Welcome to Python

What is a programming language?

The same idea at three levels

Here is how "store the number 97" might look at each level. Notice how each step up is easier for a person to read:

Why not just program in English?

English is full of words that mean more than one thing. "Make it bigger" could mean the text, the window, or the number. A computer cannot guess what you meant, so a programming language has exact rules: every line can mean only one thing. That is why spelling, parentheses, and quotes matter so much in code.

What kind of language is Python?

What these words look like in practice

• Interpreted: you click Run and the program starts right away. There is no separate "build" step that makes an .exe file first.
• Interactive: at the >>> prompt, typing 5 * 4 and pressing Enter shows 20 immediately. It is like a very powerful calculator.
• Object-oriented: in Python, every value (a number, a word, a list) is an object that comes with its own tools. For example, "hello".upper() gives "HELLO". You will use these tools all year.
• Open source: the code that makes Python work is public. Anyone can read it, suggest fixes, or learn from it.

"Hello, World!" in three languages

Programmers traditionally start a new language with a program that prints Hello, World! Compare how much you need to write.

print("Hello, World!")

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

#include <stdio.h>

int main(void) {
    printf("Hello, World!\n");
    return 0;
}

Reading the Python line piece by piece

• print is a function: a named tool that does a job. Its job is to show something on the screen.
• The parentheses ( ) hold what you are giving the function to work with.
• The quotes " " mark the text as a string: words to show exactly as written.

Leave out the quotes and Python thinks Hello is the name of a variable. print(Hello) stops with NameError: name 'Hello' is not defined, because nothing named Hello was ever created.

Where Python came from

Ideas Python borrowed from ABC

ABC was designed to be easy for beginners. Python kept some of its best ideas, including using indentation to group lines together and having simple, readable keywords. Guido also fixed things he did not like about ABC, such as how hard it was to add new features to it.

The goal: programming for everybody

In 1999 Guido wrote a proposal called "Computer Programming for Everybody." It described goals for Python: an easy and intuitive language that is still powerful, open source so anyone can help, code that reads like plain English, and good for everyday tasks. Those goals are a big reason Python is used in classrooms like this one.

Why is it called "Python"?

You will see the jokes in examples

Because of the Monty Python connection, Python examples often use the words spam and eggs as sample variable names, the same way math books use x and y. Those words come from a famous Monty Python sketch about a restaurant that puts spam in everything. If you see spam = 5 in an online example, now you know why.

Python through the years

Click a year to read what happened.

How to read a version number

A version like 3.14.0 has three parts:

• 3 is the major version. It changes only when the language changes in big ways that can break old code (like 2 to 3).
• 14 is the minor version. A new one comes out about once a year with new features. Code from 3.13 almost always still works in 3.14.
• 0 is the micro (bug fix) version. These fix problems without adding new features.

Check your own version: type python --version in a terminal (on some Macs, python3 --version).

Python 2 vs. Python 3

print "Hello"

print("Hello")

Other changes you might notice in old code

The division change is a good example of why Python 3 was worth the trouble. In Python 2, 7 / 2 silently gave 3, which surprised many beginners. Python 3 gives the answer you expect from math class.

Who runs Python today?

How a change gets into Python

1. Someone writes a PEP explaining the idea, why it is needed, and how it would work.
2. The community discusses it in public, and people point out problems or improvements.
3. The Steering Council accepts or rejects it.
4. If accepted, developers write and test the code, and it appears in a future version.

A PEP you will hear about: PEP 8 is Python's official style guide. It explains things like using 4 spaces for indentation and writing variable names in lowercase_with_underscores. Following it makes your code look like everyone else's, which makes it easier to read.

What is Python used for?

Python is a general-purpose language, so it shows up almost everywhere. Here are areas python.org lists, with tools you may hear about:

A few more places Python shows up

• Artificial intelligence: popular machine-learning libraries such as PyTorch and TensorFlow are used mainly from Python.
• Automation: short Python scripts rename hundreds of files, fill in spreadsheets, or download data while you do something else.
• Hardware projects: the Raspberry Pi, a small low-cost computer made for learning, uses Python for many of its projects, like controlling lights and sensors.

What happens when you click Run?

Follow one line through the steps

Say your file hello.py contains print(2 + 3).

1. You write the source code and click Run.
2. Python checks and compiles the whole file. It confirms the grammar is correct (the parentheses match, nothing is misspelled in a way that breaks the rules) and turns it into bytecode.
3. The virtual machine runs the bytecode: it adds 2 and 3 to get 5, then calls print with 5.
4. Output: 5 appears on the screen.

Two kinds of stops

Common runtime errors you will meet

Grammar problems are SyntaxErrors. A missing parenthesis, a missing quote, or a missing colon after if are all caught before anything runs. Wrong indentation shows up as an IndentationError, which is a special kind of SyntaxError.

Reading an error message: look at the last line first. It names the error type and gives a short explanation. The line above it shows the code that caused it, and the "line" number tells you where to look.

Two ways to run Python

Side by side

Tips for the prompt: you never type the >>> yourself; Python shows it to say "I'm ready." When a line needs more (like after an if line), the prompt changes to ... to say "keep going." Type exit() to leave.

The Zen of Python

import this

What a few lines mean in real code

• Readability counts. Code is read many more times than it is written: by your teacher, your partner, and you next week. Compare:

  a = 12 * 3          # hard to understand
  total_cost = price * quantity   # easy to understand

• Simple is better than complex. If there is a short, clear way to do something, use it instead of a clever, confusing one.
• Errors should never pass silently. (Another line from the list.) It is better for a program to stop with an error message than to quietly give a wrong answer. This is why Python stops and tells you when something goes wrong.

Check yourself

Choose an answer, then click Check. This is practice only. Nothing is graded or recorded.

Python at a glance

Remember

• We use Python 3. print always needs parentheses.
• Python reads your whole file first. A SyntaxError means nothing ran.
• The >>> prompt shows values automatically. A script needs print().
• Readability counts: write code that other people can understand.

Key words to know

source code (what you write) · bytecode (Python's in-between form) · interpreter / virtual machine (what runs it) · SyntaxError (grammar mistake, nothing runs) · runtime error (problem found while running) · interactive mode (>>>) · script (.py file) · PEP (a proposal to change Python)