Assertion patterns in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assert that the transaction was successful.

Blockchain / Solidity

assert transaction.status == [1]

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

AFalse

BTrue

CNone

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using False instead of True

Using 0 or None which do not represent success

The assertion checks if the transaction status is True, meaning it succeeded.

2fill in blank

medium

Complete the code to assert the balance increased by the expected amount.

Blockchain / Solidity

assert new_balance == old_balance + [1]

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

Afee

Bgas

Cnonce

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Using fee or gas which are costs, not amounts received

Using nonce which is unrelated to balance

The balance should increase by the amount transferred, not fees or other values.

3fill in blank

hard

Fix the error in the assertion that checks the event log contains the expected event.

Blockchain / Solidity

assert [1] in transaction.events

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

Aevent_log

Btransaction.event

Cexpected_event

Dtransaction.logs

Attempts:

3 left

💡 Hint

Common Mistakes

Using transaction.event which is not a valid attribute

Using event_log or transaction.logs which may not exist

The assertion must check if the expected_event is in the transaction's events list.

4fill in blank

hard

Fill both blanks to assert the smart contract's state variable equals the expected value after the call.

Blockchain / Solidity

assert contract.[1] == [2]

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

AstateVariable

BexpectedValue

CactualValue

Dbalance

Attempts:

3 left

💡 Hint

Common Mistakes

Using actualValue which is not defined

Using balance which is unrelated here

The assertion compares the contract's stateVariable to the expectedValue after execution.

5fill in blank

hard

Fill all three blanks to assert the transaction reverted with the expected error message.

Blockchain / Solidity

with pytest.raises([1]) as excinfo:
    contract.call_function()
assert [2] in str(excinfo.[3])

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

AVirtualMachineError

B"revert"

Cvalue

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong exception type

Checking wrong attribute of excinfo

Missing quotes around 'revert'

The code asserts that a VirtualMachineError is raised, and the error message contains "revert". The exception message is accessed via excinfo.value.

Practice

1.

What is the main purpose of an assertion in blockchain smart contract testing?

easy

A. To optimize the contract's gas usage

B. To log information without affecting execution

C. To deploy the contract to the blockchain

D. To check if a condition is true and stop execution if false

Solution

1. Step 1: Understand assertion role

   Assertions verify conditions during execution to ensure correctness.

2. Step 2: Identify assertion effect

   If the condition is false, assertions stop execution and revert changes to prevent errors.

3. Final Answer:

   To check if a condition is true and stop execution if false -> Option D

4. Quick Check:

   Assertions stop execution on false condition [OK]

Hint: Assertions stop code if condition fails, preventing errors [OK]

Common Mistakes:

• Thinking assertions only log info without stopping
• Confusing assertions with deployment steps
• Assuming assertions optimize gas usage

2.

Which of the following is the correct syntax for an assertion in Solidity?

?

easy

A. assert: balance >= amount;

B. assert(balance >= amount);

C. assert(balance < amount);

D. assert(balance = amount);

Solution

1. Step 1: Recall Solidity assertion syntax

   Assertions use the function-like syntax: assert(condition);

2. Step 2: Check condition correctness

   Condition should be a boolean expression, here balance >= amount is valid.

3. Final Answer:

   assert(balance >= amount); -> Option B

4. Quick Check:

   assert(condition) uses parentheses and semicolon [OK]

Hint: Assertions use assert(condition); with parentheses and semicolon [OK]

Common Mistakes:

• Using colon instead of parentheses
• Using assignment (=) instead of comparison (>=)
• Writing incorrect comparison operators

3.

Consider this Solidity code snippet:

uint balance = 100;
uint amount = 150;
assert(balance >= amount);

What will happen when this code runs?

medium

A. Execution continues normally

B. balance is updated to amount

C. Execution stops and reverts due to failed assertion

D. Compilation error due to invalid assertion

Solution

1. Step 1: Evaluate assertion condition

   balance is 100, amount is 150, so balance >= amount is false.

2. Step 2: Understand assertion behavior on false

   Assertion fails, so execution stops and reverts all changes.

3. Final Answer:

   Execution stops and reverts due to failed assertion -> Option C

4. Quick Check:

   False assertion stops execution [OK]

Hint: False assertion stops execution and reverts changes [OK]

Common Mistakes:

• Assuming code continues after failed assertion
• Thinking assertion causes compile error
• Believing balance changes automatically

4.

Identify the error in this Solidity assertion statement:

assert(balance = amount);

medium

A. Using assignment operator instead of comparison

B. Missing semicolon at the end

C. Incorrect function name, should be require()

D. balance and amount are undefined

Solution

1. Step 1: Analyze operator usage

   The single equals sign (=) assigns value, not compares values.

2. Step 2: Correct operator for assertion

   Assertions require comparison operators like == or >= to check conditions.

3. Final Answer:

   Using assignment operator instead of comparison -> Option A

4. Quick Check:

   Use '==' for comparison in assertions [OK]

Hint: Use '==' for comparison, not '=' assignment in assertions [OK]

Common Mistakes:

• Confusing '=' with '==' in conditions
• Thinking require() must replace assert() always
• Ignoring missing semicolon (which is present here)

5.

You want to assert that a user's balance is positive and provide a clear error message if not. Which assertion pattern is best?

?

hard

A. require(balance > 0, "Balance must be positive");

B. assert(balance > 0, "Balance must be positive");

C. assert(balance > 0);

D. if(balance <= 0) revert("Balance must be positive");

Solution

1. Step 1: Understand assertion vs require

   In Solidity, assert is for internal errors without messages; require allows error messages.

2. Step 2: Choose assertion with message

   require(condition, message) is best to check user input and show clear error.

3. Final Answer:

   require(balance > 0, "Balance must be positive"); -> Option A

4. Quick Check:

   Use require() for checks with error messages [OK]

Hint: Use require() to assert with clear error messages [OK]

Common Mistakes:

• Using assert() with message (not supported)
• Ignoring error messages in assertions
• Using if + revert instead of require() unnecessarily