Basics

Variables & Data Types

# Variables (no declaration needed)
x = 5                    # int
y = 3.14                 # float
name = "Alice"           # str
is_valid = True          # bool
items = [1, 2, 3]        # list
coords = (10, 20)        # tuple
unique = {1, 2, 3}       # set
person = {"name": "Bob"} # dict
nothing = None           # NoneType

String Operations

s = "Hello World"
s.lower()                # "hello world"
s.upper()                # "HELLO WORLD"
s.strip()                # remove whitespace
s.replace("H", "J")      # "Jello World"
s.split()                # ["Hello", "World"]
s.startswith("He")       # True
s.endswith("ld")         # True
len(s)                   # 11
s[0]                     # "H"
s[-1]                    # "d"
s[0:5]                   # "Hello"

# F-strings (Python 3.6+)
name = "Alice"
age = 30
f"My name is {name} and I'm {age}"
f"{age:.2f}"             # format to 2 decimals

Type Conversion

int("42")                # 42
float("3.14")            # 3.14
str(100)                 # "100"
list("abc")              # ['a', 'b', 'c']
tuple([1, 2])            # (1, 2)

Data Structures

Lists (mutable, ordered)

fruits = ["apple", "banana", "cherry"]

# Access
fruits[0]                # "apple"
fruits[-1]               # "cherry"
fruits[1:3]              # ["banana", "cherry"]

# Modify
fruits.append("orange")  # add to end
fruits.insert(1, "kiwi") # insert at index
fruits.remove("banana")  # remove by value
fruits.pop()             # remove & return last
fruits.pop(0)            # remove & return at index
del fruits[0]            # delete by index

# Operations
len(fruits)              # length
fruits.sort()            # sort in place
sorted(fruits)           # return sorted copy
fruits.reverse()         # reverse in place
fruits.count("apple")    # count occurrences
fruits.index("cherry")   # find index
"apple" in fruits        # True/False

# List comprehension
squares = [x**2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]

Tuples (immutable, ordered)

point = (10, 20)
x, y = point             # unpacking
single = (1,)            # single element tuple needs comma

Sets (mutable, unordered, unique)

s = {1, 2, 3, 3}         # {1, 2, 3}
s.add(4)                 # add element
s.remove(2)              # remove (error if not exists)
s.discard(2)             # remove (no error)

# Operations
a = {1, 2, 3}
b = {3, 4, 5}
a | b                    # union: {1, 2, 3, 4, 5}
a & b                    # intersection: {3}
a - b                    # difference: {1, 2}
a ^ b                    # symmetric diff: {1, 2, 4, 5}

Dictionaries (key-value pairs)

person = {"name": "Alice", "age": 30}

# Access
person["name"]           # "Alice"
person.get("age")        # 30
person.get("city", "NYC") # default value

# Modify
person["age"] = 31       # update
person["city"] = "NYC"   # add new key
del person["age"]        # delete key
person.pop("name")       # remove & return

# Operations
person.keys()            # dict_keys(['name', 'age'])
person.values()          # dict_values(['Alice', 30])
person.items()           # dict_items([('name', 'Alice'), ...])
"name" in person         # True

# Dict comprehension
squares = {x: x**2 for x in range(5)}

Control Flow

If Statements

if x > 0:
    print("positive")
elif x < 0:
    print("negative")
else:
    print("zero")

# Ternary operator
result = "even" if x % 2 == 0 else "odd"

Loops

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

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

for fruit in fruits:
    print(fruit)

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

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

# Break and continue
for i in range(10):
    if i == 3:
        continue             # skip to next iteration
    if i == 8:
        break                # exit loop
    print(i)

Match Statement (Python 3.10+)

match status:
    case 200:
        print("OK")
    case 404:
        print("Not Found")
    case _:
        print("Unknown")

Functions

Basic Functions

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

def add(a, b=0):             # default parameter
    return a + b

def info(*args, **kwargs):   # variable arguments
    print(args)              # tuple of positional args
    print(kwargs)            # dict of keyword args

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

Decorators

def decorator(func):
    def wrapper(*args, **kwargs):
        print("Before")
        result = func(*args, **kwargs)
        print("After")
        return result
    return wrapper

@decorator
def say_hello():
    print("Hello!")

Object-Oriented Programming

Classes

class Person:
    # Class variable
    species = "Homo sapiens"
    
    def __init__(self, name, age):
        # Instance variables
        self.name = name
        self.age = age
    
    def greet(self):
        return f"Hi, I'm {self.name}"
    
    def __str__(self):
        return f"Person({self.name}, {self.age})"

# Create instance
p = Person("Alice", 30)
print(p.greet())
print(p)

# Inheritance
class Student(Person):
    def __init__(self, name, age, student_id):
        super().__init__(name, age)
        self.student_id = student_id

File Handling

# Read file
with open("file.txt", "r") as f:
    content = f.read()           # read entire file
    lines = f.readlines()        # list of lines
    
    for line in f:               # iterate lines
        print(line.strip())

# Write file
with open("file.txt", "w") as f:
    f.write("Hello\n")
    f.writelines(["Line 1\n", "Line 2\n"])

# Append
with open("file.txt", "a") as f:
    f.write("Appended text\n")

# JSON
import json
data = {"name": "Alice", "age": 30}
json.dumps(data)                 # to string
json.loads('{"name": "Alice"}')  # from string

with open("data.json", "w") as f:
    json.dump(data, f)           # to file

with open("data.json", "r") as f:
    data = json.load(f)          # from file

Error Handling

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError as e:
    print(f"Value error: {e}")
except Exception as e:
    print(f"Error: {e}")
else:
    print("No error occurred")
finally:
    print("Always executes")

# Raise exceptions
raise ValueError("Invalid value")

Common Built-in Functions

abs(-5)           # 5
min(1, 2, 3)      # 1
max(1, 2, 3)      # 3
sum([1, 2, 3])    # 6
round(3.14159, 2) # 3.14
pow(2, 3)         # 8

len([1, 2, 3])
sorted([3, 1, 2])
reversed([1, 2, 3])
enumerate(["a", "b"])
zip([1, 2], ["a", "b"])
all([True, False])
any([True, False])

type(x)
isinstance(x, int)

Useful Modules

Math

import math
math.pi                  # 3.141592...
math.ceil(3.2)           # 4
math.floor(3.8)          # 3
math.sqrt(16)            # 4.0
math.factorial(5)        # 120

Random

import random
random.random()          # random float [0.0, 1.0)
random.randint(1, 10)    # random int [1, 10]
random.choice([1, 2, 3]) # random element
random.shuffle(lst)      # shuffle list in place
random.sample(lst, 3)    # pick 3 unique elements

Datetime

from datetime import datetime, timedelta

now = datetime.now()
today = datetime.today()
dt = datetime(2024, 12, 25, 10, 30)

now.year                 # 2024
now.strftime("%Y-%m-%d") # "2024-12-25"
datetime.strptime("2024-12-25", "%Y-%m-%d")

# Timedelta
tomorrow = now + timedelta(days=1)
week_ago = now - timedelta(weeks=1)

Collections

from collections import Counter, defaultdict, deque

# Counter
c = Counter([1, 1, 2, 3, 3, 3])
c.most_common(2)         # [(3, 3), (1, 2)]

# defaultdict
d = defaultdict(int)     # default value is 0
d["key"] += 1            # no KeyError

# deque (double-ended queue)
dq = deque([1, 2, 3])
dq.append(4)             # add to right
dq.appendleft(0)         # add to left
dq.pop()                 # remove from right
dq.popleft()             # remove from left

Itertools

from itertools import combinations, permutations, product

list(combinations([1, 2, 3], 2))    # [(1,2), (1,3), (2,3)]
list(permutations([1, 2, 3], 2))    # [(1,2), (1,3), (2,1), ...]
list(product([1, 2], ["a", "b"]))   # [(1,'a'), (1,'b'), ...]

Comprehensions

# List comprehension
[x**2 for x in range(10)]
[x for x in range(20) if x % 2 == 0]
[x if x > 0 else 0 for x in range(-5, 5)]

# Dict comprehension
{x: x**2 for x in range(5)}
{k: v.upper() for k, v in dict.items()}

# Set comprehension
{x % 3 for x in range(10)}

# Generator expression (memory efficient)
(x**2 for x in range(1000000))

Common Patterns

a, b = b, a

x, y, z = 1, 2, 3

if not my_list:
    # preferred

value = dictionary.get(
    "key", "default"
)

if 0 < x < 10:
    print("x is between 
    0 and 10")

if (n := len(items)) > 5:
    print(f"List has 
    {n} items")