Assert statement usage in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the assert statement in Python?

The assert statement checks if a condition is true. If it is false, it stops the program and raises an AssertionError. It helps catch bugs early by verifying assumptions.

beginner

How do you write an assert statement with a custom error message?

Use assert condition, 'error message'. If the condition is false, Python shows the message with the AssertionError.

beginner

What happens when an assert statement fails?

Python raises an AssertionError and stops running the program unless the error is caught by a try-except block.

intermediate

Can assert statements be disabled? If yes, how?

Yes, running Python with the -O (optimize) flag disables assert statements, so they do not run and do not check conditions.

intermediate

Is it good practice to use assert for data validation in production code?

No, assert is mainly for debugging and testing. For production, use proper error handling because assert can be disabled.

What does this code do?

assert 5 > 3

A Ignores the statement

B Raises an error because 5 is not greater than 3

C Prints 5 and 3

D Checks if 5 is greater than 3 and continues if true

The assert checks if 5 > 3, which is true, so the program continues without error.

What error is raised when an assert condition fails?

A ValueError

B TypeError

C AssertionError

D RuntimeError

When an assert condition is false, Python raises an AssertionError.

How can you disable assert statements in Python?

A Run Python with the -O flag

B Set a global variable

C Use a try-except block

D You cannot disable assert statements

Running Python with the -O (optimize) flag disables assert statements.

Which is the correct way to add a message to an assert?

A assert condition : 'message'

B assert condition, 'message'

C assert(condition, 'message')

D assert 'message' if condition

The syntax is assert condition, 'message' to show a custom error message.

Should assert be used for checking user input in production?

A No, use proper error handling

B Yes, always

C Only for numbers

D Only if input is from a file

Assert is for debugging, not for production input validation.

Explain how the assert statement works and when it should be used.

Think about how assert helps catch mistakes early.

Describe how to write an assert statement with a custom error message and why that might be helpful.

Custom messages make errors clearer.

Practice

1. What does the assert statement do in Python?

easy

A. Prints a message when a condition is false

B. Runs a loop until a condition is true

C. Defines a function to check conditions

D. Checks if a condition is true and stops the program if false

Solution

1. Step 1: Understand the purpose of assert

   The assert statement tests a condition and raises an error if the condition is false.

2. Step 2: Compare options with assert behavior

   Only Checks if a condition is true and stops the program if false correctly describes assert's behavior of stopping the program when the condition is false.

3. Final Answer:

   Checks if a condition is true and stops the program if false -> Option D

4. Quick Check:

   Assert checks condition and stops if false [OK]

Hint: Assert stops program if condition is false [OK]

Common Mistakes:

• Thinking assert runs loops
• Confusing assert with print
• Believing assert defines functions

2. Which of the following is the correct syntax for an assert statement with a message?

easy

A. assert x > 0, "x must be positive"

B. assert (x > 0) "x must be positive"

C. assert x > 0; "x must be positive"

D. assert x > 0: "x must be positive"

Solution

1. Step 1: Recall assert syntax

   The correct syntax is assert condition, message with a comma separating condition and message.

2. Step 2: Check each option

   Only assert x > 0, "x must be positive" uses a comma correctly between condition and message.

3. Final Answer:

   assert x > 0, "x must be positive" -> Option A

4. Quick Check:

   Assert syntax uses comma before message [OK]

Hint: Use comma between condition and message in assert [OK]

Common Mistakes:

• Using colon or semicolon instead of comma
• Missing comma before message
• Putting message without quotes

3. What will be the output of this code?

def check_age(age):
    assert age >= 18, "Age must be at least 18"
    return "Access granted"

print(check_age(20))
print(check_age(16))

medium

A. Access granted\nAccess granted

B. Access granted\nAssertionError: Age must be at least 18

C. AssertionError: Age must be at least 18\nAccess granted

D. No output, program crashes immediately

Solution

1. Step 1: Analyze first function call

   check_age(20) passes the assert since 20 >= 18, so it returns "Access granted" and prints it.

2. Step 2: Analyze second function call

   check_age(16) fails the assert because 16 < 18, so it raises AssertionError with the message.

3. Final Answer:

   Access granted\nAssertionError: Age must be at least 18 -> Option B

4. Quick Check:

   Assert stops program on false condition with error [OK]

Hint: Assert stops at first false condition with error [OK]

Common Mistakes:

• Thinking both print statements run
• Ignoring the error message
• Assuming assert prints message without error

4. Find the error in this code snippet:

assert x > 10 "x should be greater than 10"

medium

A. Missing comma between condition and message

B. Missing parentheses around condition

C. Message should be a variable, not a string

D. Assert cannot have a message

Solution

1. Step 1: Check assert syntax

   Assert requires a comma between the condition and the message string.

2. Step 2: Identify the error

   The code misses the comma, causing a syntax error.

3. Final Answer:

   Missing comma between condition and message -> Option A

4. Quick Check:

   Comma separates condition and message in assert [OK]

Hint: Always put a comma before the assert message [OK]

Common Mistakes:

• Using colon or space instead of comma
• Thinking parentheses are mandatory
• Believing assert can't have messages

5. You want to check a list of numbers to ensure all are positive using assert. Which code correctly uses assert inside a loop to do this?

nums = [3, 5, -1, 7]
for n in nums:
    ?

hard

A. assert n > 0; f"Number {n} is not positive"

B. assert n > 0: f"Number {n} is not positive"

C. assert n > 0, f"Number {n} is not positive"

D. assert (n > 0), "Number n is not positive"

Solution

1. Step 1: Understand assert in loop context

   We want to check each number and stop if any is not positive, showing which one failed.

2. Step 2: Check syntax correctness

   assert n > 0, f"Number {n} is not positive" uses correct assert syntax with a comma and f-string for message. Others use invalid punctuation.

3. Final Answer:

   assert n > 0, f"Number {n} is not positive" -> Option C

4. Quick Check:

   Assert syntax: condition, message with comma [OK]

Hint: Use comma and f-string for assert message in loops [OK]

Common Mistakes:

• Using colon or semicolon instead of comma
• Not using f-string for variable message
• Putting message outside assert statement