Python Cheatsheet — Syntax, Data Types & Common Patterns

Variables & Types

name = "Alice"        # str
age = 30              # int
price = 9.99          # float
active = True         # bool
items = [1, 2, 3]     # list
coords = (10, 20)     # tuple (immutable)
user = {"name": "Alice", "age": 30}  # dict
unique = {1, 2, 3}    # set

String Formatting

# f-strings (recommended)
f"Hello, {name}! Age: {age}"
f"Price: {price:.2f}"
f"{'hello':>20}"      # Right-align

# .format()
"Hello, {}".format(name)

Lists

nums = [1, 2, 3, 4, 5]
nums.append(6)          # Add to end
nums.insert(0, 0)       # Insert at index
nums.pop()              # Remove last
nums.remove(3)          # Remove first occurrence
nums.sort()             # Sort in place
sorted(nums)            # Return sorted copy
nums[1:3]               # Slice: [2, 3]
nums[::-1]              # Reverse

Dictionaries

d = {"a": 1, "b": 2}
d["c"] = 3              # Add/update
d.get("x", 0)           # Get with default
d.keys()                # Dict keys
d.values()              # Dict values
d.items()               # Key-value pairs
{**d, "d": 4}           # Merge (Python 3.9+: d | {"d": 4})

Comprehensions

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

# Dict comprehension
word_len = {w: len(w) for w in words}

# Set comprehension
unique_chars = {c for c in "hello"}

Functions

def greet(name: str, greeting: str = "Hello") -> str:
    return f"{greeting}, {name}!"

# Lambda
double = lambda x: x * 2

# *args and **kwargs
def func(*args, **kwargs):
    print(args)    # Tuple of positional args
    print(kwargs)  # Dict of keyword args

Error Handling

try:
    result = int("abc")
except ValueError as e:
    print(f"Error: {e}")
except Exception as e:
    print(f"Unexpected: {e}")
finally:
    print("Always runs")