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.