Transfer and approve flow in Blockchain / Solidity - Mini Project: Build & Apply

Transfer and approve flow

📖 Scenario: You are building a simple blockchain token system where users can transfer tokens to each other and approve others to spend tokens on their behalf.

🎯 Goal: Create a basic token transfer and approval flow using a dictionary to track balances and allowances.

📋 What You'll Learn

Create a dictionary called balances with exact user balances

Create a dictionary called allowances to track approved spending

Write a function transfer to move tokens between users

Write a function approve to set spending allowance

Write a function transfer_from to spend tokens using allowance

Print the final balances after transfers and approvals

💡 Why This Matters

🌍 Real World

This project models how cryptocurrencies and tokens manage transfers and approvals on blockchains.

💼 Career

Understanding transfer and approve flows is essential for blockchain developers working on smart contracts and token standards.

1

Set up initial balances

Create a dictionary called balances with these exact entries: 'Alice': 100, 'Bob': 50, 'Charlie': 75.

Blockchain / Solidity

# Create the balances dictionary with exact values
# Your code here

Need a hint?

Use curly braces to create a dictionary with keys and values exactly as shown.

2

Set up allowances dictionary

Create an empty dictionary called allowances to track approved spending.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50, 'Charlie': 75}
# Create an empty allowances dictionary
# Your code here

Need a hint?

Use empty curly braces to create an empty dictionary.

3

Write transfer, approve, and transfer_from functions

Write three functions: transfer(sender, receiver, amount) to move tokens if sender has enough balance; approve(owner, spender, amount) to set allowance; and transfer_from(spender, owner, receiver, amount) to transfer tokens using allowance if allowed and owner has enough balance.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50, 'Charlie': 75}
allowances = {}
# Write transfer, approve, and transfer_from functions below
# Your code here

Need a hint?

Use dictionary methods like get to safely access balances and allowances.

4

Perform transfers, approvals, and print final balances

Call transfer('Alice', 'Bob', 20), then approve('Bob', 'Charlie', 15), then transfer_from('Charlie', 'Bob', 'Alice', 10). Finally, print the balances dictionary.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50, 'Charlie': 75}
allowances = {}

def transfer(sender, receiver, amount):
    if balances.get(sender, 0) >= amount:
        balances[sender] -= amount
        balances[receiver] = balances.get(receiver, 0) + amount

def approve(owner, spender, amount):
    if owner not in allowances:
        allowances[owner] = {}
    allowances[owner][spender] = amount

def transfer_from(spender, owner, receiver, amount):
    allowed = allowances.get(owner, {}).get(spender, 0)
    if allowed >= amount and balances.get(owner, 0) >= amount:
        balances[owner] -= amount
        balances[receiver] = balances.get(receiver, 0) + amount
        allowances[owner][spender] -= amount

# Call transfer, approve, transfer_from and print balances
# Your code here

Need a hint?

Make sure to call the functions in the correct order and print the balances dictionary.