← QCE Digital Solutions Unit 3

Reliability of a solution

A solution is reliable when it behaves correctly and consistently every time it is used, even with invalid input or missing files or data. It handles errors without crashing.

Part of: QCE Digital Solutions Unit 3 — Innovative Digital Solutions — Module 73

Understanding Reliability of a solution in QCE Digital Solutions Unit 3

What it does: The loop keeps checking the input and re-prompts until it is valid, so bad input never crashes the program. This function depends only on its input, so double(4) returns 8 every time it is used, which makes it consistent and predictable. A reliable solution handles invalid input without crashing and lets the user try again. try/except lets the program cope with missing files instead of crashing. Reliability includes handling missing files or data. A crash in that situation makes the solution unreliable. Reliable code does not assume users behave perfectly. It handles errors like ZeroDivisionError. Reliability needs correct behaviour every time. One unexplained failure shows it is inconsistent.

Common mistakes: An except clause must end with a colon. Without it the program has a syntax error and the missing-file handling never runs. It should be except FileNotFoundError:. Python has no catch keyword. Error handling starts with try:, which lets the program recover from bad input instead of crashing.

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This module's key terms are also in a printable crossword (Modules 251–337) and word search (Modules 194–337).

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Learning

Duo

Which statement best describes a reliable solution?

It works correctly only the first time it is runIt behaves correctly and consistently every time it is used

Matches

f = open("data.txt") lines = f.read()

↓

No error handling: if data.txt is missing the program crashes with FileNotFoundError, so it is not reliable. Wrap it in try/except.

Applying

Meanings

Term

reliability

Meaning

A solution is reliable when it behaves correctly and consistently every time it is used, even with invalid input or missing files or data. It handles errors without crashing.

Practice

value = "abc"

Testing

Gaps

except :

Check Answer

Spots

2 f = open('data.txt')

3except FileNotFoundError

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Quick Questions

1. Which statement best describes a reliable solution?

A. It behaves correctly and consistently every time it is used ✓

B. It works correctly only the first time it is run

Reliability means correct, consistent behaviour every time, not just when everything goes well once.

2. A user types 'abc' where a number is expected. Which response shows reliability?

A. Show an error message and ask for the number again ✓

B. Let the program crash with a ValueError

A reliable solution handles invalid input without crashing and lets the user try again.

3. A program opens 'scores.csv' but the file is missing. Which code is more reliable?

A. try: open('scores.csv') with except FileNotFoundError ✓

B. f = open('scores.csv') with no error handling

try/except lets the program cope with missing files instead of crashing.

4. Which is an example of consistent results?

A. The same input sometimes gives different outputs

B. The same input always gives the same output ✓

A reliable solution produces predictable results for the same input each time it is used.

5. True or false: A solution that crashes when a file is missing is still reliable if it works when the file exists.

A. True

B. False ✓

Reliability includes handling missing files or data. A crash in that situation makes the solution unreliable.

Match the Pairs

try: f = open("scores.txt") except FileNotFoundError: print("File missing") → Handles a missing file with an error message instead of crashing, which makes the solution more reliable

assert calc_total(2, 3) == 5 → A test that checks the function gives the correct result, helping confirm the solution is reliable

A reliable solution deals with invalid input by asking again rather than crashing → while not age.isdigit(): age = input("Enter age: ")

A reliable solution gives consistent results: the same input always produces the same output → def double(n): return n * 2

f = open("data.txt") lines = f.read() → No error handling: if data.txt is missing the program crashes with FileNotFoundError, so it is not reliable. Wrap it in try/except.

age = int(input("Age: ")) → Typing text such as 'abc' causes a ValueError and the program crashes. The input needs validation or try/except so invalid input is handled.

Key Term

reliability

A solution is reliable when it behaves correctly and consistently every time it is used, even with invalid input or missing files or data. It handles errors without crashing.

Fill in the Blanks

1. try: f = open("scores.txt") except FileNotFoundError:

Hint: Type the built-in error name raised when the file being opened does not exist.

2. while not age.isdigit(): age = input("Age: ")

Hint: Type the keyword that keeps repeating the prompt until the input is valid.

3. assert average([2, 4, 6]) == 4

Hint: Type the keyword that tests a function gives the expected result and raises an error if it does not.

4. try: n = int(input("Number: ")) except ValueError:

Hint: Type the error name raised when text such as letters cannot be converted to a whole number.

5. if os.path.exists("data.csv"): f = open("data.csv")

Hint: Type the os.path function name that checks whether a file is present before the program opens it.

Spot the Error

Question 1

A try:

B f = open('data.txt')

C except FileNotFoundError

D print('File missing')

Line C — An except clause must end with a colon. Without it the program has a syntax error and the missing-file handling never runs. It should be except FileNotFoundError:

Question 2

A catch:

B n = int(input('Age: '))

C except ValueError:

D print('Enter a number')

Line A — Python has no catch keyword. Error handling starts with try:, which lets the program recover from bad input instead of crashing.

Question 3

A age = -1

B while age < 0 and age > 120:

C age = int(input('Age: '))

Line B — No number is both below 0 and above 120, so the loop never runs and invalid ages are never rejected. It should use or.

Question 4

A def double(n):

B return n + 2

C assert double(3) == 6

D assert double(0) == 0

Line B — Adding 2 gives 5 for double(3), so the assert fails. The function must return n * 2 to give correct results every time.

Question 5

A def average(nums):

B if len(nums) == 0:

C return 1 / 0

D return sum(nums)/len(nums)

Line C — Returning 1 / 0 causes a ZeroDivisionError, so an empty list crashes the program. The guard should return a safe value such as 0.

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