Mutable vs Immutable

addTwo.py

def addTwo(val):
  val += 2 

x = 1
addTwo(x) 

print(x)

# Output 1 
# Expected 3 

list.py

favoriteNumbers = [1, 2, 3, 4] 

# Update the first element to 0
favoriteNumbers[0] = 0 
# Add 5 to the list 
favoriteNumbers += [5] 
print(favoriteNumber) 

# Output [0, 2, 3, 4, 5]