Lists three possibilities to consider if a function is not working.
- Describe each possibility in your own words.
- Define "precondition" and "postcondition" as part of your description.
- Create your own example of each possibility in Python code. List the code for each example, along with sample output from trying to run it.
Precondition: Conditions that must be true before the function executes.
Postcondition: Once the function executes, certain guarantees (postconditions) should hold true.
2. Function Logic Issue (Postcondition Violation):
This happens when the function executes but does not produce the expected results, violating its postconditions.
Precondition: The function executes without error, but the result may not be as expected.
Postcondition: The expected guarantees after the function execution are not met.
3. Return Value or Usage Issue:
This occurs when the function returns incorrect values or when the returned values are not used properly.
Precondition: The function executes without error, and the return value is generated.
Postcondition: The return value should be correct, and it should be used appropriately in the rest of the program.
Example Code:
# Argument-related Issue (Precondition Violation)
def calculate_area(length, width):
return length * width
# Calling the function with incorrect arguments
# Expected: Length and width must be numeric values
area = calculate_area("5", 3)
print("Area:", area)Output (Argument-Related Issue Explaination):
TypeError: can't multiply sequence by non-int of type 'str'
Explanation: The function 'calculate_are'a expects numeric values for 'length' and 'width', but it's called with a string argument for 'length', violating the precondition.
# Function Logic Issue (Postcondition Violation)
def calculate_area(length, width):
# Incorrect calculation
return length + width
length = 5
width = 3
area = calculate_area(length, width)
print("Area:", area)Output (Function Logic Issue Explanation):
Area: 8
Explanation: The function 'calculate_area' is intended to calculate the area of a rectangle, but it adds 'length'and 'width' instead of multiplying them, violating the postcondition.
# Return Value or Usage Issue
def calculate_area(length, width):
return length * width
length = 5
width = 3
area = calculate_area(length, width)
# Incorrect usage of the return value
# Expected: Printing the area
print("Area calculated:", area[0])
Output (Return Value or Usage Issue Explanation):
TypeError: 'int' object is not subscriptable
Explanation: The return value from 'calculate_area' is an integer, but the code tries to access it as if it were a list or tuple, leading to a TypeError.
Now Can some explain to me How do you approach designing functions to minimize the risk of precondition and postcondition violations, especially in larger codebases?
For the pre-condition example, we could make it explicit by using an assert:
def calculate_area(length, width):
# pre-condition: input must be numbers
from numbers import Number
assert isinstance(length, Number) and isinstance(width, Number), "Wrong input type!"
return length * width
area = calculate_area("5", 3)
print("Area:", area)
# output
AssertionError Traceback (most recent call last)
Cell In[23], line 10
6 return length * width
8 # Calling the function with incorrect arguments
9 # Expected: Length and width must be numeric values
---> 10 area = calculate_area("5", 3)
11 print("Area:", area)
Cell In[23], line 5, in calculate_area(length, width)
2 def calculate_area(length, width):
3 # pre-condition: input must be numbers
4 from numbers import Number
----> 5 assert isinstance(length, Number) and isinstance(width, Number), "Wrong input type!"
6 return length * width
AssertionError: Wrong input type!