Getting Started with Python

October 6, 20224 min read

Basics of Python

Math

print(round(2.9))  # 3
print(abs(-2.9))  # absolute value

Variables

iq = 190
user_age = iq / 4
 
print(user_age, "age")  # 47.5 age

Constants

Are represented by all capital letters

PI = 3.14

Strings

print("Hello World")  # Hello World
print("Hello" + " World")  # Hello World
print("hello" * 3)  # hellohellohello
print("hello"[0])  # h
print("Hello" + str(3))  # Hello3

Escape Sequences

we use \ to escape a character

\n - new line

\t - tab

\\ - backslash

\" - double quote

weather = "It\'s \"kind of\" sunny"
print(weather)  # It's "kind of" sunny

Formatted Strings

We want to be able to display strings with variables in them. We can do this by adding an f before the string and then adding curly braces around the variable name.

name = "John"
age = 55
print(f"Hi {name}. You are {age} years old.")  # Hi John. You are 55 years old.
 
# or
 
print("Hi {}. You are {} years old.".format(name, age))

String Indexes

String is memory are stored as a sequence of characters. We can access each character by using an index. The first character has an index of 0.

selfish = "me me me"
print(selfish[0])  # m
print(selfish[1])  # e
print(selfish[2])  # space

We can also use a range of indexes to get a substring.

selfish = "me me me"
print(selfish[0:2])  # me
print(selfish[0:5])  # me me
print(selfish[0:])  # me me me
 
numbers = "0123456789"
print(numbers[0:])  # 0123456789
print(numbers[::2])  # 02468 # every second number
print(numbers[:5])  # 01234 # up to firth index

You can use negative indexes to start from the end of the string.

numbers = "0123456789"
print(numbers[-1])  # 9
print(numbers[-2])  # 8
 
## reverse a string
print(numbers[::-1])  # 9876543210

This works like so: string[start:end:step]

if there is no start index, it will start at the beginning

if there is no end index, it will go to the end

if there is no step, it will go by 1

if there is no start or end index, it will go by 1 (default behaviour)

Immutability

Is the concept of not being able to change a string. We can't change a character in a string. We can only create a new string. You can not reassign part of a string.

For example, you can't do this:

greeting = "Hello"
greeting[0] = "J"  # TypeError: 'str' object does not support item assignment

You can only do this:

greeting = "Hello"
greeting = "J" + greeting[1:]
print(greeting)  # Jello

Built-in Functions and Methods

List of built in functions: https://docs.python.org/3/library/functions.html

For example, len() is a built-in function that returns the length of a string.

Tell us how many characters are in a string.

greeting = "Hello"
print(len(greeting))  # 5
 
numbers = "123456789"
print(len(numbers)) # 9

Methods have a dot after the variable name. They are functions that are built into the variable type.

List of string methods: https://www.w3schools.com/python/python_ref_string.asp

We can use the upper() method to convert a string to uppercase.

quote = 'to be or not to be'
print(quote.upper())
# TO BE OR NOT TO BE

Running these methods will NOT mutate the original string. They will return a new string. Howerver, we can reassign the variable to the new string.

quote = 'to be or not to be'
print(quote.upper())
# TO BE OR NOT TO BE
 
print(quote)
# to be or not to be
 
quote2 = quote.upper()
print(quote2)
# TO BE OR NOT TO BE

Lists

Are similar to arrays in other languages. They are a collection of items.

They are mutable.

are denoted by square brackets []

can access items by using the index

friends = ["Kevin", "Karen", "Jim", "Oscar", "Toby"]
print(friends[0])  # Kevin
print(friends[1])  # Karen

Matrix

A matrix is a 2D list. It is a list of lists.

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
 
print(matrix[0][1])  # 2

We can access the values in the matrix by using the indexes.

List Methods

List of list methods: https://www.w3schools.com/python/python_ref_list.asp

We can use methods like reverse() to reverse a list.

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
 
matrix.reverse()
 
new_matrix = matrix
 
print(new_matrix)
# [[7, 8, 9], [4, 5, 6], [1, 2, 3]]