Using Thonny's Debugger

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Del curso dictado por Rice University

Python Data Representations

399 calificaciones

Rice University

Python Data Representations

399 calificaciones

Curso 2 de 4 en SpecializationIntroduction to Scripting in Python

This course will continue the introduction to Python programming that started with Python Programming Essentials. We'll learn about different data representations, including strings, lists, and tuples, that form the core of all Python programs. We will also teach you how to access files, which will allow you to store and retrieve data within your programs. These concepts and skills will help you to manipulate data and write more complex Python programs. By the end of the course, you will be able to write Python programs that can manipulate data stored in files. This will extend your Python programming expertise, enabling you to write a wide range of scripts using Python This course uses Python 3. While most Python programs continue to use Python 2, Python 3 is the future of the Python programming language. This course introduces basic desktop Python development environments, allowing you to run Python programs directly on your computer. This choice enables a smooth transition from online development environments.

De la lección

Basics of Lists

This module will teach you the basics of Python's list data type. Lists are used to hold a sequence of data within programs.

Using Thonny's Debugger8:17

Debugging an Example Program8:42

Conoce a los instructores

Scott Rixner
Professor
Computer Science
Joe Warren
Professor
Computer Science

[MUSIC]

Well, welcome to class.

In the first course in this specialization,

many of you developed your programs using CodeSkulptor 3.

Now, this had the advantage that it was no installation, it just ran on your browser.

And it's extremely convenient to use.

However, for this course and the remaining two courses in the specialization,

we're going to require that you use a desktop IDE.

So you can get experience working with your file system and

working with packages.

In this video, I'm going to introduce one of the IDEs that we recommend,

that beginning Python students use.

This IDE is called Thonny.

Thonny is designed to be easy to install and easy to use.

It has a very nice interface for installing Python packages.

And probably for this video, most importantly, it has a simple debugger that

you can use to understand the behavior of your Python programs.

So let's take a look at Thonny.

Okay, let's get started.

So I followed the instructions in the class reading on using Thonny.

I've gone to Thonny.org.

I've clicked the download link for my system, which is Windows, and

I've installed Thonny.

Now, the window that spawns has basically the same kind of structure

that CodeSkulptor 3 did.

Here we have a code pane, where I have typed in some Python code.

Here we have the console, where the output from running our program appears.

In the top we have some controls, and

the controls are kind of shown here as a set of buttons.

But we also have pulldown menus that give this small precise control over Thonny.

So the simplest thing we can do is we just type run, for

this small Python program that I've written here.

And one of the first things we're going to notice here, is that Thonny always

asks us to save our Python program the first time that we run it.

So we'll call this hello_world.py and we'll save it.

And we ran it, and sure enough there's Hello world.

So now let's go back and look at some of the features that Thonny's debugger has.

Okay, let's take a look at Thonny's debugger.

So the first thing we have to understand is what is a debugger?

It's just simply a computer program that allows us to test and debug a program.

Thonny has a very simple but nice debugger,

that allows to evaluate a python program step by step.

So let's try a little more complicated example.

So I'm going to click on these other tabs that are open, and

switch over to our second program.

It's a sequence of print statements,

that we can use to kind of show off some of the features of Thonny's debugger.

So the controls for the debugger can be accessed up here with these buttons.

So what I am going to do is,

instead of clicking run, I'm going to click debug current script.

And what Thonny does is it highlights the next piece of your Python program,

that we're getting ready to evaluate.

So we're going to evaluate this docstring,

we can hit basically step over, go to the next thing.

And so now we're ready to evaluate this print step.

So what the operation step over is going to do,

it's going to evaluate everything in this yellow box in one step.

So when I step over, what you're going to see is,

that I evaluated what's in here and printed out, Hello Thonny.

I can also step over this and

now I'm ready to evaluate the last print statement.

So what we're going to do now is we're going to use a second option,

which is step into.

Step into was going to now walk through evaluating the pieces of this

print function.

So let's do that.

So it says we need to evaluate the entire expression.

Then we need to evaluate secret number, which is just what the secret number is.

Do we now devaluate this arithmetic expression?

And Thonny's going to show us basically how the pieces of this arithmetic

expression are evaluated, and the order in which this evaluation takes place.

So we're going to evaluate one, you get back one.

Then, we're going to evaluate two times three.

So we have to evaluate two, that's two.

We have to evaluate three, that's three.

Then we're going to evaluate two times three, that's six.

Then we're going to evaluate one plus six, that's seven.

And then when we're done, we're going to print both those out.

And we're going to get the secret number of seven.

So notice here that when we step into a piece of code,

we're evaluating the components one step at a time.

If you're tired of kind of evaluating all of the little pieces,

you can use the final button which is step out.

And that basically exits a thing that you're currently evaluating, and

jumps back out to the next higher level Okay,

let's try Thonny's debugger out on a more interesting program,

one that uses concepts that you've been working on this week.

In particular, I've written a little code here that takes a list of numbers and

computes the smallest number in the list.

Now, notice in Python we can do this with a single function min, but

we've just done this as an example,

so you can see how evaluating something with loops and lists, would work.

So I'm going to start off by just running the code, and

you'll notice that it basically says that the minimal of this example list is 1.1.

So I have a list of numbers here, and sure enough the smallest number is 1.1.

So now I'm going to run the debugger,

we're going to see how this program is actually evaluated inside python.

So I'm going to click the debug script.

And I'm going to use Step over here, till we get to something interesting.

So we're going to define both our functions, then we're going to call and

run example.

So now we want to step into that.

And the thing that's interesting that happen here when we're actually evaluating

run_example, notice that Thonny popped up a new window that corresponds to

the evaluation of run_example.

So now we're going to evaluate run example.

And I'm going to step over this.

And you can see here,

we've actually bound the local variable example list to the list we provided.

And we're going to print out that example list, we're going to print a blank line.

And now the next nice thing that happens,

is we're going to actually evaluate list_num.

So I'm going to step into that.

And we notice that what happens is,

example_list gets replaced by the actual list.

And now we're going to dive into the evaluation of list_minimum, so

again we popped up a new window.

The thing to notice here, the parameter numbers,

is actually bound to the particular list we're interested in.

So now, I'm going to step over and get to the for loop.

To evaluate the for loop, the interpieces of it,

we're going to need to step into that.

So I'm going to step into that.

And I notice when I do my first evaluation, the for loop,

num is bound to 4.6, that's the first number in the list, okay?

I'm going to go through and evaluate that if statements.

So I have to go through and compute the value for num and min_num.

It's 4.6 and infinity.

So yeah, 4.6 is less than infinity, so that's true.

So now we need to evaluate the body of the if statement.

So we're going to go through and do that, 4.6.

And notice how min_num is now 4.6.

We've also jumped on now to working on the second number in the list.

So I'm going to just step over these now, and so you can watch what happens, okay.

On this iteration num is 2.1, min_num is 4.6.

So on the next iteration min_num is now 2.1.

And as we just step over these various evaluations,

you'll see that eventually 1.1 is assigned to min_num.

So now we'll step over that and we're ready to return back.

Notice we get 1.1 back.

So now when we step back over there, what we see is that min_num is now 1.1.

And one more line, and sure enough the minimum of this example list is 1.1.

So that was a very detailed example of how you can use this debugger,

to actually understand how your program is executing.

Now, that's nice for seeing how correct programs work.

This is a lot like the way Python Tutor worked.

But the real value of this debugger is to go through and

actually run it on programs, that don't work the way you intend.

So in the next video, we'll look at some examples of programs that are broken.

And how we can use the debugger to help understand what the issue is.

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