Running Python on Your Own Machine

This course runs Python in a browser sandbox. On your computer, the experience is slightly different but the language is identical. You install CPython from python.org, open an interactive interpreter with >>>, or run scripts directly from a terminal.

Installing and the interactive interpreter

Install Python from python.org (on Windows, tick "Add python.exe to PATH"). Then type python3 in a terminal on macOS or Linux (py on Windows) to open the interactive interpreter. Its >>> prompt runs one line at a time and shows the result straight away, which is the quickest way to try an idea. exit() leaves it.

At the >>> prompt

>>> 2 ** 10
1024
>>> name = 'Ada'
>>> name.upper()
'ADA'

A session in the interpreter

Scripts and arguments

For anything longer, save your code in a .py file and run it with python3 hello.py (py hello.py on Windows).

# greet.py
import sys

def main():
    # argv[0] is the script name, rest are user input
    if len(sys.argv) < 2:
        return 1
    print(f"Hello, {sys.argv[1]}!")
    return 0

# On your machine: python3 greet.py Alice

Simulating a CLI script

sys.argv is the command line split into words. argv[0] is always the path to the running file. The rest are the words you typed after it, in order: the terminal splits on spaces, and quotes keep words together, so python3 greet.py "Ada Lovelace" gives one argument, not two.

Exit codes

# Convention: 0 = success
# Any non-zero value signals an error.
import sys

def main():
    return 0   # your program's work goes here

code = main()
sys.exit(code)

Standard exit pattern

Virtual environments

In a terminal

# Create (on Windows: py -m venv .venv)
python3 -m venv .venv
# Activate (Linux/macOS)
source .venv/bin/activate
# Activate (Windows)
.venv\Scripts\activate
# Inside the venv, python and pip are the venv's own
pip install requests

Isolating dependencies

Each project gets its own .venv so packages never clash globally. Pin exact versions in requirements.txt for reproducible builds.

In a terminal

# save what this project needs
pip freeze > requirements.txt

# on another machine, install exactly that
pip install -r requirements.txt

Saving and reinstalling requirements.txt

Writing it the way others expect: PEP 8

PEP 8 is Python's style guide, and most Python code you will read follows it:

Indent with four spaces.
Name functions and variables in snake_case, and classes in CapWords.
Keep lines short, around 79 characters.
Put two blank lines between top-level functions and classes.

Python on your own machine