Validating Function Arguments
When creating functions in Python, it's important to verify that the arguments passed to your functions are valid before proceeding with the function's main logic. In this step, we'll learn several techniques for validating function arguments.
Creating a New Python File
Let's create a new file to work with validation concepts:
- Click on the "File" menu in the WebIDE
- Select "New File"
- Enter
validate_args.py as the filename
- Click "OK"
Basic Validation with Conditionals
The simplest way to validate arguments is to use conditional statements. Let's start with some basic validations:
def calculate_rectangle_area(length, width):
"""Calculate the area of a rectangle, validating inputs."""
## Validate that inputs are numbers
if not isinstance(length, (int, float)):
raise TypeError("Length must be a number")
if not isinstance(width, (int, float)):
raise TypeError("Width must be a number")
## Validate that inputs are positive
if length <= 0:
raise ValueError("Length must be positive")
if width <= 0:
raise ValueError("Width must be positive")
## Calculate the area
return length * width
## Test with valid inputs
try:
area = calculate_rectangle_area(5, 3)
print(f"Area of rectangle: {area}")
except (TypeError, ValueError) as e:
print(f"Error: {e}")
## Test with invalid types
try:
area = calculate_rectangle_area("5", 3)
print(f"Area of rectangle: {area}")
except (TypeError, ValueError) as e:
print(f"Error: {e}")
## Test with invalid values
try:
area = calculate_rectangle_area(5, -3)
print(f"Area of rectangle: {area}")
except (TypeError, ValueError) as e:
print(f"Error: {e}")
Save and run:
python3 validate_args.py
Output:
Area of rectangle: 15
Error: Length must be a number
Error: Width must be positive
This function validates both the types and values of its arguments before performing any calculations. When invalid arguments are detected, appropriate error messages are raised.
Using Assertions for Validation
Another way to validate arguments is to use assertions. Assertions are statements that raise an AssertionError if a condition is not met:
def calculate_discount(price, discount_percent):
"""Calculate the discounted price."""
## Assert that inputs are valid
assert isinstance(price, (int, float)), "Price must be a number"
assert isinstance(discount_percent, (int, float)), "Discount must be a number"
assert price >= 0, "Price cannot be negative"
assert 0 <= discount_percent <= 100, "Discount must be between 0 and 100"
## Calculate the discount
discount_amount = price * (discount_percent / 100)
return price - discount_amount
## Test with valid inputs
try:
discounted_price = calculate_discount(100, 20)
print(f"Discounted price: ${discounted_price}")
except AssertionError as e:
print(f"Error: {e}")
## Test with invalid discount percentage
try:
discounted_price = calculate_discount(100, 120)
print(f"Discounted price: ${discounted_price}")
except AssertionError as e:
print(f"Error: {e}")
Save and run:
python3 validate_args.py
Output:
Discounted price: $80.0
Error: Discount must be between 0 and 100
Assertions are useful for development and debugging, but they can be disabled in production code, so they're not always the best choice for validation in real-world applications.
Using Type Hints for Documentation
Python 3.5+ supports type hints, which can help document the expected types of function arguments and return values. While type hints don't perform runtime validation by themselves, they provide useful documentation and can be checked by external tools like mypy:
def calculate_average(numbers: list[float]) -> float:
"""Calculate the average of a list of numbers."""
if not numbers:
raise ValueError("Cannot calculate average of empty list")
if not all(isinstance(n, (int, float)) for n in numbers):
raise TypeError("All elements must be numbers")
return sum(numbers) / len(numbers)
## Test with valid input
try:
avg = calculate_average([1, 2, 3, 4, 5])
print(f"Average: {avg}")
except (TypeError, ValueError) as e:
print(f"Error: {e}")
## Test with empty list
try:
avg = calculate_average([])
print(f"Average: {avg}")
except (TypeError, ValueError) as e:
print(f"Error: {e}")
## Test with non-numeric elements
try:
avg = calculate_average([1, 2, "3", 4, 5])
print(f"Average: {avg}")
except (TypeError, ValueError) as e:
print(f"Error: {e}")
Save and run:
python3 validate_args.py
Output:
Average: 3.0
Error: Cannot calculate average of empty list
Error: All elements must be numbers
Note that the type hints (list[float] and -> float) don't perform any validation by themselves - we still need to write our own validation code. They serve as documentation and can be checked by external tools.
Creating a Robust Function with Validation
Now, let's apply all these techniques to build a robust function that calculates the total cost of items with a discount:
def calculate_total_cost(items=None, tax_rate=0, discount=0):
"""
Calculate the total cost of items with tax and discount.
Args:
items: List of (item_name, price) tuples
tax_rate: Tax rate percentage (0-100)
discount: Discount percentage (0-100)
Returns:
A dictionary with the total, subtotal, tax amount, and discount amount
"""
## Validate items
if items is None:
items = []
if not isinstance(items, list):
raise TypeError("Items must be a list")
## Validate each item in the list
for i, item in enumerate(items):
if not isinstance(item, tuple) or len(item) != 2:
raise ValueError(f"Item {i} must be a tuple of (name, price)")
name, price = item
if not isinstance(name, str):
raise TypeError(f"Name of item {i} must be a string")
if not isinstance(price, (int, float)):
raise TypeError(f"Price of item {i} must be a number")
if price < 0:
raise ValueError(f"Price of item {i} cannot be negative")
## Validate tax_rate and discount
if not isinstance(tax_rate, (int, float)):
raise TypeError("Tax rate must be a number")
if not isinstance(discount, (int, float)):
raise TypeError("Discount must be a number")
if not (0 <= tax_rate <= 100):
raise ValueError("Tax rate must be between 0 and 100")
if not (0 <= discount <= 100):
raise ValueError("Discount must be between 0 and 100")
## Calculate the total
subtotal = sum(price for _, price in items)
discount_amount = subtotal * (discount / 100)
tax_amount = (subtotal - discount_amount) * (tax_rate / 100)
total = subtotal - discount_amount + tax_amount
return {
"subtotal": subtotal,
"discount_amount": discount_amount,
"tax_amount": tax_amount,
"total": total
}
## Test with valid inputs
shopping_cart = [
("Laptop", 1000),
("Mouse", 25),
("Keyboard", 45)
]
try:
result = calculate_total_cost(shopping_cart, tax_rate=8.5, discount=10)
print("Shopping Cart Total:")
for key, value in result.items():
print(f" {key.replace('_', ' ').title()}: ${value:.2f}")
except (TypeError, ValueError) as e:
print(f"Error: {e}")
## Test with invalid item
try:
invalid_cart = [
("Laptop", 1000),
("Mouse", "twenty-five"), ## Invalid price
("Keyboard", 45)
]
result = calculate_total_cost(invalid_cart)
print(result)
except (TypeError, ValueError) as e:
print(f"Error with invalid item: {e}")
Save and run:
python3 validate_args.py
Output:
Shopping Cart Total:
Subtotal: $1070.00
Discount Amount: $107.00
Tax Amount: $81.86
Total: $1044.86
Error with invalid item: Price of item 1 must be a number
This function demonstrates robust validation by:
- Checking the types of all inputs
- Validating the range of numeric values
- Providing detailed error messages
- Setting reasonable defaults for optional parameters
- Using docstrings to document the expected inputs and return values
By implementing thorough validation in your functions, you can prevent errors, provide better feedback to users, and make your code more robust and maintainable.
