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Introduction and getting started

Python is a programming language you can type and run straight away.

python --version: Check which version of Python is installed.
python script.py: Run a Python file from the command line.
print(): Show something on the screen.

Syntax

Python reads spaces at the start of a line to know how things belong together.

if x == 5:: A line that ends with a colon starts a new block.
print(x): Indented lines belong to the block above them.
# a comment: Text after # is ignored by Python.

Comments

# This is a comment: Python ignores this line.
# print("not run"): You can comment out code to stop it running.

Variables

= saves a value under a name. Variable names cannot start with a number and cannot have spaces.

x = 5: Save the number 5 as x.
name = "Ana": Save text as name.
x, y = 5, 6: Assign two values at the same time.
x = y = 7: Assign the same value to two names.

Data types

Python works out the type from the value you save.

type(5): Find out the type of a value.
int: Whole numbers like 1, 2, 3.
float: Numbers with a decimal point like 3.5.
str: Text like "hello".
bool: True or False.
list: A collection in order.
tuple: A collection that cannot be changed.
set: A collection with no duplicates.
dict: Values saved under keys.

Numbers

x = 5: An integer.
y = 3.14: A float.
x + y: Add numbers together.
x - y: Subtract numbers.
x * y: Multiply numbers.
x / y: Divide numbers.
x // y: Divide and keep the whole number.
x % y: Get the remainder.
x ** y: Raise to a power.

Casting

Turn one type of value into another.

int("5"): Turn text into an integer.
float("3.5"): Turn text into a float.
str(5): Turn a number into text.
bool(0): Turn a value into True or False.

Strings

"hello".upper(): Make all letters uppercase.
"HELLO".lower(): Make all letters lowercase.
len("hello"): Get the length of text.
"hello"[0]: Get the first character.
"hello" in "hello world": Check if text contains something.
"a,b".split(","): Split text into a list.
"a".replace("a","b"): Replace part of the text.

Booleans

True: A true value.
False: A false value.
5 > 3: Compare values to get True or False.
5 == 5: Check if two values are equal.
5 != 3: Check if two values are not equal.
bool("hello"): Most values become True. Zero and empty become False.

Operators

+ - * /: Arithmetic operators.
== != < > <= >=: Comparison operators.
and or not: Logical operators.
in not in: Membership operators.
is is not: Identity operators.

Lists

Lists hold values in order and can be changed.

fruits = ["apple", "pear"]: Create a list.
fruits[0]: Get the first item.
len(fruits): Get how many items.
fruits.append("plum"): Add an item to the end.
fruits.remove("pear"): Remove an item.
fruits[0] = "banana": Change an item.
fruits.pop(): Remove the last item.

Tuples

Tuples hold values in order but cannot be changed.

colors = ("red", "blue"): Create a tuple.
colors[0]: Get the first item.
len(colors): Get how many items.
"red" in colors: Check if an item is in the tuple.

Sets

Sets hold values with no duplicates and no set order.

nums = {1, 2, 3}: Create a set.
nums.add(4): Add an item.
nums.remove(1): Remove an item.
len(nums): Get how many items.
1 in nums: Check if an item is in the set.

Dictionaries

Dictionaries save values under keys.

me = {"name": "Ana", "age": 14}: Create a dictionary.
me["name"]: Get a value by key.
me["age"] = 15: Change a value.
me["city"] = "Paris": Add a new key and value.
me.keys(): Get all keys.
me.values(): Get all values.
len(me): Get how many key value pairs.

If...Else

Run code only when a condition is True or False.

if x > 5:: Run if condition is True.
elif x == 5:: Run if previous was False and this is True.
else:: Run if all conditions are False.
if x > 5 and x < 10:: Combine conditions.
if x > 5 or x < 2:: Either condition can be True.

Match

Compare a value against several patterns. Available in Python 3.10 and later.

match day:: Start a match statement.
case "Mon":: Run if day matches this pattern.
print("start"): Code to run for this case.
case _:: A default case when nothing matches.

While loops

Keep running code while a condition is True.

while x < 5:: Repeat while condition is True.
x += 1: Change something so the loop ends.
break: Leave the loop straight away.
continue: Skip the rest of this round and go to the next.

For loops

Loop through a sequence of items.

for i in range(3):: Loop three times.
for fruit in fruits:: Loop through each item in a list.
for c in "abc":: Loop through each character in text.
break: Leave the loop.
continue: Skip to the next item.

Functions

Group code together and reuse it by name.

def greet():: Define a function with no input.
print("hi"): Code that runs when called.
greet(): Call the function.
def greet(name):: Define a function that takes an input.
return name: Return a value from the function.
greet("Ana"): Call with an argument.

Range

range() gives a sequence of numbers to use in loops.

range(5): Numbers from 0 up to but not including 5.
range(1, 5): Numbers from 1 up to but not including 5.
range(1, 10, 2): Numbers from 1 stepping by 2.
list(range(5)): Turn range into a list to see the numbers.

Arrays

Python has lists. Some libraries also use arrays for numbers.

nums = [1, 2, 3]: Create a list to hold numbers.
len(nums): Get the number of items.
nums.append(4): Add a new number.
nums[0] = 0: Change a number.

Iterators

An iterator lets you go through items one by one.

fruits = ["apple", "pear"]: A list is iterable.
it = iter(fruits): Get an iterator from the list.
next(it): Get the next item.
for fruit in fruits:: A for loop uses iterators behind the scenes.

Modules

Modules let you reuse code from other files or built in tools.

import math: Import a module.
math.sqrt(16): Use something from the module.
from math import sqrt: Import just one thing.
import random: Another common module.
random.randint(1, 10): Get a random number.

Dates

from datetime import datetime: Import the datetime tools.
datetime.now(): Get the current date and time.
datetime.now().year: Get just the year.
datetime.now().strftime("%Y-%m-%d"): Format the date as text.

Math

import math: Import math tools.
math.pi: Get pi.
math.sqrt(25): Get the square root.
math.ceil(3.2): Round up.
math.floor(3.8): Round down.
abs(-5): Get the absolute value.

JSON

Work with JSON data in Python.

import json: Import the json module.
data = {"name": "Ana"}: Python dictionary.
json.dumps(data): Turn a dictionary into JSON text.
json.loads("{\"name\": \"Ana\"}"): Turn JSON text into a Python dictionary.

RegEx

Search and match text patterns.

import re: Import the re module.
re.search("a", "apple"): Search for a pattern.
re.findall("a", "apple"): Find all matches.
re.match("a", "apple"): Match at the start of text.
re.sub("a", "b", "apple"): Replace matches.

Try...Except

Handle errors so your program does not crash.

try:: Try to run code that might cause an error.
x = int("abc"): This will cause an error.
except ValueError:: Run this if a ValueError happens.
print("not a number"): Show a friendly message.
else:: Run this if no error happened.
finally:: Run this no matter what.

String formatting

"Hello {}".format("Ana"): Format with curly braces.
f"Hello {name}": Format with f strings.
"Hello %s" % "Ana": Format with percent signs.
"{0} {1}".format("Hello", "Ana"): Format with numbered placeholders.

User input

Ask the user a question and get their answer.

name = input("What is your name? "): Ask for input and save it as text.
age = int(input("How old are you? ")): Get input and turn it into a number.
print("Hi", name): Use the answer.

Output

Something went wrong

What each line did

Settings

Python dictionary