Python Cheat Sheet

Variables & Data Types

# Variables (dynamically typed)
name = "Alice"
age = 30
height = 5.7
is_student = True

# Type checking
type(name)  # <class 'str'>

# Type conversion
int("123")    # 123
str(123)      # "123"
float("3.14") # 3.14

Strings

Lists

# Create list
fruits = ["apple", "banana", "cherry"]

# Access elements
fruits[0]        # "apple"
fruits[-1]       # "cherry" (last item)
fruits[1:3]      # ["banana", "cherry"] (slicing)

# Modify list
fruits.append("orange")       # Add to end
fruits.insert(1, "mango")     # Insert at index
fruits.remove("banana")       # Remove by value
fruits.pop()                  # Remove and return last
fruits.pop(0)                 # Remove and return at index

# List operations
len(fruits)                   # Get length
"apple" in fruits             # Check membership
fruits.sort()                 # Sort in place
sorted(fruits)                # Return sorted copy

# List comprehension
squares = [x**2 for x in range(10)]
evens = [x for x in range(10) if x % 2 == 0]

Dictionaries

# Create dictionary
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

# Access values
person["name"]           # "Alice"
person.get("name")       # "Alice"
person.get("job", "N/A") # "N/A" (default if key missing)

# Modify dictionary
person["age"] = 31       # Update value
person["job"] = "Engineer" # Add new key-value

# Dictionary methods
person.keys()            # Get all keys
person.values()          # Get all values
person.items()           # Get (key, value) pairs

# Dictionary comprehension
{x: x**2 for x in range(5)}

Control Flow

# If-elif-else
if age < 18:
    print("Minor")
elif age < 65:
    print("Adult")
else:
    print("Senior")

# For loop
for fruit in fruits:
    print(fruit)

for i in range(5):      # 0 to 4
    print(i)

for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

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

Functions

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

# Default parameters
def greet(name="World"):
    return f"Hello, {name}!"

# Multiple return values
def get_coordinates():
    return 10, 20

x, y = get_coordinates()

# *args and **kwargs
def sum_all(*args):
    return sum(args)

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

# Lambda functions
square = lambda x: x**2
add = lambda x, y: x + y

File Operations

# Read 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())

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

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

Common Libraries

Exception Handling

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")
except Exception as e:
    print(f"Error: {e}")
else:
    print("No errors occurred")
finally:
    print("This always executes")