Overview

A short reference for the Python you use in cs17. Each page covers one function or idea, in plain words, with an example you can run and the mistake people make most often.

How to read a page

Section	What it tells you
Synopsis	How to write the call. Words in the call, like `value`, are placeholders.
Description	What it does, and anything surprising.
Parameters	What each value in the parentheses means.
Return value	What you get back, and its type.
Example	Code and exactly what it prints. Typed answers to `input` are shown after the prompt.
Common mistake	Code that goes wrong, what Python says, and the fix.

Every example was run with Python 3.12, the version on your laptop, so the output and error messages match what you will see.

Want more detail? Every page links to the official Python documentation. You can also type help(print) at the Python prompt: run python3 with no file name, and leave with exit().

Built-in functions

Functions that are always there. You never import them.

Name	What it does
print	Shows values on the screen.
input	Asks the user to type something, and returns it as a string.
int	Converts a value to a whole number.
float	Converts a value to a number with a decimal point.
str	Converts a value to a string (text).
type	Tells you the type of a value.
len	Returns how many items a string or list has.
range	Makes a sequence of whole numbers, most often for a `for` loop.
round	Rounds a number.
abs	Returns how far a number is from zero.
min	Returns the smallest of several values.
max	Returns the largest of several values.
sum	Adds up all the numbers in a list.

Strings

Working with text. Most of these are methods, called with a dot: text.lower().

Name	What it does
f-strings	Puts values into text.
str.lower	Returns a copy of a string in lowercase.
str.upper	Returns a copy of a string in capitals.
str.strip	Removes spaces (or other characters) from both ends of a string.
str.split	Breaks a string into a list of parts.
str.join	Joins a list of strings into one string.
str.replace	Returns a copy of a string with one piece of text swapped for another.
str.startswith	Checks how a string begins (or, with `endswith`, how it ends).
in	Checks whether a piece of text, or an item, is inside another.

Lists

A list holds several values in order: [3, 1, 2].

Name	What it does
list.append	Adds an item to the end of a list.
list.pop	Removes an item from a list and returns it.
list.sort	Puts a list in order.
Indexing and slicing	Gets one item, or a range of items, from a string or list by position.

math

Maths beyond + - * /. Start the file with import math.

Name	What it does
math.sqrt	Returns the square root of a number.
math.floor	Rounds a number down.
math.ceil	Rounds a number up.

random

Random numbers and choices, for games and simulations. Start the file with import random.

Name	What it does
random.randint	Picks a random whole number between two numbers.
random.choice	Picks a random item from a list or a string.