Why testing prevents costly bugs in Blockchain / Solidity - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple test function that checks if a smart contract function returns the expected value.

Blockchain / Solidity

def test_get_balance():
    expected = 100
    result = get_balance()
    assert result [1] expected

Drag options to blanks, or click blank then click option'

A==

B!=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' causes the test to fail even if the function is correct.

Using comparison operators like '>' or '<' does not check for exact equality.

The test checks if the result equals the expected value using ==. This confirms the function works as intended.

2fill in blank

medium

Complete the code to simulate a transaction and check if the balance updates correctly.

Blockchain / Solidity

def test_transaction():
    initial_balance = 100
    amount = 20
    new_balance = initial_balance - [1]
    assert new_balance == 80

Drag options to blanks, or click blank then click option'

Ainitial_balance

Bamount

C20

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Subtracting the initial balance from itself results in zero.

Using a fixed number instead of the variable reduces flexibility.

The code subtracts the amount from the initial_balance to get the new balance.

3fill in blank

hard

Fix the error in the test that checks if a smart contract function raises an exception on invalid input.

Blockchain / Solidity

def test_invalid_input():
    try:
        process_transaction(-10)
    except [1]:
        passed = True
    else:
        passed = False
    assert passed

Drag options to blanks, or click blank then click option'

ATypeError

BZeroDivisionError

CValueError

DIndexError

Attempts:

3 left

💡 Hint

Common Mistakes

Catching the wrong exception type causes the test to fail.

Not catching any exception leads to unhandled errors.

The function should raise a ValueError when given a negative amount, so the test catches that exception.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps transaction IDs to their amounts, but only for amounts greater than 50.

Blockchain / Solidity

transactions = {tx_id: amount for tx_id, amount in tx_list if amount [1] 50 and amount [2] 0}

Drag options to blanks, or click blank then click option'

A>

B<

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' reverses the filter logic.

Using '<=' instead of '>=' allows negative amounts.

The comprehension filters amounts greater than 50 and greater than or equal to 0 to include valid transactions.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps user addresses to their balances, but only include users with balances greater than zero.

Blockchain / Solidity

user_balances = { [3].[1]: [3].[2] for [3] in users if [3].[2] > 0 }

Drag options to blanks, or click blank then click option'

Aaddress

Bbalance

Cuser

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names causes errors.

Not filtering for positive balances includes invalid data.

The comprehension uses user to get address and balance for each user, filtering for positive balances.

Practice

1. Why is testing especially important in blockchain development?

easy

A. Because it helps find bugs early to avoid costly mistakes

B. Because blockchain code never changes once written

C. Because testing slows down the development process

D. Because blockchain does not handle real assets

Solution

1. Step 1: Understand blockchain risks

   Blockchain handles real assets and trust, so bugs can cause big losses.

2. Step 2: Role of testing

   Testing finds bugs early, preventing costly mistakes before deployment.

3. Final Answer:

   Because it helps find bugs early to avoid costly mistakes -> Option A

4. Quick Check:

   Testing prevents costly bugs = C [OK]

Hint: Testing finds bugs early to save money and trust [OK]

Common Mistakes:

• Thinking blockchain code never changes
• Believing testing slows development
• Ignoring real asset risks

2. Which of the following is the correct syntax to define a simple test function in Solidity?

easy

A. test function balance() { /* code */ }

B. function testBalance() public { /* test code */ }

C. def testBalance(): pass

D. function testBalance() private { /* test code */ }

Solution

1. Step 1: Recall Solidity function syntax

   Functions start with 'function', name, visibility, and body in braces.

2. Step 2: Check options

   function testBalance() public { /* test code */ } matches Solidity syntax with 'public' visibility for test functions.

3. Final Answer:

   function testBalance() public { /* test code */ } -> Option B

4. Quick Check:

   Solidity test function syntax = B [OK]

Hint: Solidity functions start with 'function' and visibility [OK]

Common Mistakes:

• Using Python syntax in Solidity
• Missing 'function' keyword
• Using wrong visibility for test functions

3. Consider this Solidity test snippet:

uint balance = 100;
function testWithdraw() public {
    uint amount = 50;
    balance -= amount;
    assert(balance == 50);
}

What will happen when testWithdraw() runs?

medium

A. The test passes because balance equals 50 after withdrawal

B. The test fails because balance is not updated

C. The code causes a compilation error

D. The test throws a runtime exception

Solution

1. Step 1: Analyze balance update

   Initial balance is 100, subtract 50, new balance is 50.

2. Step 2: Check assertion

   Assert checks balance == 50, which is true, so test passes.

3. Final Answer:

   The test passes because balance equals 50 after withdrawal -> Option A

4. Quick Check:

   balance after subtract 50 = 50 [OK]

Hint: Check variable updates before assertions [OK]

Common Mistakes:

• Assuming balance is unchanged
• Confusing compile errors with runtime logic
• Ignoring assert condition

4. This Solidity test function is meant to check if a user can withdraw funds:

function testWithdraw() public {
    uint balance = 100;
    uint amount = 150;
    balance -= amount;
    assert(balance >= 0);
}

What is the main problem with this test?

medium

A. The test function is missing visibility

B. The assert condition is always true

C. It allows balance to go negative, which is invalid in Solidity

D. The amount is less than balance, so test is useless

Solution

1. Step 1: Check balance subtraction

   Subtracting 150 from 100 causes underflow (negative balance).

2. Step 2: Understand Solidity behavior

   Solidity unsigned integers cannot be negative; underflow causes error or wrap.

3. Final Answer:

   It allows balance to go negative, which is invalid in Solidity -> Option C

4. Quick Check:

   Unsigned int underflow = error [OK]

Hint: Check for underflow or overflow in tests [OK]

Common Mistakes:

• Ignoring unsigned integer limits
• Assuming assert always passes
• Missing visibility keyword

5. You want to write automated tests for a smart contract that manages token transfers. Which approach best prevents costly bugs?

hard

A. Write tests that check only successful transfers

B. Write tests only after deployment

C. Skip tests and rely on manual code review

D. Write tests that check both successful and failed transfers

Solution

1. Step 1: Understand test coverage needs

   Testing only successes misses bugs in failure cases.

2. Step 2: Importance of failure tests

   Tests for failed transfers catch edge cases and prevent costly errors.

3. Final Answer:

   Write tests that check both successful and failed transfers -> Option D

4. Quick Check:

   Test successes and failures = D [OK]

Hint: Test both success and failure cases to catch all bugs [OK]

Common Mistakes:

• Testing only success cases
• Skipping automated tests
• Testing after deployment too late