Python Tools & Reference

Quick-access syntax cheat sheet, code examples, and common mistakes to avoid.

Variables & Data Types

Variable

x = 5

Assign a value to a variable

String

name = "Alex"

Text enclosed in quotes

Integer

age = 25

Whole number without decimals

Float

price = 9.99

Number with a decimal point

Boolean

is_active = True

True or False value

Conversion

int("5") / str(10) / float("3.14")

Convert between data types

Input & Output

Output

print("Hello")

Display output to the console

Input

input("Enter name: ")

Get user input as a string

Format

f"Hi {name}"

Insert variables into strings with f-strings

Operators

Arithmetic

+ - * / // % **

Math operations: add, sub, mul, div, floor div, mod, power

Comparison

== != > < >= <=

Compare values, returns True or False

Logical

and or not

Combine boolean expressions

Assignment

= += -=

Assign and update variable values

Control Flow

if x > 0:

Run code only when condition is True

if…else

if x > 0: … else: …

Choose between two paths

if…elif…else

if …: elif …: else:

Choose among multiple conditions

for

for item in list:

Iterate over a sequence of items

while

while x > 0:

Repeat as long as condition is True

break / continue

break / continue

Exit loop early or skip to next iteration

Functions

Define

def greet(name):

Define a reusable block of code

Return

return value

Send a result back from a function

Call

greet("Alex")

Execute a function with arguments

Built-in

len() / type() / range()

Common built-in functions for length, type, and ranges

1. Hello World with User Input

Ask the user for their name and greet them personally.

Beginner

hello.py

# Ask the user for their name
name = input("What is your name? ")

# Greet them
print("Hello, " + name + "!")
print(f"Welcome to Python, {name}!")

2. Simple Calculator

Add, subtract, multiply, or divide two numbers.

Beginner

calculator.py

# Simple calculator
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))

print(f"Add: {num1 + num2}")
print(f"Subtract: {num1 - num2}")
print(f"Multiply: {num1 * num2}")
print(f"Divide: {num1 / num2}")

3. Number Guessing Game

The computer picks a random number and you try to guess it.

Intermediate

guessing_game.py

# Number guessing game
import random

secret = random.randint(1, 10)
guess = 0

while guess != secret:
    guess = int(input("Guess a number (1-10): "))
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
    else:
        print("You got it!")

4. Grade Checker

Use if/elif/else to determine a letter grade from a score.

Beginner

grades.py

# Grade checker with if/elif/else
score = int(input("Enter your score (0-100): "))

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Your grade is: {grade}")

5. Star Triangle

Use a nested for loop to print a triangle pattern of stars.

Intermediate

star_triangle.py

# Star triangle pattern
rows = 5

for i in range(1, rows + 1):
    print("*" * i)

# Output:
# *
# **
# ***
# ****
# *****

6. Greeting Machine

Define a function that builds a personalized greeting message.

Beginner

greeting.py

# Greeting machine function
def greet(name, time_of_day):
    message = f"Good {time_of_day}, {name}!"
    return message

# Call the function
print(greet("Alex", "morning"))
print(greet("Sam", "evening"))

# Reusable for any name and time
your_name = input("Your name: ")
your_time = input("Morning/Afternoon/Evening? ")
print(greet(your_name, your_time))

Forgetting Colons

WRONG if x > 0

RIGHT if x > 0:

Always add a colon after if, for, while, def, elif, else statements.

Using = Instead of ==

WRONG if x = 5:

RIGHT if x == 5:

Use = for assignment and == for comparison. They are not interchangeable!

Indentation Errors

WRONG

if True:

print("a")

print("b") ← mixed tabs & spaces

RIGHT

if True:

print("a")

print("b")

Python uses indentation to define blocks. Use consistent spaces (4 per level), never mix tabs and spaces.

Forgetting to Convert input()

WRONG age = input("Age: ") + 1

RIGHT age = int(input("Age: ")) + 1

input() always returns a string. Use int() or float() to convert it before doing math.

Using Reserved Words as Variable Names

WRONG class = "Math"

RIGHT course = "Math"

Words like class, def, return, if, for are reserved by Python. Pick descriptive, non-conflicting names.

Off-by-One Errors in range()

WRONG for i in range(1, 5): ← prints 1-4, not 1-5

RIGHT for i in range(1, 6): ← prints 1-5

range(start, stop) stops before the stop value. Use range(1, 6) to include 5.

Forgetting Parentheses on print()

WRONG print "Hello"

RIGHT print("Hello")

In Python 3, print is a function and always requires parentheses. The old syntax is Python 2 only.

Not Closing Strings or Brackets

WRONG name = "Alex

RIGHT name = "Alex"

Every opening quote, parenthesis, or bracket must have a matching closing one. Use an editor with syntax highlighting to catch these.