Calculate Percentages and Discounts with Python

This tutorial will guide you through calculating percentages and discounts using Python. We’ll cover the basic formulas and build three progressively more complex programs.

Example 1: Calculating Percentage of a Value

The first example will calculate the percentage of a given value. We’ll use the formula: Percentage = (Part / Whole) 100

def calculate_percentage(part, whole):
  """Calculates the percentage of a value."""
  if whole == 0:
    return "Error: Cannot divide by zero."
  percentage = (part / whole)  100
  return percentage

# Get input from the user
try:
  part_value = float(input("Enter the part value: "))
  whole_value = float(input("Enter the whole value: "))
except ValueError:
  print("Invalid input. Please enter numbers.")
  exit()

# Calculate the percentage
result = calculate_percentage(part_value, whole_value)

# Print the result
if isinstance(result, str):
    print(result)
else:
    print(f"The percentage is: {result:.2f}%")

Explanation:

• We define a function `calculate_percentage` that takes `part` and `whole` as input.
• Inside the function, we first check if `whole` is zero to prevent division by zero errors.
• If `whole` is not zero, we calculate the percentage using the formula.
• We get user input for `part_value` and `whole_value` using `input()` and convert them to floating-point numbers using `float()`.
• We use a `try-except` block to handle potential `ValueError` exceptions if the user enters non-numeric input.
• We call the `calculate_percentage` function with the input values and store the result.
• We print the calculated percentage formatted to two decimal places using an f-string and `:.2f`.

Common Mistakes and Corrections:

• Mistake: Division by zero. Correction: Added a check for `whole == 0` to prevent the error.
• Mistake: Not handling invalid input. Correction: Used a `try-except` block to catch `ValueError` if the user enters non-numeric input.

# Example Usage:
# Enter the part value: 20
# Enter the whole value: 100
# The percentage is: 20.00%

Output:

Enter the part value: 20
Enter the whole value: 100
The percentage is: 20.00%

Example 2: Calculating Discount Amount

This example calculates the discount amount given a price and a discount percentage. The formula is: Discount Amount = (Discount Percentage / 100) Price

def calculate_discount(price, discount_percentage):
  """Calculates the discount amount for a given price and discount percentage."""
  discount_amount = (discount_percentage / 100)  price
  return discount_amount

# Get input from the user
try:
  price = float(input("Enter the price: "))
  discount_percentage = float(input("Enter the discount percentage: "))
except ValueError:
  print("Invalid input. Please enter numbers.")
  exit()

# Calculate the discount amount
discount = calculate_discount(price, discount_percentage)

# Print the result
print(f"The discount amount is: ${discount:.2f}")

Explanation:

• Similar to the previous example, we define a function `calculate_discount` to calculate the discount amount.
• We get the price and discount percentage from the user.
• We calculate the discount amount using the formula.
• We print the discount amount formatted to two decimal places.

# Example Usage:
# Enter the price: 100
# Enter the discount percentage: 10
# The discount amount is: $10.00

Output:

Enter the price: 100
Enter the discount percentage: 10
The discount amount is: $10.00

Example 3: Calculating Final Price after Discount

This example combines the previous two examples to calculate the final price after applying a discount. The formula is: Final Price = Original Price – Discount Amount

def calculate_final_price(price, discount_percentage):
  """Calculates the final price after applying a discount."""
  discount_amount = (discount_percentage / 100)  price
  final_price = price - discount_amount
  return final_price

# Get input from the user
try:
  price = float(input("Enter the original price: "))
  discount_percentage = float(input("Enter the discount percentage: "))
except ValueError:
  print("Invalid input. Please enter numbers.")
  exit()

# Calculate the final price
final_price = calculate_final_price(price, discount_percentage)

# Print the result
print(f"The final price is: ${final_price:.2f}")

Explanation:

• We define a function `calculate_final_price` that takes the original price and discount percentage as input.
• Inside the function, we first calculate the discount amount using the formula.
• Then, we calculate the final price by subtracting the discount amount from the original price.
• We print the final price formatted to two decimal places.

# Example Usage:
# Enter the original price: 100
# Enter the discount percentage: 10
# The final price is: $90.00

Output:

Enter the original price: 100
Enter the discount percentage: 10
The final price is: $90.00