Python Basics

Variables

Python is dynamically typed — you don't declare a type, it's inferred automatically.

name = "Alice"
age = 25
height = 5.6
is_student = True

Naming rules

• Must start with a letter or underscore (not a number)
• Case-sensitive — age and Age are different
• Convention: snake_case for variables and functions

Data Types

Check a variable's type anytime with type(x).

print(type(42))      # <class 'int'>
print(type("hi"))    # <class 'str'>

Printing & Input

print("Hello, world!")

name = input("What's your name? ")
print(f"Hi, {name}!")   # f-string: embeds variables directly in text

f-strings are the standard way to format strings in modern Python:

age = 25
print(f"You are {age} years old.")
print(f"Next year you'll be {age + 1}.")

Operators

Arithmetic

3 + 2    # 5   addition
3 - 2    # 1   subtraction
3 * 2    # 6   multiplication
3 / 2    # 1.5 division (always returns float)
3 // 2   # 1   floor division (rounds down)
3 % 2    # 1   modulus (remainder)
3 ** 2   # 9   exponent

Comparison

3 == 2   # False
3 != 2   # True
3 > 2    # True
3 <= 2  # False

Logical

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

Conditionals

Python uses indentation (not curly braces) to define code blocks. This is mandatory, not a style choice.

age = 18

if age >= 18:
    print("Adult")
elif age >= 13:
    print("Teen")
else:
    print("Child")

Loops

for loop — iterate over a sequence

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

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

while loop — repeat while a condition is true

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

Loop control

for i in range(10):
    if i == 3:
        continue   # skip this iteration
    if i == 7:
        break      # exit the loop entirely
    print(i)

Lists

Ordered, changeable collections — the most commonly used data structure.

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

fruits.append("date")        # add to end
fruits.remove("banana")      # remove by value
fruits.insert(0, "mango")    # insert at position
fruits.pop()                 # remove & return last item

print(fruits[0])             # access by index
print(fruits[-1])            # last item
print(fruits[1:3])           # slice (items 1 and 2)
print(len(fruits))           # number of items

for fruit in fruits:
    print(fruit)

Dictionaries

Store data as key-value pairs — useful for structured, labeled data.

person = {"name": "Alice", "age": 25}

print(person["name"])        # access value by key
person["city"] = "Manila"    # add a new key-value pair
person["age"] = 26           # update existing value

for key, value in person.items():
    print(key, value)

Functions

Reusable blocks of code that take inputs and (optionally) return outputs.

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

message = greet("Alice")
print(message)

Default arguments

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

print(greet("Bob"))             # Hello, Bob!
print(greet("Bob", "Hi"))       # Hi, Bob!

Comments

# This is a single-line comment

"""
This is a
multi-line comment
(often used as documentation)
"""

Quick Reference Cheat Sheet

Suggested Next Steps

1. Practice writing small scripts that combine loops + conditionals (e.g., FizzBuzz)
2. Learn list comprehensions: [x*2 for x in range(5)]
3. Explore error handling with try / except
4. Learn how to work with files (open(), reading/writing text)
5. Get comfortable with built-ins: len(), sum(), sorted(), max(), min()