Foundations of Python Programming: Functions First

The len function, when applied to a string, returns the number of characters in a string.

To get the last letter of a string, you might be tempted to try something like this:

That won’t work. It causes the runtime error IndexError: string index out of range. The reason is that there is no letter at index position 6 in "Banana". Since we started counting at zero, the six indexes are numbered 0 to 5. To get the last character, we have to subtract 1 from the length. Give it a try in the example above.

Typically, a Python programmer would combine lines 2 and 3 from the above example into a single line:

lastch = fruit[len(fruit)-1]

Though, from what you just learned about using negative indices, using fruit[-1] would be a more appropriate way to access the last index in a list.

You can still use the len function to access other predictable indices, like the middle character of a string.

fruit = "grape"
midchar = fruit[len(fruit)//2]
# the value of midchar is "a"

As with strings, the function len returns the length of a list (the number of items in the list). However, since lists can have items which are themselves sequences (e.g., strings), it important to note that len only returns the top-most length.

Note that alist[0] is the string "hello", which has length 5.