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Now before you start getting really committed to print and start making

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friendship bracelets for this print statements,

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there's another tool that I really want to talk about

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and it's a tool that's bigger, it's fancier, it's more complex.

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So what is it?

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It's a debugger and we've already seen debuggers a little bit.

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So I've already shown you pythontutor.com

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and you've also seen us use the debugger in the Thonny editor right here.

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Now, in this example, I'm going to stick with the web version of Python Tutor

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just so that you don't have to install anything extra.

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But if we go ahead and comment out the previous block and uncomment

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the last one, you can see that we have yet another problem to solve

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and yet another piece of code, that's not doing what it's supposed to do.

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So in this case,

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we've got a function that's supposed to mutate a list so that it takes a list as

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an input

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and then it's supposed to multiply each of the items in the list

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so the final output should be something like 2, 4,

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6, 10, and so on and so forth.

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Basically taking each of these numbers and multiplying them by 2,

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which happens on this line of code. But unfortunately when we run it,

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we just get a list with a single value, 26. So what's going on?

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Let's try and debug it.

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So I want you to copy these lines of code and then paste it into Python Tutor

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and then go ahead and click Visualize Execution.

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Now that it's inside the debugger, we can do a lot of things with it.

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We can step through the code to see it line by line. So for example,

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the first thing that happens is we create a function called mutate and this

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mutate gets past a input called a_list.

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So now inside this function, mutate,

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we've got a variable called a_list and it's this list with these values.

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Now, as we keep going, we can see we create a b_list, which is an empty list

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and then we use the for-loop to loop through each of the items in the list.

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So the first time this item is equal to the first item in the list,

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which is 1,

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and then we multiply it by 2 to create this new_item variable,

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which is now equal to 2.

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And now the loop goes back to the beginning and we keep doing this with each of

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the numbers in the list. We multiply 2 to each of the numbers.

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Now, once we're done with that loop, the next line that happens is this one.

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This b_list gets appended with the new item and finally,

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we print out the b_list. Now this is where the problem occurs.

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Notice how b_list is just a list with one value.

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And that is in fact what gets printed. So with this debugger,

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we've managed to see line line what the computer is doing,

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which is really helpful in itself.

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But we can actually make this process even simpler. Inside

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most debuggers, you can put a breakpoint

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which tells the computer to stop what you're doing at this particular line.

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And then at that moment in time,

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I want to examine what all the variables and all the functions are doing.

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So in our line, the key problem happens right here.

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If I click on this line, you can see it becomes red

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and this is where it happens, this is the break point basically.

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So now I can scroll over to that point in time I can examine the values of all of

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my variables. a_list is still what I expect it to be,

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but b_list is completely empty. And this item happens to be on the last item

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number 13 and the new item is doing what it needs to do,

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multiplying it by 2,

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but none of them are being added to my list because if I click next,

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you can see this b_list is equal to just one value.

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And it happens to be the last value that new_item was equal to.

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So by using this debugger,

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you should now have a good idea of what the problem is.

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Pause the video and try to fix this code so that the mutated list with all of

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its six values gets printed. Pause the video now.

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All right.

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So the simple problem is that when we are adding the new item to this b_list,

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we're doing it outside the loop.

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It's when this loop has completed and at the very end

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do we actually call this line.

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So what we needed to do to fix this code is just to indent that line over.

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And now when we run the code,

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you can see that the proper output gets printed

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and if we change this code in there, the debugger, if we edited it right here,

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then when we visualize it, you can see if I put a breakpoint here

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that right line of code gets executed six times

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and if we go to that line of code and we hit next,

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you can see that the b_list is now getting new values added each time as we

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would expect it to. And when we examine all of our variables

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especially this b_list variable,

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it's exactly what we expect it to equal each step of the code.

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So a debugger can be really useful.

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It's almost like you've put a print statement on all the variables in your

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code,

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and you can isolate a particular time point to see what each of the values are.

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And as we become even more experienced,

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we're going to be using the debugger more and more.