Python Cheat Sheet

Table of Contents

1. Basics
   • 1.1 Variables and Data Types
   • 1.2 Data Type Conversion
2. Operators
   • 2.1 Arithmetic
   • 2.2 Comparison
   • 2.3 Logical
3. Strings
   • 3.1 String Methods
   • 3.2 Slicing Strings
4. Control Flow
   • 4.1 If / Else Statements
   • 4.2 Loops
   • 4.3 List Comprehension
5. Functions
   • 5.1 Basic Functions
   • 5.2 *args
   • 5.3 **kwargs
6. Data Structures
   • 6.1 Lists
     • 6.1.1 List Methods
     • 6.1.2 Slicing Lists
   • 6.2 Dictionaries
     • 6.2.1 Dictionary Methods
     • 6.2.2 Dictionary Iteration
   • 6.3 Tuples
   • 6.4 Sets
7. Modules and Imports
8. Error Handling

1. Basics

1.1 Variables and Data Types

# Variables
## Primitives:
x = 1  # Integer
y = 1.5  # Float
name = "Kai"  # String
is_male = True  # Boolean
no_value = None  # NoneType (represents the absence of a value)

## Data Structures
cities = ["Amsterdam", "Stockholm", "Manila"]  # List (ordered, mutable collection)
coordinates = (59.329, 18.068)  # Tuple (ordered, immutable collection)
person = {"name": "Kai", "city": "Amsterdam"}  # Dictionary (key-value pairs)
unique_numbers = {1, 2, 3, 4, 5}  # Set (unordered, unique elements)

# Check types
type(x)  # <class 'int'>
type(y)  # <class 'float'>
type(name)  # <class 'str'>
type(is_male)  # <class 'bool'>
type(no_value)  # <class 'NoneType'>

type(fruits)  # <class 'list'>
type(coordinates)  # <class 'tuple'>
type(person)  # <class 'dict'>
type(unique_numbers)  # <class 'set'>

1.2 Data Type Conversion

int(2.5)  # 2
float(5)  # 5.0
str(100)  # '100'
bool(0)  # False
bool(10)  # True: should be true for any non 0 number both positive and negative

2. Operators

2.1 Arithmetic

a = 10 + 5  # Addition
b = 10 - 5  # Subtraction
c = 10 * 5  # Multiplication
d = 10 / 5  # Division (always returns float)
e = 10 // 3  # Floor division (will return integer)
f = 10 % 3  # Modulo (find the remainder)
g = 2 ** 3  # Exponentiation (2 to the power of 3 in this example)

2.2 Comparison

1 == 2  # False
1 != 2  # True
1 > 2  # False
1 <= 2  # True

2.3 Logical

True and False  # False
True or False  # True
not True  # False

3. Strings

3.1 String Methods

my_string = "Hello, Kai!"

# Length of a String:
len(my_string)  # 11

# Reversing a String
my_string[::-1]  # "!iaK ,olleH"

# Changing case:
my_string.lower()  # "hello, kai!" (Converts to lowercase)
my_string.upper()  # "HELLO, KAI!" (Converts to uppercase)

my_string2 = "hello, kai!"
my_string2.capitalize()  # "Hello, kai!" (Capitalizes the first letter of the string)
my_string2.title()  # "Hello, Kai!" (Converts the first character of each word to uppercase)

# Finding Substrings:
my_string.find("Kai")  # 7 (Returns the index of the first occurrence of a substring)
my_string.index("Kai") # 7 (Returns the index of the first occurrence (same as find))

# Finding Substrings with start and end parameters
my_string.index("o", 0, 5)  # 4 (Searches for "o" only between index 0 (inclusive) and 5(exclusive))

# Replacing Substrings:
my_string.replace("Kai", "World")  # "Hello, World!" (original was "Hello, Kai!")

# String Splitting and Joining:
my_string4 = "apple,banana,mango"
# (Splits a string into a list based on a delimiter, in this case a ',')
my_string4.split(",")  # ['apple', 'banana', 'mango'] (string to list)

fruits = ['apple', 'banana', 'mango']
# (Joins elements of a list into a string with a delimiter, in this case with a ', ')
my_string = ", ".join(fruits)  # "apple, banana, mango" (list to string)

# Check String Contents:
my_string.startswith("Hello")  # True
my_string.endswith("Kai!")  # True
"abcdef123".isalnum()  # True if all characters are letters and numbers only. no space or special characters 
"333".isdigit()  # True if all characters are digits. no space or special characters.
"abc".isalpha()  # True if all characters must be latin letters. no space, numbers, or special characters)
"   ".isspace()  # True if all characters must be spaces)

# Checking for Lowercase and Uppercase
"hello".islower()  # True
"HELLO".isupper()  # True

# Removing whitespaces:
to_strip = "  Hello, Kai!  "
to_strip.strip()  # "Hello, Kai!" (Removes both leading and trailing spaces)
to_strip.lstrip()  # "Hello, Kai!  " (Removes leading spaces)
to_strip.rstrip()  # "  Hello, Kai!" (Removes trailing spaces)

# Counting Substrings:
my_string3 = "Hello, Kai! Hello, Kai!"
my_string3.count("Kai")  # 2 (Counts the number of occurrences of a substring)

3.2 Slicing Strings

Slicing is used to access a portion of a string (substring). You can specify a range of indices to create a new substring without modifying the original string. The slice includes characters from the starting index up to (but not including) the ending index.

Syntax:

string[start:end:step]

• start: Index to start the slice (inclusive).
• end: Index to end the slice (exclusive).
• step: (Optional) The increment between each index.

# Basic Slicing
sliced_string = "Hello, Kai!"[0:5]  # Gets characters from index 0 to 4 (inclusive)
print(sliced_string)  # Output: "Hello"

# Slicing with Start Only
sliced_string = "Hello, Kai!"[7:]  # Gets characters from index 7 to the end
print(sliced_string)  # Output: "Kai!"

# Slicing with End Only
sliced_string = "Hello, Kai!"[:5]  # Gets characters from the start to index 5 (exclusive)
print(sliced_string)  # Output: "Hello"

# Slicing with Step
sliced_string = "Hello, Kai!"[::2]  # Gets every second character
print(sliced_string)  # Output: "Hlo ai!"

# Negative Indices
sliced_string = "Hello, Kai!"[-5:]  # Gets the last 5 characters
print(sliced_string)  # Output: "Kai!"

4. Control Flow

4.1 If/Else Statements

if x > 0:
    print("x is greater than 0")
elif x == 0:
    print("x is equal to 0")
else:
    print("x is less than 0")

4.2 Loops

# While Loop
count = 0
while count < 5:
    print(count)
    count += 1

# For Loop using a range
for i in range(5):  
    print(i) # Output: 0, 1, 2, 3, 4

# Loop with steps of 2
for i in range(0, 10, 2):
    print(i) # Output: 0, 2, 4, 6, 8

# Loop with steps of -1 (reverse)
for i in range(5, 0, -1):
    print(i) # Output: 5, 4, 3, 2, 1

# Looping through a list
cities = ["Amsterdam", "Leeuwarden", "Den Haag", "Arnhem"]
for city in cities:
    print(city) # Output: Each city in the list

# Looping through a list with index using enumarate
cities = ["Amsterdam", "Leeuwarden", "Den Haag", "Arnhem"]
for index, city in enumerate(cities):
     print(f"{index}: {city}")  # Output: 0: Amsterdam, 1: Leeuwarden, etc.

# Looping through multiple lists using zip
fruits = ["apple", "banana", "mango"]
prices = [5, 4.50, 7]
for fruit, price in zip(fruits, prices):
    print(f"{fruit} cost €{price}.") # Output: apple cost €5, etc.

# Looping through a string
my_string = "Amsterdam"
for i in my_string:
    print(i) #Output: A, m, s, t, e, r, d, a, m

# Exiting loop early using break:
for i in range(5):
    if i == 3:
        break  # Exits loop when i is 3
    print(i)    

4.3 List Comprehension

# Create a list by doubling each number in a range of 5
doubled = [x*2 for x in range(5)]  # [0, 2, 4, 6, 8]

# List comprehension with condition
# Create a list of even numbers between 0 and 9
even_numbers = [x for x in range(10) if x % 2 == 0]  # [0, 2, 4, 6, 8]

# List comprehension with multiple conditions
# Generate a list of tuples where:
# 1. The first number is even and the second number is odd 
# 2. Both numbers are between 0 (inclusive) and 6 (exclusive)
# 3. Exclude pairs where the sum of the two numbers is divisible by 3
result = [(i, j) for i in range(6) if i % 2 == 0 for j in range(6) if j % 2 != 0 and (i + j) % 3 != 0]
print(result)
# Output: [(0, 1), (0, 5), (2, 3), (2, 5), (4, 1), (4, 3)]

5. Functions

5.1 Basic Functions

# Basic Function
def greet(name):
    return f"Hello, {name}!"

print(greet("Kai"))  # Output: Hello, Kai!

# Function with default parameter
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Kai"))            # Output: Hello, Kai!
print(greet("Kai", "Hi"))     # Output: Hi, Kai!

# Return multiple values:
def calculate(a, b):
    sum_result = a + b
    diff_result = a - b
    return sum_result, diff_result

result_sum, result_diff = calculate(10, 5)
print(result_sum)  # Output: 15
print(result_diff)  # Output: 5

5.2 *args

• *args: Allows a function to accept any number arguments

def add_numbers(*args):
    total = 0 
    for arg in args:
        total += arg
    return total

# Example usage
print(add_numbers(1, 2, 3))      # Output: 6
print(add_numbers(5, 10, 15, 20)) # Output: 50

5.3 **kwargs

• **kwargs: Allows a function to accept any number keyword arguments

def describe_person(**kwargs):
    for key, value in kwargs.items():
        print("{}: {}".format(key, value))

describe_person(name="Kai", pet="cat", city="Amsterdam")
# Output:
# name: Kai
# pet: cat
# city: Amsterdam

6. Data Structures

6.1 Lists

6.1.1 List Methods

# Create a list
my_list = [1, 2, 3, 4, 5]

# Access elements
my_list[0]  # 1 
my_list[-1]  # 5 (last element)
my_list[-2]  # 4 (second to last element)

# Modify elements
my_list[1] = 10

# List methods
my_list.append(6)  # Add element to the end of the list
my_list.remove(10)  # Remove element "10" not the element in index 10
my_list.pop() # Removes and returns the last element from the list
del my_list[2]  # Removes element at index 2 (3)
my_list.extend([7, 8])  # Extends the list by appending elements from another list
my_list.sort()  # Sorts the list in ascending order

# Reverse list
my_list[::-1] # Output [5, 4, 3, 2, 1]

# Built-in Functions
len(my_list)  # will return the length of 5
max(my_list)  # 5
min(my_list)  # 1
sum(my_list)  # will sum the total of all the elements of a list, in this case, 15
sum([1, 2, 'three', 4, 5]) # will return a TypeError, sum requires numeric arguments
sorted([3, 1, 2])  # Returns a new sorted list: Output: [1, 2, 3] 

6.1.2 Slicing Lists

Slicing is used to access a portion (sublist) of a list. You can specify a range of indices to create a new list or to modify an existing list.

Syntax:

list[start:end:step]

• start: Index to start the slice (inclusive).
• end: Index to end the slice (exclusive).
• step: (Optional) The increment between each index.

# Basic Slicing
my_list = [1, 2, 3, 4, 5, 6, 7]
sublist = my_list[2:5]  # Gets elements at indices 2, 3, 4 (values 3, 4, 5)
print(sublist)  # Output: [3, 4, 5]

# Slicing with Start Only
my_list = [1, 2, 3, 4, 5, 6, 7]
sublist = my_list[2:]  # Gets elements starting at index 2 until the end of the list
print(sublist)  # Output: [3, 4, 5, 6, 7]

# Slicing with End Only
my_list = [1, 2, 3, 4, 5, 6, 7]
sublist = my_list[:4]  # Gets elements from the start up to index 4 (exclusive)
print(sublist)  # Output: [1, 2, 3, 4]

# Slicing with Step
my_list = [1, 2, 3, 4, 5, 6, 7]
sublist = my_list[0:7:2]  # Gets every second element
print(sublist)  # Output: [1, 3, 5, 7]

# Negative Indices
my_list = [1, 2, 3, 4, 5, 6, 7]
sublist = my_list[-4:-1]  # Gets elements at indices -4, -3, -2 (values 4, 5, 6)
print(sublist)  # Output: [4, 5, 6]

# Removing a Sublist
# You can remove a range of elements from a list using the `del` statement:
my_list = [1, 2, 3, 4, 5, 6, 7]
del my_list[2:5]  # Removes elements at indices 2, 3, and 4 (values 3, 4, 5)
print(my_list)  # Output: [1, 2, 6, 7]

6.2 Dictionaries

6.2.1 Dictionary Methods

# Create a dictionary
my_dict = {"name": "Kai", "profession": "Software Engineer"}

# Access values
my_dict["name"]  # Output: Kai

# Add or modify values
my_dict["city"] = "Amsterdam" # if the key exists, then the value will be overwritten

# Dictionary methods
my_dict.keys()  # dict_keys(['name', 'profession', 'city'])
my_dict.values()  # dict_values(['Kai', 'Software Engineer', 'Amsterdam'])

6.2.2 Dictionary Iteration

my_dict = {"name": "Kai", "profession": "Software Engineer"}

for key in my_dict:
    print(key)

for value in my_dict.values():
    print(value)

for key, value in my_dict.items():
    print(f"{key}: {value}")

6.3 Tuples

What is the difference between a list and a tuple?

A list is mutable, meaning you can add, remove, or change the elements that are definied in it. A tuple is immutable, meaning that once defined, you cannot add, remove, or change the elements defined in it.

# Create a tuple
my_tuple = (1, 2, 3)

# Access elements 
my_tuple[0]  # 1

# Modify element
my_tuple[1] = 10 # Raises TypeError: 'tuple' object does not support item assignment

6.4 Sets

# Creating a set
cities = {'Amsterdam', 'Rotterdam', 'The Hague', 'Amsterdam', 'Utrecht'} # 'Amsterdam' will only appear once

# Adding an element
cities.add('Eindhoven')

# Removing an element
cities.remove('Rotterdam')

# Checking membership
'Amsterdam' in cities  # Returns True

# Set operations
set_a = {'Amsterdam', 'Rotterdam', 'The Hague'}
set_b = {'Rotterdam', 'Utrecht'}
print(set_a | set_b)  # Union: {'Amsterdam', 'Rotterdam', 'The Hague', 'Utrecht'}
print(set_a & set_b)  # Intersection: {'Rotterdam'}

7. Modules and Imports

# Import a module
import math
print(math.sqrt(16))  # 4.0

# Import specific function
from math import pi
print(pi)  # 3.141592653589793

# Renaming imported module with 'as' 
import numpy as np

8. Error Handling

Error handling in Python is done using try and except blocks, allowing you to manage exceptions that may occur during program execution.

try:
    # Code that may raise an error
    result = 10 / 0
except ZeroDivisionError:
    print("You can't divide by zero!")