Python Basics

We all need to start somewhere, so how about doing it here. This guide covers the fundamental Python basics including operators, data types, variables, and core functions.

Math Operators

From highest to lowest precedence:

Examples of expressions:

# Multiplication has higher precedence than addition
# So this evaluates as: 2 + (3 * 6) = 2 + 18 = 20
2 + 3 * 6

20

# Parentheses override operator precedence
# This evaluates as: 5 * 6 = 30
(2 + 3) * 6

30

2 ** 8

256

23 // 7

3

23 % 7

2

(5 - 1) * ((7 + 1) / (3 - 1))

16.0

4 + 2 * 3

Augmented Assignment Operators

Examples:

# Augmented assignment: equivalent to greeting = greeting + ' world!'
greeting = 'Hello'
greeting += ' world!'
greeting

'Hello world!'

# Increment a number by 1
number = 1
number += 1
number

2

# Replicate list elements: equivalent to my_list = my_list * 3
my_list = ['item']
my_list *= 3
my_list

['item', 'item', 'item']

x = 5
x += 3

Walrus Operator

The Walrus Operator allows assignment of variables within an expression while returning the value of the variable

Example:

# Walrus operator assigns and returns value in one expression
# my_var is assigned "Hello World!" and then printed
print(my_var:="Hello World!")

Hello World!

my_var="Yes"
print(my_var)

Yes

print(my_var:="Hello")

Hello

The Walrus Operator, or Assignment Expression Operator was firstly introduced in 2018 via PEP 572, and then officially released with Python 3.8 in October 2019.

Data Types

Understanding data types is one of the most important Python basics. Python has nine core built-in data types that cover almost everything you’ll need:

Quick Examples

# Numbers
age = 25                    # int
price = 19.99              # float
coordinate = 2 + 3j        # complex

# Text
name = "Alice"             # str

# Boolean
is_student = True          # bool

# None
result = None              # NoneType

# Collections
scores = [85, 92, 78]      # list
person = {'name': 'Bob', 'age': 30}  # dict
coordinates = (10, 20)     # tuple
unique_ids = {1, 2, 3}     # set

For a comprehensive guide with visual examples and detailed explanations of when to use each type, see: Python Data Types: A Visual Guide for Beginners.

Concatenation and Replication

String concatenation:

# String concatenation: adjacent strings are automatically joined
'Alice' 'Bob'

'AliceBob'

String replication:

# String replication: repeat string multiple times
'Alice' * 5

'AliceAliceAliceAliceAlice'

Variables

Variables are a fundamental part of Python basics. You can name a variable anything as long as it obeys the following rules:

1. It can be only one word.

# bad
my variable = 'Hello'

# good
var = 'Hello'

2. It can use only letters, numbers, and the underscore (_) character.

# bad
%$@variable = 'Hello'

# good
my_var = 'Hello'

# good
my_var_2 = 'Hello'

3. It can’t begin with a number.

# this wont work
23_var = 'hello'

4. Variable name starting with an underscore (_) are considered as “unuseful”.

# _spam should not be used again in the code
_spam = 'Hello'

Comments

Inline comment:

# This is a comment

Multiline comment:

# This is a
# multiline comment

Code with a comment:

a = 1  # initialization

Please note the two spaces in front of the comment.

Function docstring:

def foo():
    """
    This is a function docstring
    You can also use:
    ''' Function Docstring '''
    """

The print() Function

The print() function is one of the first Python basics you’ll learn. It writes the value of the argument(s) it is given. […] it handles multiple arguments, floating point-quantities, and strings. Strings are printed without quotes, and a space is inserted between items, so you can format things nicely:

print('Hello world!')

Hello world!

a = 1
print('Hello world!', a)

Hello world! 1

The end keyword

The keyword argument end can be used to avoid the newline after the output, or end the output with a different string:

# Use end parameter to change what comes after each print statement
phrase = ['printed', 'with', 'a', 'dash', 'in', 'between']
for word in phrase:
    print(word, end='-')  # Use '-' instead of newline

printed-with-a-dash-in-between-

The sep keyword

The keyword sep specify how to separate the objects, if there is more than one:

# Use sep parameter to specify separator between multiple arguments
print('cats', 'dogs', 'mice', sep=',')  # Comma-separated output

cats,dogs,mice

The input() Function

This function takes the input from the user and converts it into a string:

# input() reads user input and returns it as a string
print('What is your name?')   # ask for their name
my_name = input()  # Wait for user to type and press Enter
print('Hi, {}'.format(my_name))

What is your name?
Martha
Hi, Martha

input() can also set a default message without using print():

my_name = input('What is your name? ')  # default message
print('Hi, {}'.format(my_name))

What is your name? Martha
Hi, Martha

It is also possible to use formatted strings to avoid using .format:

# input() can display a prompt message directly
my_name = input('What is your name? ')  # Prompt and read in one call
print(f'Hi, {my_name}')  # f-string for string formatting

What is your name? Martha
Hi, Martha

The len() Function

Evaluates to the integer value of the number of characters in a string, list, dictionary, etc.:

# len() returns the number of characters in a string
len('hello')  # Returns 5

5

# len() returns the number of items in a list
len(['cat', 3, 'dog'])  # Returns 3 (three items)

3

Test of emptiness example:

a = [1, 2, 3]

# bad: unnecessary len() check
if len(a) > 0:  # evaluates to True
    print("the list is not empty!")

the list is not empty!

# good: direct boolean evaluation (Pythonic way)
if a:  # evaluates to True if list is not empty
    print("the list is not empty!")

the list is not empty!

The str(), int(), and float() Functions

These functions allow you to change the type of variable. For example, you can transform from an integer or float to a string:

# Convert integer to string
str(29)  # Returns '29'

'29'

str(-3.14)

'-3.14'

Or from a string to an integer or float:

# Convert string to integer
int('11')  # Returns 11

11

# Convert string to float
float('3.14')  # Returns 3.14

3.14

result = int('42')
type(result)

Relevant Links

• Python Data Types: A Visual Guide for Beginners
• Python Comprehensions Step-by-Step
• Control Flow
• Functions
• Lists and Tuples
• Dictionaries
• Sets
• String Formatting