Python Reference

name = input("What is your name? ")
print(f"Hello, {name}!")

age_text = input("How old are you? ")

print(age_text + 1)      # ❌ error (string + number)

age = int(input("How old are you? "))
print(age + 1)           # ✅ works

width = float(input("Enter the width (m): "))
height = float(input("Enter the height (m): "))

area = width * height
print(f"Area = {area} square metres")

number = input("Enter a number: ")
print(number * 2)   # prints text twice, not doubled

number = int(input("Enter a number: "))
print(number * 2)

# ❌ Hard to read
name = input("Enter your name")

# ✅ Clear
name = input("Enter your name: ")

# ❌ Asked twice
print(input("Name: "))
print(input("Name: "))

# ✅ Asked once, reused
name = input("Name: ")
print(name)
print(name)

• + adds numbers
• + joins strings
• == compares values
• = assigns values

• and → both must be true
• or → at least one must be true
• not → flips true/false

if condition:
    # indented block runs only when condition is True
    do_something()

• The colon : at the end of the line
• Indentation under the if/elif/else

1. Python checks the if condition.
2. If it’s True, Python runs that block and skips the rest.
3. If it’s False, Python checks the first elif.
4. It keeps checking elif blocks in order until one is True.
5. If none are True, Python runs the else block (if there is one).

temperature = int(input("Temperature (°C): "))

if temperature >= 30:
    print("It is hot today.")

number = int(input("Enter a number: "))

if number % 2 == 0:
    print("Even")
else:
    print("Odd")

score = int(input("Score (0-100): "))

if score >= 85:
    print("A")
elif score >= 70:
    print("B")
elif score >= 50:
    print("C")
else:
    print("D")

age = int(input("Age: "))
has_ticket = input("Do you have a ticket? (yes/no): ").lower()

if age >= 13 and has_ticket == "yes":
    print("You can enter.")
else:
    print("You cannot enter.")

# ❌ Wrong
# if score = 50:

# ✅ Right
# if score == 50:

# ❌ Wrong
# if age >= 13
#     print("OK")

# ✅ Right
# if age >= 13:
#     print("OK")

# ❌ Wrong (mixed indentation)
if True:
  print("A")
    print("B")

# ✅ Right (same indentation level)
if True:
    print("A")
    print("B")

• repeat actions cleanly
• scale your program easily
• change behaviour by updating one line

for i in range(5):
    print(i)

for i in range(1, 6):
    print(i)

for i in range(0, 10, 2):
    print(i)

count = 0

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

# ❌ Infinite loop
count = 0
while count < 5:
    print(count)

word = "python"

print(word[0])   # p
print(word[1])   # y
print(word[-1])  # n (last character)

text = "computer science"

print(text[0:8])     # computer
print(text[9:])      # science
print(text[:8])      # computer

# ❌ Not allowed
word = "cat"
# word[0] = "b"

word = "cat"
word = "b" + word[1:]
print(word)

answer = input("Yes or no? ")

if answer == "yes":
    print("OK")

answer = input("Yes or no? ").strip().lower()

if answer == "yes":
    print("OK")

teams = ["red", "blue", "green"]
teams[1] = "yellow"
print(teams)   # ['red', 'yellow', 'green']

pets = ["dog", "cat", "fish"]

for pet in pets:
    print(pet)

pets = ["dog", "cat", "fish"]

for i in range(len(pets)):
    print(i, pets[i])

coords = (10, 20)
days = ("Mon", "Tue", "Wed")

names = ["Ava", "Noah", "Zoe"]
# print(names[3])  # ❌ error

names = ["Ava", "Noah", "Zoe"]

# ❌ Wrong idea:
# names.remove(1)

# ✅ Correct:
names.remove("Noah")
print(names)

profile = {
    "name": "Ava",
    "year": 9,
    "favourite_subject": "Science"
}

print(profile["name"])
print(profile["year"])
print(profile["favourite_subject"])

scores = {
    "Ava": 85,
    "Noah": 72,
    "Zoe": 90
}

for name, score in scores.items():
    print(name, score)

names = ["Ava", "Noah", "Ava", "Zoe", "Noah"]
unique_names = set(names)

print(unique_names)

used_names = {"Ava", "Noah", "Zoe"}
name = input("Enter a name: ")

if name in used_names:
    print("Already used")
else:
    print("Available")

profile = {"name": "Ava", "year": 9}

# ❌ Wrong idea:
# print(profile[0])

# ✅ Correct:
print(profile["name"])

scores = {"Ava": 85}

name = input("Enter name exactly: ")
print(scores[name])

numbers = [3, 1, 3, 2]
print(set(numbers))

empty_dict = {}
empty_set = set()

print(type(empty_dict))
print(type(empty_set))

def greet():
    print("Hello!")

greet()
greet()

def greet(name):
    print("Hello", name)

greet("Ava")
greet("Noah")

def add_print(a, b):
    print(a + b)

add_print(3, 4)

def add_return(a, b):
    return a + b

result = add_return(3, 4)
print(result)

def greet():
    print("Hello")

# ❌ Nothing happens

def greet(name):
    print(name)

# greet()  ❌ error

def add(a, b):
    print(a + b)

total = add(2, 3)
print(total)  # prints None

import random

number = random.randint(1, 10)
print(number)

from random import randint

number = randint(1, 10)
print(number)

# ❌ Wrong:
# number = random.randint

# ✅ Correct:
number = random.randint(1, 10)

Traceback (most recent call last):
  File "main.py", line 7, in <module>
    print(score)
NameError: name 'score' is not defined

age_text = input("Enter your age: ")

print("DEBUG: age_text is", age_text)
print("DEBUG: type is", type(age_text))

age = int(age_text)
print("DEBUG: age is", age)

print("DEBUG: total =", total)

age = 14

if age > 14:
    print("You can go")
else:
    print("You cannot go")

1. Choose one test input (like age = 14).
2. Predict what should happen before you run it.
3. Run it and compare the result.
4. Add debug prints to show the values used in decisions.

score = 10
print(scroe)

score = 10
print(score)

a = 5
b = 7
c = 2

total = a + b
total = c

print(total)

a = 5
b = 7
c = 2

total = a + b + c
print(total)