Your first program

A program is a text that contains instructions for the computer, and the computer carries them out exactly as they are written: one after the other, from top to bottom. For the computer to understand them, they have to be written in a language with fixed rules, a programming language. Here you learn Python, a language that reads well and is widely used in science, data analysis and AI.

You do not need to install anything. Every box with code on this page is a small editor: you can change the code and start it with Run (or Ctrl+Enter), and whatever the program prints appears below it. The first time, it takes a few seconds until Python has loaded.

Hello, world

The first program in almost every language prints a greeting. In Python it is a single line:

print("Hello, world!")

print is a function, a ready-made piece of program that does one job, here: writing something on the screen. What the function should print goes in the round brackets after it. The quotation marks tell Python that Hello, world! is text and not a command; such a piece of text is called a string.

Change the text between the quotation marks and run the program again. With Reset you can always go back to the original code.

Line by line

A program may have as many lines as you like, and Python carries them out in the order in which they appear:

print("First line")
print("Second line")
print()
print("After an empty line")

A print() with nothing in the brackets prints an empty line. You can also give print several values separated by commas; it then puts a space between each of them:

print("A day has", 24, "hours.")

The 24 has no quotation marks here because it is a number and not text. What this difference is about is the subject of the next lesson.

Comments

Python ignores everything on a line after a #. Such comments are notes for the people who read the code, including yourself in a few weeks:

# greeting for the first day
print("Welcome!")  # appears on the screen

When something goes wrong

Python is strict: a single missing quotation mark is enough, and the program does not start. Run this code and see what happens:

print("Hello, world!)

Instead of output you get an error message. It looks daunting at first but says fairly precisely what is wrong: on which line Python got stuck (line 1), at which point (the ^ underneath) and what kind of error it is. A SyntaxError means that the text breaks the rules of the language, here because the string never ends. Add the missing quotation mark and the program runs.

A second error that often happens at the start:

Print("Hello, world!")

Python distinguishes between upper and lower case. There is no function called Print, and that is why Python reports a NameError: it does not know this name.

Errors are part of programming, for professionals too. People who program spend a good part of their time reading error messages and improving their code accordingly. How to read every message line by line, and which errors come up most often, is shown later in Reading error messages.

Takeaways

• A program is a sequence of instructions that Python carries out from top to bottom.
• print(...) prints what is in the brackets. Text goes in quotation marks, numbers do not.
• A # starts a comment, which Python skips.
• An error message names the line, the place and the kind of error; the last line tells you the most.