string[-6:] # from the end - 6 to the end
'world!'
>>> my_string[-10:-4] # from the end - 10 to the end - 4
'lo, wo'
You can also use just a single negative number to get an item counting backwards from the end of a list. For example, to get the last item from a list:
>>> my_list = [1, 3, 3, 7]
>>> my_list[-1]
7
Python slices also have a third, optional argument, called “step” or “stride”, separated by a second colon. This lets you skip elements of a list or even reverse them. For example:
>>> my_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> my_list[::2] # move forward by 2, or skip every other index
[0, 2, 4, 6, 8]
>>> my_list[::-1] # move backward by 1, and easy way to reverse a list
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> my_list[1:7:2] # get every other index between 1 and 7
[1, 3, 5]
You can use a slice to get a subset of items from any data type that maintains an order, such as a list or tuple, but not from any non-ordered data types, such as dict or set.