Why testing prevents costly bugs in Blockchain / Solidity - See It in Action

Why Testing Prevents Costly Bugs in Blockchain

📖 Scenario: Imagine you are building a simple blockchain smart contract simulation. Bugs in blockchain code can cause huge financial losses. Testing your code carefully helps catch mistakes early, saving money and trust.

🎯 Goal: You will create a small blockchain transaction record, set a limit for valid transactions, filter out invalid ones using a comprehension, and print the valid transactions. This shows how testing and filtering prevent costly errors.

📋 What You'll Learn

Create a dictionary called transactions with exact entries for 5 users and their transaction amounts

Create a variable called max_amount set to 1000

Use a dictionary comprehension called valid_transactions to keep only transactions less than or equal to max_amount

Print the valid_transactions dictionary

💡 Why This Matters

🌍 Real World

In blockchain development, testing transaction limits prevents sending or accepting invalid amounts that could cause financial loss.

💼 Career

Blockchain developers must write safe, tested code to avoid costly bugs that can damage reputation and cause legal issues.

1

DATA SETUP: Create the transactions dictionary

Create a dictionary called transactions with these exact entries: 'Alice': 500, 'Bob': 1500, 'Charlie': 700, 'Diana': 1200, 'Eve': 300

Blockchain / Solidity

# Create the transactions dictionary with exact entries
# Your code here

Hint

Use curly braces {} to create a dictionary with the exact keys and values.

2

CONFIGURATION: Set the maximum allowed transaction amount

Create a variable called max_amount and set it to 1000

Blockchain / Solidity

transactions = {'Alice': 500, 'Bob': 1500, 'Charlie': 700, 'Diana': 1200, 'Eve': 300}
# Set max_amount to 1000
# Your code here

Hint

Just assign 1000 to the variable max_amount.

3

CORE LOGIC: Filter valid transactions using dictionary comprehension

Create a dictionary comprehension called valid_transactions that includes only entries from transactions where the amount is less than or equal to max_amount

Blockchain / Solidity

transactions = {'Alice': 500, 'Bob': 1500, 'Charlie': 700, 'Diana': 1200, 'Eve': 300}
max_amount = 1000
# Create valid_transactions dictionary comprehension
# Your code here

Hint

Use {key: value for key, value in dict.items() if condition} format.

4

OUTPUT: Print the valid transactions

Write a print statement to display the valid_transactions dictionary

Blockchain / Solidity

transactions = {'Alice': 500, 'Bob': 1500, 'Charlie': 700, 'Diana': 1200, 'Eve': 300}
max_amount = 1000
valid_transactions = {user: amount for user, amount in transactions.items() if amount <= max_amount}
# Print the valid_transactions dictionary
# Your code here

Hint

Use print(valid_transactions) to show the filtered dictionary.

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