Python Basic Syntax

Data types tell Python what kind of data you are working with. Just like in real life, you handle numbers differently than text - you can add numbers together, but you cannot add "hello" + 5 (that does not make sense!). Python has several built-in data types, and understanding them is essential for writing correct programs.

Understanding Each Type

# Integer examples
age = 25              # Positive integer
temperature = -10     # Negative integer
count = 0             # Zero is also an integer
big_number = 1000000  # Python handles big numbers easily

print(age + 5)        # 30 (math works!)

# Float examples
price = 19.99         # Money usually needs decimals
pi = 3.14159          # Mathematical constants
temperature = 98.6    # Body temperature
percentage = 0.85     # 85% as a decimal

# Be careful: division always returns a float!
result = 10 / 4       # 2.5 (not 2!)
print(type(result))   # <class 'float'>

# String examples - both quote styles work
name = "Alice"        # Double quotes
city = 'New York'     # Single quotes

# Use the other quote type to include quotes in your string
message = "She said 'hello'"
quote = 'He replied "goodbye"'

# Empty string (no characters)
empty = ""

# Strings can be joined (concatenated)
full_name = "John" + " " + "Doe"
print(full_name)      # John Doe

# Boolean examples (note the capital T and F!)
is_student = True     # Yes, they are a student
has_license = False   # No, they do not have a license

# Booleans come from comparisons
age = 20
is_adult = age >= 18  # True (20 is greater than or equal to 18)
print(is_adult)       # True

# Common use: checking conditions
is_raining = True
if is_raining:
    print("Bring an umbrella!")

Checking Data Types

Not sure what type a variable is? Use the type() function to find out:

# Check data types with type()
age = 25
price = 19.99
name = "Python"
is_active = True

print(type(age))       # <class 'int'>
print(type(price))     # <class 'float'>
print(type(name))      # <class 'str'>
print(type(is_active)) # <class 'bool'>

# Useful for debugging!
mystery = "42"
print(type(mystery))   # <class 'str'> - it is text, not a number!

Type Conversion (Casting)

Sometimes you need to convert data from one type to another. This is called "casting" or "type conversion":

# Converting strings to numbers
user_input = "42"       # This is a string (text)
number = int(user_input)  # Convert to integer: 42
decimal = float(user_input)  # Convert to float: 42.0

print(number + 10)       # 52 (now we can do math!)

# Converting numbers to strings
age = 25
age_text = str(age)      # Convert to string: "25"
message = "I am " + age_text + " years old"
print(message)           # I am 25 years old

# Converting float to int (WARNING: this truncates, not rounds!)
price = 9.99
whole = int(price)       # 9 (drops the .99, does not round to 10!)
print(whole)             # 9

text = "hello"
number = int(text)  # ERROR! "hello" is not a number

Operators are special symbols that perform operations on values. Just like in math class, you can add, subtract, multiply, and divide. But Python also has operators for comparing values, combining conditions, and more!

Arithmetic Operators

These operators do math, just like you learned in school:

Practical Examples

# Real-world arithmetic examples

# Calculate total price
price = 29.99
quantity = 3
total = price * quantity
print(f"Total: ${total}")   # Total: $89.97

# Split a bill
bill = 85.50
people = 4
each_pays = bill / people
print(f"Each person pays: ${each_pays}")  # $21.375

# How many full boxes needed?
items = 25
items_per_box = 6
full_boxes = items // 6     # 4 (floor division)
leftover = items % 6        # 1 (remainder)
print(f"{full_boxes} full boxes, {leftover} items left over")

# Check if number is even or odd
number = 17
if number % 2 == 0:
    print("Even")
else:
    print("Odd")   # Odd (17 divided by 2 has remainder 1)

Comparison Operators

Comparison operators compare two values and return True or False. They are essential for making decisions in your code:

x = 10
y = 5

# Equal to (use == not =)
print(x == y)   # False - is 10 equal to 5? No!
print(x == 10)  # True - is 10 equal to 10? Yes!

# Not equal to
print(x != y)   # True - is 10 different from 5? Yes!

# Greater than / Less than
print(x > y)    # True - is 10 greater than 5? Yes!
print(x < y)    # False - is 10 less than 5? No!

# Greater than or equal / Less than or equal
print(x >= 10)  # True - is 10 greater than or equal to 10? Yes!
print(x <= y)   # False - is 10 less than or equal to 5? No!

Logical Operators

Logical operators let you combine multiple conditions. Think of them as connecting words:

Here is how logical operators work in practice with real-world scenarios:

# Logical operators in action
a = True
b = False

print(a and b)  # False (both must be True, but b is False)
print(a or b)   # True (at least one is True, and a is True)
print(not a)    # False (opposite of True)
print(not b)    # True (opposite of False)

# Real-world examples
age = 25
has_license = True
has_car = False

# Can you drive? Need to be 18+ AND have a license
can_drive = age >= 18 and has_license
print(f"Can drive: {can_drive}")  # True

# Can you get to work? Need a car OR a bus pass
has_bus_pass = True
can_commute = has_car or has_bus_pass
print(f"Can commute: {can_commute}")  # True

# Is the store closed?
is_open = False
is_closed = not is_open
print(f"Store closed: {is_closed}")  # True

Assignment Operators

These operators assign values to variables. The shorthand versions save typing and make your code cleaner:

# Basic assignment
score = 0

# Long way vs shorthand
score = score + 10    # Add 10 to score
score += 10           # Same thing, shorter!

# All shorthand operators
x = 10
x += 5    # x = x + 5  -> 15
x -= 3    # x = x - 3  -> 12
x *= 2    # x = x * 2  -> 24
x /= 4    # x = x / 4  -> 6.0
x //= 2   # x = x // 2 -> 3.0
x **= 2   # x = x ** 2 -> 9.0

Comments are notes in your code that Python completely ignores when running your program. They are written for humans - to help you (and others) understand what your code does and why you wrote it that way. Think of comments as sticky notes you leave for your future self!

Single-Line Comments

Use the hash symbol (#) to start a comment. Everything after the # on that line is ignored:

# This entire line is a comment - Python ignores it
name = "Alice"  # You can also add comments at the end of code

# Comments help explain your logic
# Calculate the area of a rectangle
width = 10
height = 5
area = width * height  # Result: 50 square units

# TODO: Add error handling for negative values
# FIXME: This breaks when height is zero
# NOTE: Measurements are in centimeters

Multi-Line Comments

For longer explanations, use triple quotes ("""). These are also called "docstrings" when used to document functions:

"""
This is a multi-line comment.
It can span as many lines as you need.
Python treats this as a string but ignores it
if you do not assign it to a variable.
"""

# Multi-line comments are great for:
# - Explaining complex algorithms
# - Documenting what a file does
# - Temporarily disabling code blocks

def calculate_area(width, height):
    """
    Calculate the area of a rectangle.
    
    This is called a "docstring" - it documents what
    the function does. Tools can read these automatically!
    
    Args:
        width: The width of the rectangle (in cm)
        height: The height of the rectangle (in cm)
    
    Returns:
        The area (width * height) in square cm
    """
    return width * height

Good vs Bad Comments

# Add 1 to x
x = x + 1

# Set name to John
name = "John"

# Loop through list
for item in items:

# Account for off-by-one error in API
x = x + 1

# Default user for testing
name = "John"

# Process items oldest first
for item in items:

Python uses indentation (usually 4 spaces) to define code blocks. This makes Python code clean and readable, but incorrect indentation causes errors. This is one of the first things that trips up beginners, so pay close attention!

Why Does Python Use Indentation?

Many programming languages use curly braces {} to group code. Python uses spaces instead, which makes code cleaner and forces you to write readable code:

if (true) {
console.log("Braces define blocks");
console.log("Indentation is optional");
}

if True:
    print("Indentation defines blocks")
    print("No braces needed!")

The Rules of Indentation

Visual Example

# Correct indentation example
age = 20

if age >= 18:               # Line ends with : so indent next line
    print("You are an adult")   # 4 spaces in
    print("You can vote")       # Same level = same block
    
    if age >= 21:           # Nested if - ends with : so indent more
        print("You can also drink")  # 8 spaces in (4 + 4)
    
    print("Back to first if block")  # Back to 4 spaces

print("Outside the if")     # No indentation = outside the if block

Common Indentation Errors

if True:
print("Missing indent!")  # IndentationError!

if True:
    print("Four spaces")
  print("Two spaces")  # IndentationError!

name = "Alice"
    print(name)  # IndentationError!

if True:
    print("Properly indented!")
    print("Same level - works!")

Every program needs to communicate with the outside world! Output lets your program display information to users, while input lets users provide information to your program. These two functions are among the most commonly used in Python.

The print() Function

The print() function displays text on the screen. It is your program's way of talking to the user:

# Basic printing - just put text in quotes
print("Hello, World!")           # Hello, World!
print('Single quotes work too')  # Single quotes work too

# Print variables
name = "Alice"
age = 25
print(name)    # Alice
print(age)     # 25

# Print multiple things at once (separated by spaces)
print("Name:", name, "Age:", age)  # Name: Alice Age: 25

F-Strings: The Best Way to Print Variables

F-strings (formatted strings) are the modern, recommended way to include variables in text. Just put an f before the quotes and wrap variables in curly braces {}:

# F-strings - put f before the quotes, variables in {}
name = "Alice"
age = 25

# Much cleaner than concatenation!
print(f"My name is {name} and I am {age} years old.")
# Output: My name is Alice and I am 25 years old.

# You can even do calculations inside {}
print(f"Next year I will be {age + 1}")  # Next year I will be 26

# Format numbers nicely
price = 19.99
print(f"Total: ${price:.2f}")  # Total: $19.99 (2 decimal places)

pi = 3.14159265359
print(f"Pi is approximately {pi:.2f}")  # Pi is approximately 3.14

Advanced print() Options

# Change what separates printed items (default is space)
print("A", "B", "C")             # A B C (default: space)
print("A", "B", "C", sep="-")    # A-B-C
print("A", "B", "C", sep="")     # ABC (no separator)
print("A", "B", "C", sep="\n")   # Each on new line

# Change what comes at the end (default is newline)
print("Hello", end=" ")   # Does NOT go to new line
print("World")            # Continues on same line
# Output: Hello World

# Print on same line in a loop
for i in range(5):
    print(i, end=" ")     # 0 1 2 3 4

The input() Function

The input() function pauses your program and waits for the user to type something. It is how your program can have a conversation with the user:

# Basic input - displays prompt and waits for user
name = input("What is your name? ")
print(f"Hello, {name}!")

# When you run this, it shows:
# What is your name? _   (cursor waits here)
# User types: Alice
# Output: Hello, Alice!

# input() always returns a string - even numbers!
age_string = input("Enter your age: ")  # User types: 25
print(type(age_string))   # <class 'str'> - it's a string!
# print(age_string + 5)   # ERROR! Cannot add string + number

# Convert to int for whole numbers
age = int(input("Enter your age: "))
print(f"Next year you will be {age + 1}")  # Works!

# Convert to float for decimals
height = float(input("Enter your height in meters: "))
print(f"Your height is {height} meters")

# Common pattern: get input and convert in one line
price = float(input("Enter the price: $"))
quantity = int(input("How many? "))
total = price * quantity
print(f"Your total is ${total:.2f}")

Complete Interactive Example

Here is a complete example showing input and output working together:

# A simple greeting program
print("Welcome to the Greeting Program!")
print("-" * 30)  # Print 30 dashes as a separator

# Get information from the user
name = input("What is your name? ")
age = int(input("How old are you? "))
city = input("What city do you live in? ")

# Display a personalized message
print()  # Empty line for spacing
print(f"Nice to meet you, {name}!")
print(f"So you are {age} years old and live in {city}.")
print(f"In 10 years, you will be {age + 10}!")
print()
print("Thanks for using this program. Goodbye!")

age = int(input("Enter age: "))  # User types "twenty"
# ERROR: ValueError: invalid literal for int()