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Basics

Variables & Data Types​

Variable Assignment​

# Simple assignment
name = "Alice"
age = 25
height = 5.6
is_student = True

# Multiple assignment
x, y, z = 1, 2, 3
a = b = c = 0

# Swapping variables
x, y = y, x

Data Types​

# Integers
count = 42
negative = -10
binary = 0b1010      # Binary: 10
octal = 0o12         # Octal: 10
hex_num = 0xFF       # Hexadecimal: 255

# Floats
price = 19.99
scientific = 1.5e-4  # 0.00015
infinity = float('inf')
not_a_number = float('nan')

# Strings
name = "John Doe"
message = 'Hello World'
multiline = """This is
a multiline
string"""

# Boolean
is_active = True
is_complete = False

# None (null equivalent)
result = None

Type Checking & Conversion​

# Check type
type(42)           # <class 'int'>
type("hello")      # <class 'str'>
isinstance(42, int)  # True

# Type conversion
int("42")          # 42
float("3.14")      # 3.14
str(42)            # "42"
bool(1)            # True
bool(0)            # False
bool("")           # False
bool("hello")      # True

Operators​

Arithmetic Operators​

# Basic arithmetic
x = 10
y = 3

addition = x + y        # 13
subtraction = x - y     # 7
multiplication = x * y  # 30
division = x / y        # 3.333...
floor_division = x // y # 3
modulo = x % y         # 1
exponentiation = x ** y # 1000

# Augmented assignment
x += 5  # x = x + 5
x -= 3  # x = x - 3
x *= 2  # x = x * 2
x /= 4  # x = x / 4
x //= 2 # x = x // 2
x %= 3  # x = x % 3
x **= 2 # x = x ** 2

Walrus Operator (Python 3.8+)​

# Assignment expression - assign and return value in one operation
# Basic syntax: (variable := expression)

# Traditional approach
data = input("Enter a number: ")
if len(data) > 0:
    print(f"You entered: {data}")

# With walrus operator
if (data := input("Enter a number: ")):
    print(f"You entered: {data}")

# Useful in loops
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
while (n := len(numbers)) > 5:
    print(f"List has {n} items, removing one...")
    numbers.pop()

# In list comprehensions
data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
squares = [square for x in data if (square := x**2) > 25]
print(squares)  # [36, 49, 64, 81, 100]

# File processing
with open('file.txt', 'r') as f:
    while (line := f.readline()):
        print(line.strip())

# Regex matching
import re
pattern = r'\d+'
text = "The price is $25.99"
if (match := re.search(pattern, text)):
    print(f"Found number: {match.group()}")

# Avoiding repeated calculations
def expensive_function(x):
    print(f"Computing for {x}")
    return x * x

# Traditional approach
x = 5
result = expensive_function(x)
if result > 20:
    print(f"Result {result} is large")

# With walrus operator
x = 5
if (result := expensive_function(x)) > 20:
    print(f"Result {result} is large")

# Dictionary operations
data = {"name": "Alice", "age": 30, "city": "New York"}
if (age := data.get("age")) and age >= 18:
    print(f"Adult: {age} years old")

# List processing
items = ["apple", "banana", "cherry", "date"]
processed = [(item, length) for item in items if (length := len(item)) > 5]
print(processed)  # [('banana', 6), ('cherry', 6)]

Comparison Operators​

x = 10
y = 5

# Comparisons
equal = x == y          # False
not_equal = x != y      # True
greater = x > y         # True
less = x < y            # False
greater_equal = x >= y  # True
less_equal = x <= y     # False

# Chained comparisons
age = 25
is_adult = 18 <= age <= 65  # True

# Identity operators
a = [1, 2, 3]
b = [1, 2, 3]
c = a

a is b      # False (different objects)
a is c      # True (same object)
a == b      # True (same content)

Logical Operators​

# Boolean logic
a = True
b = False

and_result = a and b    # False
or_result = a or b      # True
not_result = not a      # False

# Short-circuit evaluation
x = 5
result = x > 0 and x < 10  # True
result = x < 0 or x > 10   # False

# Membership operators
numbers = [1, 2, 3, 4, 5]
has_three = 3 in numbers      # True
no_six = 6 not in numbers    # True

Control Structures​

If/Else Statements​

age = 18

# Basic if-else
if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

# elif (else if)
if age < 13:
    print("Child")
elif age < 18:
    print("Teenager")
else:
    print("Adult")

# Ternary operator
status = "Adult" if age >= 18 else "Minor"

# Multiple conditions
has_license = True
if age >= 16 and has_license:
    print("Can drive")
elif age >= 16:
    print("Can get a license")
else:
    print("Too young to drive")

For Loops​

# Iterate over a list
fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(fruit)

# Range function
for i in range(5):          # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):       # 1, 2, 3, 4, 5
    print(i)

for i in range(0, 10, 2):   # 0, 2, 4, 6, 8
    print(i)

# Enumerate (get index and value)
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Enumerate with start
for i, fruit in enumerate(fruits, 1):
    print(f"{i}. {fruit}")

# Iterate over dictionary
person = {"name": "John", "age": 30}
for key in person:
    print(key, person[key])

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

While Loops​

# Basic while loop
count = 0
while count < 5:
    print(count)
    count += 1

# While with else
count = 0
while count < 3:
    print(count)
    count += 1
else:
    print("Loop completed normally")

# Infinite loop with break
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == 'quit':
        break
    print(f"You entered: {user_input}")

Loop Control​

# Break and continue
for i in range(10):
    if i == 3:
        continue  # Skip this iteration
    if i == 7:
        break     # Exit loop
    print(i)

# Nested loops with labels (using functions)
def find_item():
    matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
    target = 5

    for row in matrix:
        for item in row:
            if item == target:
                return f"Found {target}"
    return "Not found"

# Pass statement (placeholder)
for i in range(5):
    if i == 2:
        pass  # Do nothing, placeholder
    print(i)

Input/Output​

# Basic printing
print("Hello, World!")

# Multiple arguments
print("Name:", "John", "Age:", 25)

# Separator and end
print("A", "B", "C", sep="-")        # A-B-C
print("Hello", end=" ")
print("World")                       # Hello World

# Formatted printing
name = "Alice"
age = 30
print(f"My name is {name} and I am {age} years old")
print("My name is {} and I am {} years old".format(name, age))
print("My name is %s and I am %d years old" % (name, age))

# Print to file
with open("output.txt", "w") as f:
    print("Hello, File!", file=f)

Input Function​

# Basic input
name = input("Enter your name: ")
print(f"Hello, {name}!")

# Input with type conversion
age = int(input("Enter your age: "))
height = float(input("Enter your height: "))

# Input validation
while True:
    try:
        age = int(input("Enter your age: "))
        if age < 0:
            print("Age cannot be negative")
            continue
        break
    except ValueError:
        print("Please enter a valid number")

# Multiple inputs
data = input("Enter three numbers separated by spaces: ").split()
numbers = [int(x) for x in data]

Comments & Documentation​

Comments​

# Single line comment
print("Hello")  # End of line comment

# Multi-line comment using multiple # symbols
# This is a longer comment
# that spans multiple lines
# to explain complex logic

"""
Multi-line comment using triple quotes
This is typically used for documentation
Can span many lines
"""

# TODO: Implement error handling
# FIXME: This function has a bug
# NOTE: This is important information

Docstrings​

def greet(name, greeting="Hello"):
    """
    Greet a person with a custom message.

    Args:
        name (str): The person's name
        greeting (str, optional): The greeting message. Defaults to "Hello".

    Returns:
        str: The formatted greeting message

    Examples:
        >>> greet("Alice")
        'Hello, Alice!'
        >>> greet("Bob", "Hi")
        'Hi, Bob!'
    """
    return f"{greeting}, {name}!"

class Person:
    """
    A class to represent a person.

    Attributes:
        name (str): The person's name
        age (int): The person's age
    """

    def __init__(self, name, age):
        """
        Initialize a Person object.

        Args:
            name (str): The person's name
            age (int): The person's age
        """
        self.name = name
        self.age = age

String Operations​

String Methods​

text = "  Hello World  "

# Case conversion
text.upper()         # "  HELLO WORLD  "
text.lower()         # "  hello world  "
text.title()         # "  Hello World  "
text.capitalize()    # "  hello world  "
text.swapcase()      # "  hELLO wORLD  "

# Whitespace handling
text.strip()         # "Hello World"
text.lstrip()        # "Hello World  "
text.rstrip()        # "  Hello World"

# Searching and checking
text.find("World")   # 8 (index of first occurrence)
text.index("World")  # 8 (raises error if not found)
text.count("l")      # 3
text.startswith("  H") # True
text.endswith("d  ") # True
"World" in text      # True

# String validation
"123".isdigit()      # True
"abc".isalpha()      # True
"abc123".isalnum()   # True
"   ".isspace()      # True

String Formatting​

name = "Alice"
age = 25
score = 95.5

# f-strings (Python 3.6+)
message = f"Hello, {name}! You are {age} years old."
formatted = f"Score: {score:.1f}%"  # Score: 95.5%
padded = f"Name: {name:>10}"        # Right-aligned in 10 chars

# str.format()
message = "Hello, {}! You are {} years old.".format(name, age)
message = "Hello, {name}! You are {age} years old.".format(name=name, age=age)
formatted = "Score: {:.1f}%".format(score)

# % formatting (older style)
message = "Hello, %s! You are %d years old." % (name, age)
formatted = "Score: %.1f%%" % score

# Format specifiers
number = 42
print(f"{number:d}")    # 42 (decimal)
print(f"{number:b}")    # 101010 (binary)
print(f"{number:x}")    # 2a (hexadecimal)
print(f"{number:04d}")  # 0042 (zero-padded)

String Slicing​

text = "Hello World"

# Basic slicing
first_five = text[:5]    # "Hello"
last_five = text[-5:]    # "World"
middle = text[2:7]       # "llo W"

# Step slicing
every_other = text[::2]  # "HloWrd"
reversed_text = text[::-1] # "dlroW olleH"

# Negative indices
last_char = text[-1]     # "d"
second_last = text[-2]   # "l"

String Splitting & Joining​

# Splitting
sentence = "apple,banana,orange"
fruits = sentence.split(",")      # ["apple", "banana", "orange"]
words = "hello world".split()     # ["hello", "world"]
parts = "a.b.c".split(".", 1)     # ["a", "b.c"] (max 1 split)

# Joining
fruits = ["apple", "banana", "orange"]
sentence = ",".join(fruits)       # "apple,banana,orange"
sentence = " and ".join(fruits)   # "apple and banana and orange"
path = "/".join(["home", "user", "docs"])  # "home/user/docs"

# String replacement
text = "Hello World"
new_text = text.replace("World", "Python")  # "Hello Python"
new_text = text.replace("l", "L", 1)        # "HeLlo World" (replace only first)

File I/O​

Reading Files​

# Read entire file
with open("file.txt", "r") as f:
    content = f.read()

# Read line by line
with open("file.txt", "r") as f:
    for line in f:
        print(line.strip())  # Remove newline characters

# Read all lines into a list
with open("file.txt", "r") as f:
    lines = f.readlines()

# Read specific number of characters
with open("file.txt", "r") as f:
    chunk = f.read(10)  # Read first 10 characters

# Read one line at a time
with open("file.txt", "r") as f:
    first_line = f.readline()
    second_line = f.readline()

Writing Files​

# Write to file (overwrite)
with open("output.txt", "w") as f:
    f.write("Hello, World!")

# Append to file
with open("output.txt", "a") as f:
    f.write("\nNew line")

# Write multiple lines
lines = ["First line", "Second line", "Third line"]
with open("output.txt", "w") as f:
    f.writelines(line + "\n" for line in lines)

# Write with print
with open("output.txt", "w") as f:
    print("Hello, World!", file=f)
    print("Second line", file=f)

File Modes & Error Handling​

# Different file modes
# "r" - read (default)
# "w" - write (overwrites)
# "a" - append
# "r+" - read and write
# "b" - binary mode (e.g., "rb", "wb")

# Error handling
try:
    with open("nonexistent.txt", "r") as f:
        content = f.read()
except FileNotFoundError:
    print("File not found!")
except PermissionError:
    print("Permission denied!")
except Exception as e:
    print(f"An error occurred: {e}")

# Check if file exists
import os
if os.path.exists("file.txt"):
    with open("file.txt", "r") as f:
        content = f.read()

Working with Paths​

import os
from pathlib import Path

# Using os module
current_dir = os.getcwd()
file_path = os.path.join(current_dir, "data", "file.txt")
directory = os.path.dirname(file_path)
filename = os.path.basename(file_path)

# Using pathlib (modern approach)
path = Path("data/file.txt")
path.exists()           # Check if exists
path.is_file()          # Check if file
path.is_dir()           # Check if directory
path.parent             # Get parent directory
path.name               # Get filename
path.suffix             # Get file extension
path.stem               # Get filename without extension

# Create directories
path.parent.mkdir(parents=True, exist_ok=True)

Code Structure & Indentation​

Indentation Rules​

# Python uses indentation to define code blocks
# Standard is 4 spaces per indentation level

if True:
    print("This is indented")
    if True:
        print("This is double indented")
    print("Back to single indent")

# Avoid mixing tabs and spaces
# Use consistent indentation throughout

# Good
def function_name():
    if condition:
        do_something()
        return result

# Bad - inconsistent indentation
def function_name():
  if condition:
      do_something()
    return result  # This will cause an IndentationError

Code Organization​

# Module-level docstring
"""
This module demonstrates Python code organization.
"""

# Imports at the top
import os
import sys
from datetime import datetime

# Constants (uppercase)
MAX_SIZE = 100
DEFAULT_NAME = "Unknown"

# Global variables (if necessary)
counter = 0

# Function definitions
def helper_function():
    """Helper function with proper spacing."""
    pass

def main_function():
    """Main function that does the work."""
    global counter
    counter += 1
    return counter

# Class definitions
class MyClass:
    """A sample class."""

    def __init__(self):
        self.value = 0

    def method(self):
        """A method with proper indentation."""
        if self.value > 0:
            return self.value
        return 0

# Main execution
if __name__ == "__main__":
    main_function()

Line Length & Continuation​

# Keep lines under 79-88 characters
# Use backslash for line continuation
long_string = "This is a very long string that needs to be " \
              "split across multiple lines for readability"

# Implicit line continuation with parentheses
total = (first_value + second_value +
         third_value + fourth_value)

# Function arguments
def long_function_name(parameter_one, parameter_two,
                      parameter_three, parameter_four):
    return parameter_one + parameter_two

# List/dict continuation
long_list = [
    "item1",
    "item2",
    "item3",
    "item4"
]

config = {
    "database_host": "localhost",
    "database_port": 5432,
    "database_name": "myapp"
}

Basic Functions​

Function Definition​

# Simple function
def greet():
    print("Hello, World!")

# Function with parameters
def greet_person(name):
    print(f"Hello, {name}!")

# Function with return value
def add_numbers(a, b):
    return a + b

# Function with default parameters
def greet_with_title(name, title="Mr."):
    return f"Hello, {title} {name}!"

# Function with multiple return values
def get_name_age():
    return "Alice", 25

name, age = get_name_age()

Function Parameters​

# Positional arguments
def describe_pet(name, animal_type):
    print(f"I have a {animal_type} named {name}")

describe_pet("Buddy", "dog")

# Keyword arguments
describe_pet(animal_type="cat", name="Whiskers")

# Default parameters
def make_shirt(size, message="I love Python"):
    print(f"Making a {size} shirt with message: {message}")

make_shirt("Large")
make_shirt("Medium", "Hello World")

# Variable arguments (*args)
def sum_all(*numbers):
    return sum(numbers)

result = sum_all(1, 2, 3, 4, 5)  # 15

# Keyword arguments (**kwargs)
def create_profile(**info):
    for key, value in info.items():
        print(f"{key}: {value}")

create_profile(name="Alice", age=30, city="New York")

Quick Reference​

Common Built-in Functions​

# Type conversion
int("42")           # 42
float("3.14")       # 3.14
str(42)            # "42"
bool(1)            # True
list("hello")      # ['h', 'e', 'l', 'l', 'o']

# Math functions
abs(-5)            # 5
min(1, 2, 3)       # 1
max(1, 2, 3)       # 3
sum([1, 2, 3])     # 6
round(3.7)         # 4
pow(2, 3)          # 8

# Sequence functions
len("hello")       # 5
reversed([1, 2, 3]) # [3, 2, 1]
sorted([3, 1, 2])  # [1, 2, 3]
enumerate(["a", "b", "c"])  # [(0, 'a'), (1, 'b'), (2, 'c')]
zip([1, 2, 3], ["a", "b", "c"])  # [(1, 'a'), (2, 'b'), (3, 'c')]

# Input/Output
input("Enter name: ")  # Get user input
print("Hello")         # Print to console

Useful Patterns​

# Check if variable is defined
try:
    variable
except NameError:
    variable = "default value"

# Get user input with validation
while True:
    try:
        age = int(input("Enter your age: "))
        break
    except ValueError:
        print("Please enter a valid number")

# Read file safely
filename = "data.txt"
try:
    with open(filename, "r") as f:
        content = f.read()
except FileNotFoundError:
    print(f"File {filename} not found")
    content = ""

# Multiple conditions
age = 25
if 18 <= age <= 65:
    print("Working age")

# String validation
text = input("Enter text: ")
if text and text.strip():
    print("Valid input")
else:
    print("Empty input")

Best Practices​

# Use descriptive variable names
user_age = 25          # Good
ua = 25               # Bad

# Use constants for magic numbers
MAX_ATTEMPTS = 3
for attempt in range(MAX_ATTEMPTS):
    # Try something
    pass

# Use f-strings for formatting
name = "Alice"
greeting = f"Hello, {name}!"  # Good
greeting = "Hello, " + name + "!"  # Less readable

# Handle exceptions appropriately
try:
    result = risky_operation()
except SpecificError:
    # Handle specific error
    pass
except Exception as e:
    # Handle general error
    print(f"Error: {e}")

# Use with statement for file operations
with open("file.txt", "r") as f:
    content = f.read()
# File is automatically closed

This cheatsheet covers the fundamental Python concepts you'll use daily. Master these basics before moving on to more advanced topics like classes, modules, and libraries.