Learn Python programming from Harvard University.  It dives more deeply into the design and implementation of web apps with Python, JavaScript, and SQL using frameworks like Django, React, and Bootstrap. Topics include database design, scalability, security, and user experience. You will learn how to write and use APIs, create interactive UIs, and leverage cloud services like GitHub and Heroku.

This course will teach you how to read and write code as well as how to test and “debug” it. It is designed for students with or without prior programming experience who’d like to learn Python specifically. Learn about functions, arguments, and return values (oh my!); variables and types; conditionals and Boolean expressions; and loops. Learn how to handle exceptions, find and fix bugs, and write unit tests; use third-party libraries; validate and extract data with regular expressions; model real-world entities with classes, objects, methods, and properties; and read and write files. Hands-on opportunities for lots of practice. Exercises inspired by real-world programming problems. No software required except for a web browser, or you can write code on your own PC or Mac.

Whereas CS50x (the original CS50 course) itself focuses on computer science more generally as well as programming with C, Python, SQL, and JavaScript, this course, aka CS50P, is entirely focused on programming with Python. You can take CS50P before CS50x, during CS50x, or after CS50x. But for an introduction to computer science itself, you should still take CS50x!

💻 Slides, source code, and more at https://cs50.harvard.edu/python

✏️ Dr. David J. Malan teaches this course.

🖥 Watch the original CS50x course: https://youtu.be/8mAITcNt710

⭐️ Course Contents ⭐️
(00:00:00) Introduction
(00:04:48) Lecture 0 - Functions, Variables
(01:50:24) Lecture 1 - Conditionals
(02:46:23) Lecture 2 - Loops
(04:07:10) Lecture 3 - Exceptions
(04:51:45) Lecture 4 - Libraries
(06:09:15) Lecture 5 - Unit Tests
(07:00:22) Lecture 6 - File I/O
(08:32:32) Lecture 7 - Regular Expressions
(10:37:35) Lecture 8 - Object-Oriented Programming
(13:28:47) Lecture 9 - Et Cetera

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HOW TO FOLLOW DAVID J. MALAN

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LICENSE

CC BY-NC-SA 4.0
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International Public License
https://creativecommons.org/licenses/by-nc-sa/4.0/

This course from Harvard University is an introduction to programming using the Python programming language.

You will learn how to read and write code as well as how to test and debug it.

And it's designed for students with or without prior programming experience who'd like to learn Python

My name is David Malan, and this is CS50's introduction to programming with Python.

Whereas CS50 itself is an introduction to the intellectual enterprises of computer science and the art of programming, this course is specifically focused on programming in Python itself.

At the beginning of the course, we'll be focused on a topic in programming known as functions and variables, mechanisms via which you can write code that solve smaller problems, but you can compose those smaller solutions into solutions to larger problems still.

We'll then transition to a look at conditionals, a way in code of expressing yourself logically, to maybe do something if some question has an answer of true or not do something if the answer is false.

We'll transition thereafter to introducing you to loops, the ability in code to do something again and again and again some number of times.

We'll then transition to something a little more technical known as exceptions.

Unfortunately, a lot can go wrong when you're writing code.

But you can write code defensively, so to speak, and actually catch those kinds of exceptions, those errors, and handle them properly so that the users you're writing code for don't actually see the same.

Thereafter, we'll take a look at libraries, third-party code written by other people often, or perhaps yourself in the past, that you can use and reuse in your own project so as to avoid reinventing the wheel again and again.

We'll look thereafter at something called unit tests.

It turns out you'll actually write code to test your own code, but you won't have to write tests for your tests.

Indeed, this is a best practice in industry, writing tests for your code so that, 1, you can be sure that your code today is, hopefully, if your tests are correct, correct itself.

But moreover, if you or someone else modifies your code tomorrow or down the line, you can rerun those same tests to ensure that those new changes have not broken anything about your own code.

We'll then take a look at something called FileIO, I-O for input and output, the ability to not just store information inside of a computer's memory, but rather save it persistently to disk, so to speak, to files and folders.

We'll then take a look at another technique known as regular expressions, whereby in Python you can define patterns and you can validate data to make sure the human types something in as you expect.

You can use regular expressions to extract data, perhaps from some data set you're trying to analyze.

We'll then take a look, ultimately, at object-oriented programming, a paradigm, a way of writing code whereby you can represent, in code, real-world entities.

And this is in addition to other paradigms of programming that we'll also explore, among them procedural programming, where you write lots of those functions, procedures, really, top to bottom to solve problems step by step, and even something known as functional programming as well.

And then at the very end of the course, we equip you with all the more tools for your toolkit, an additional building blocks, additional vocabulary via which after the same course you can go off on your own and either take other courses or solve projects of your own using all of these mechanisms.

Now, this course itself assumes no prior programming background, so you don't have to have written a single line of code in Python or any language yet.

But this is also a course that you can take before, during, or even after CS50 itself if you'd like to get all the more versed with Python.

Each week via the course's lectures will we introduce you to any number of concepts that we'll then drill down more deeply into in the form of problem sets each week, that is, programming projects that will enable you to apply some of those lessons learned to problems of your very own.

And by the end of the course, you'll have solved so many problems that ideally are representative of problems you'll eventually encounter in the real world.

Whether you aspire to solve code in the technical world, or perhaps in the arts, the humanities, the social sciences, the natural sciences, or beyond, you'll have ultimately the vocabulary and the technical skills via which to approach the same.

And this is CS50's introduction to programming with Python.

My name is David Malan, and this is our week on functions and variables.

But odds are, many of you, most of you, have never actually programmed before.

Let me go ahead here and open up my computer, and on it a program called Visual Studio Code or VS Code, which is just a very popular program nowadays for actually writing code.

Now, you don't have to write code using this particular tool.

In fact, all we need at the end of the day is a so-called text editor, a program for writing text.

And heck, if you really want, you could even use something like Google Docs or Microsoft Word.

You'd have to save it in the right format.

But really, at the end of the day, all you need is a program for writing text because that's what code is, text.

Now, within this particular program, I'm going to have the ability to create 1 or more files via this top portion of the screen.

And I'm going to do so by diving right in and doing this at the bottom of my screen.

At the bottom of my screen is a so-called terminal window.

And this is a command line interface, or CLI, interface to the underlying computer, be it your Mac or your PC or even some server in the cloud.

And what I'm going to do here is literally write code and then the name of the file that I want to code, for instance, hello.py.

As we'll soon see, any program that you write in Python generally has a file name that ends in .py to indicate to the computer that it's indeed a program written in Python.

Now, you'll see here at the top of my screen, I have a blinking cursor, a line 1, which is where the very first line of my code is going to go, and then just a tab that reminds me of the name of this file, hello.py.

And without even knowing much Python, I'm going to write my very first program here as follows.

Print, open parenthesis, quote, hello, comma, world, close quote, and close parenthesis.

And you'll see that at my keyboard, some of my thoughts were finished for me.

I only had to type 1 parenthesis, and the other 1 automatically appeared.

And that's just a feature that we'll see of tools like this tool here.

Now, even if you've never programmed before, odds are you can guess, infer what this simple program is going to do.

Print, open parenthesis, quote, hello world, close quote, close parenthesis.

Indeed, when I run this program, ultimately, it's just going to say, hello to the world.

And in fact, this is a very famous, perhaps the most canonical, program that you can write as your very first program in Python or any other language.

But on my Mac, my PC, even my phone, I'm generally in the habit, like you, of running programs by double clicking an icon or just tapping on the screen.

And in fact, that's because my interface to at least my current Mac or PC or some server in the cloud is, again, only a CLI, Command Line Interface, which even though it might feel like it's a step back from the menus and buttons and icons that you and I take for granted every day, you'll find, we think, that it's ultimately a much more powerful interface and incredibly popular to use among programmers in the real world.

So to run this program, I'm going to have to use a command.

And I'm going to move my cursor back down to the bottom of the screen here, where previously I already ran 1 command, the command code, which has the effect of opening VS Code in my computer.

And then I passed in the name of the file that I wanted to code up.

Now I have a chance to type a second command.

And you'll see I see a second dollar sign.

Now, the dollar sign here doesn't indicate any kind of currency or money.

It just is the symbol that's generally used to indicate your prompt, where the command line interface wants you to put those commands.

Now, the command I can run here is going to be this.

Well, it turns out that when I actually write code in a language like Python, it's, of course, stored in that file, hello.py.

But I need to interpret the code, top to bottom, left to right, so that the computer knows what to do.

Indeed, at the end of the day, even if you don't really know much about computers, you've probably heard that computers only understand zeros and ones, the so-called binary system.

Well, if that's the case, then something that says print and parentheses and quote unquote, hello world, is not surely zeros and ones.

We have to somehow translate it into the zeros and ones that the computer understands.

Now fortunately, so long as you've installed such a program in advance, there's a program as well as a language called Python.

So Python is not only a language in which we're going to write code, it's also a program, otherwise known as an interpreter, that you install for free on your own Mac or PC or some server in the cloud.

And you can then run that program, that interpreter, passing to it as input the name of your file, like mine here, hello.py.

And then that program, that interpreter, will handle the process of reading it top to bottom, left to right, and translating it effectively into those 0's and 1's that the computer can understand.

I already typed out Python of hello.py, but I didn't yet hit Enter.

And that's what's now going to kick off this command.

And hopefully, if I didn't mess any of this up, I should see my very first program's output to the screen.

So if you, too, have typed exactly that same code and have executed exactly that same command, you will have written your very first program, in this case, in Python.

Well, now let's take a step back and consider what is it that we actually just did, and What is it we're looking here on the screen?

Well, first and foremost, in most any programming language, you tend to have access to what are called functions.

A function is like an action or a verb that lets you do something in the program.

And generally speaking, any language comes with some predetermined set of functions, some very basic actions or verbs that the computer will already know how to do for you, that the language really will know how to do for you.

And you, the programmer, the human, can use those functions at will to get the computer to do those things.

Now, the program in question here, hello.py, is using 1 function.

That function is, of course, going to be this function print.

And that print function, of course, doesn't print some preordained string of text.

That is to say, it prints whatever it is you want it to print.

And here, too, do we have another piece of terminology in the world of programming, namely arguments.

An argument is an input to a function that somehow influences its behavior.

The people who invented Python, of course, didn't necessarily know what it is you and I are going to want to print to the screen.

So they designed this print function, using these parentheses, with the ability to take as input some string of text, be it in English or any other human language, that is what you want this function ultimately to print onto the screen.

And what is it that the program's ultimately doing on the screen?

It's showing us, hello world, on the screen.

And that's generally in programming known as a side effect.

In this case, it's something that appears on the screen.

And functions, therefore, can indeed have these side effects.

1 of the things they can do as this verb or action is to display on the screen as a side effect something like those words that we wanted, hello world.

But honestly, when you're learning how to program, and even once you've learned how to program years later, you're going to make mistakes.

And those mistakes, of course, are referred to a term you might already know, which is that of a bug.

And they can take so many forms and take comfort, perhaps, in knowing that over the coming weeks, you're going to make so many mistakes.

You're going to have so many bugs in your code, just like I did and just as I still do.

And those bugs themselves are just mistakes that are problems for you to solve.

And over the weeks to come, we're going to give you a lot of tools, both mental and technical, via which you can solve those problems.

If when writing your program for the first time it doesn't even work that first time, it will, with time, with practice, and with experience.

So let me deliberately now make a mistake that there was a non-zero chance I might have done accidentally already, but I got lucky.

Let me go ahead and just suppose I forgot to include something like the closing parenthesis at the end of this line of code.

But now that I've pointed it out, it's pretty obvious that it's missing that closed parenthesis.

But even little, seemingly minor details like that, that you and I as humans wouldn't really care about.

And if you're sending an email or a text message, whatever, it's just a typo.

It is going to be a big deal to the computer.

A computer is going to take you literally.

And if you don't finish your thought in the way the language expects, it's not going to necessarily run at all.

I'm going to go ahead here and clear my screen down at the bottom just so I can start fresh.

And I'm going to go ahead and run this version of my program after having made that change by deleting the parenthesis.

I'm going to go ahead and type Python again of hello.py.

And this time when I hit Enter, I'm hoping I'm going to see hello world.

But here we have an error on the screen, a so-called syntax error, which refers to my having made a mistake at my keyboard.

And This 1, fortunately, is pretty straightforward.

It indeed says that this open parenthesis was never closed.

And so that's probably pretty intuitive now what I need to do.

Unfortunately, sometimes the error messages we'll see in the coming weeks are not going to be nearly that user friendly.

But there, too, again, with experience, with practice, will you get better at debugging such programs.

Let me now make sure that I indeed fixed it correctly.

Let me go ahead, run now hello.py, and hit Enter, and voila, we're back in business.

Well, let me pause here and see if we have any questions now about Python itself, writing, or running even the simplest of these programs.

Could I write code inside a Word or, for example, Microsoft Excel?

And allow me to very explicitly say to the entire internet that you should not write code with Microsoft Word.

I mentioned that only because it's a tool via which you can write text.

And code is, At the end of the day, just text, but it's not the right tool for the job.

We don't need boldfacing, underlining, paragraphs, and the like.

We generally want something much simpler than Microsoft Word or Google Docs.

And so VS Code is an example of just a more general purpose text editor.

Its purpose in life is to allow you, the human, to edit text.

Nowadays, these text editors come with many more features.

In fact, you'll notice that even in my code here, even though it's just 1 line, there's a bit of color to it.

The word print, for me, is appearing in blue.

And we'll see, as we might write more lines of code, more and more of the lines will come to life in various colors.

Now, that's just 1 feature of a text editor.

We'll see, too, that it has features like this built-in terminal window.

It's going to have a built-in tool for debugging or finding problems with code.

And it's just a very popular tool nowadays.

But there are many, many others out there.

You're welcome to use them for this course and beyond.

We just happen to use this 1 in large part, too, because you can also use VS Code nowadays for free in the cloud.

How about 1 other question here on programming with Python or Hello World or syntax more generally?

That was time to ask if it is not possible to run the code without using the terminal window.

GALPINKIN I think I heard, is it not if it's possible to run the program without the terminal window?

So in this environment, as I've configured my computer, I can only run these Python programs via the terminal window.

Now, that's good for me, the programmer, or the person who's trying to learn how to program.

But it's not very good if you want to ship this software and have other people use your actual code.

You can absolutely write programs and then allow other people to use not a command line interface, but a graphical user interface or GUI, G-U-I.

This is just 1 mechanism, and perhaps I think the best 1 with which to start writing code, because eventually, it's going to give us a lot more control.

Allow me to forge ahead here, but please feel free to continue asking questions along the way, if only via the chat.

And let's consider now how we might go about improving this program.

Let's go about improving this program to make it a little more interactive and not just assume that everyone is going to want to be greeted more generically as Hello World.

Let's see if I can't get this program to say something like Hello David, or Hello Jeremiah, or Hello Horatio, or whatever the actual user's name is.

Well, to do this, I'm going to go back up to hello.py, and I'm going to add another line of code at the very top that simply says, for instance, what's your name, quote unquote, with an extra space at the end.

So I'm printing to the user, asking them a question for some input.

But now I need another function to actually get input from the user.

And perfectly, enough Python comes with a function named input.

So here I'm going to go ahead and call a function, input, open paren, close paren.

And that's going to prompt the user with just a blinking cursor waiting for them to type something in.

Now, it turns out if I read the documentation for the input function, it actually takes an argument itself.

I don't need to use print separately and then prompt the user for input.

So I can actually simplify this code before we even use it.

I'm going to go ahead here and take that same string from print, put it as an argument to the input function, and get rid of the print altogether.

And in fact, that print would have added a new line anyway.

So now I've just got a prompt where the user's cursor is going to end up blinking at the end of the line asking them, what's your name?

In my terminal window, I'm going to run Python of hello.py, enter.

It seems that this new function input is indeed prompting me, the human, for input.

So I'm going to type in my name, David, and hit Enter.

Unfortunately, it doesn't really do anything with my name.

All right, well, I could fix this, right?

I could go up to line 2, and I could change world to David.

And then back in my terminal window here, I can do Python of hello.py, Enter.

Of course, this isn't really working as intended here.

Let me go ahead and try pretending to be my colleague Carter here.

I'm going to go ahead and hit Enter, and I'll see, of course, hello, Carter.

Well, obviously not, because I've hardcoded, so to speak.

I've written literally my name inside of the string.

So we need some way now of actually getting back what the user's input is and doing something with it, ultimately.

And for this, we're going to leverage another feature of programming, specifically a feature of some functions, which is that they can have return values as well.

If you think of input as being, again, this action, this verb, you can actually personify it as maybe a person, like a friend of yours that you've asked a question of.

And you've asked your friend to go get input from someone else.

And if your friend comes back knowing that person's name, well, wouldn't it be nice if they handed that name back to you?

That's kind of what we need metaphorically the function to do, is get the user's input and then hand it back to me so that I, the programmer, can do something with it.

But if it's going to be handed back to me, I kind of want to put it somewhere so that I can then print it back on the screen.

I need to do the equivalent of take out a piece of paper or a Post-it note, write down on this piece of paper what it is the human has said, so that I can then feed it into as input that print function.

And to do that, we're going to need 1 more feature of programming, namely variables.

And odds are, most everyone's familiar with variables from math class way back when, x and y and z and the like.

Well, programming has that same capability, this ability to create a variable, in this case, in the computer's memory, not just on a piece of paper.

And that variable can store a value, a number, some text, even an image or video or more.

A variable is just a container for some value inside of a computer or inside of your own program.

So how do I go about expressing myself in this way?

Well, I think what I'm going to do is introduce a variable that's a little more interestingly named than x or y.

But I'm going to use a better name than a typical mathematical variable here.

And I'm going to literally call my variable name.

Well, in programming, because I have a whole keyboard in front of me, I can use more descriptive terms to describe what it is I'm writing.

And now, though, there's an opportunity to consider a specific piece of syntax.

We've seen quotes, all of which are necessary when passing inputs to a function.

But this equal sign here that's in between input on the right and name on the left is actually important.

And it's technically not an equal sign per se.

It doesn't mean equality as much as it means assignment.

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Harvard CS50’s Introduction to Programming with Python – Full University Course