9.15. 👩‍💻 Don’t Mutate A List That You Are Iterating Through¶

So far we’ve shown you how to iterate through a list:

As well as accumulate a list by appending items:

You may be tempted now to iterate through a list and accumulate some data into it or delete data from it as you’re traversing the list, however that often becomes very confusing. In the following code we will filter out all words that begin with P, B, or T.

Activity: CodeLens 9.15.3 (ac8_12_3)

In the code above, we iterated through the indexes, and deleted each item that begins with a bad letter. However, we run into a problem because as we delete content from the list, the list becomes shorter. Eventually, we have an issue indexing on line 4. Try stepping through it in codelens to see what’s going on.

We can also try to accumulate a list that we’re iterating through as well. What do you think will happen here?

Activity: CodeLens 9.15.4 (ac8_12_4)

Now try stepping through this code. When we come across a color that begins with a vowel, that color is added to the end of the list. The python interpreter doesn’t make a copy of the sequence at the beginning and iterate through that copy. It actually asks for the next item in the sequence at the top of each iteration. But here we are adding a new item to the end of the list before we get to the end of the list, so there’s always a next item. We would have an infinite loop.

To prevent the infinite loop, we’ve added a break once the list has six strings in it. You’ll learn about break and continue later in the book.

The main message here is that you should not mutate a list while you’re iterating through it! You’ll get errors, infinite loops, or, worse, semantic errors: your code may run and produce very surprising results.