Building a DeFi Lending Application with Python: A Step-by-Step Guide

DeFi has unlocked a new frontier in financial technology, offering tools and protocols that allow developers to build innovative, transparent, and flexible applications. One of the foundational elements in this ecosystem is decentralized lending and borrowing, which enables users to interact with financial services in a non-custodial and permissionless manner.

In this guide, we continue our exploration of building a DeFi lending application using Python. We’ll dive into the practical steps of depositing collateral, borrowing assets, and managing loans—all while maintaining clarity and practicality for developers.

Depositing Collateral

To interact with a lending pool like Aave, the first step is depositing collateral. This collateral serves multiple purposes:

It allows you to earn yield on the deposited assets.
It enables you to borrow other assets based on the collateral’s value.
It contributes to the overall liquidity of the protocol.

In our example, we use Wrapped Ether (WETH) as collateral. Here’s how the process works in code:

def main():
    account = get_account()
    erc20_address = config["networks"][network.show_active()]["weth_token"]

    if network.show_active() in ["mainnet-fork"]:
        get_weth(account=account)

    lending_pool = get_lending_pool()

The script begins by fetching the user’s account and the WETH token address from the configuration. If you’re on a local testnet, it also converts ETH to WETH using the get_weth function. This ensures you have the necessary tokens to proceed.

Interacting with the Lending Pool

The get_lending_pool function retrieves the address of the lending pool contract:

def get_lending_pool():
    lending_pool_addresses_provider = interface.ILendingPoolAddressesProvider(
        config["networks"][network.show_active()]["lending_poll_addresses_provider"]
    )
    lending_pool_address = lending_pool_addresses_provider.getLendingPool()
    lending_pool = interface.ILendingPool(lending_pool_address)
    return lending_pool

This function uses the LendingPoolAddressesProvider to fetch the current address of the lending pool. This is a common pattern in DeFi: using a provider contract to retrieve up-to-date addresses without hardcoding them.

Approving Token Transfers

Before depositing, you must approve the lending pool to spend your tokens:

approve_erc20(amount, lending_pool.address, erc20_address, account)

The approve_erc20 function handles this:

def approve_erc20(amount, lending_pool_address, erc20_address, account):
    print("Approving ERC20...")
    erc20 = interface.IERC20(erc20_address)
    tx_hash = erc20.approve(lending_pool_address, amount, {"from": account})
    tx_hash.wait(1)
    print("Approved!")
    return True

This step is essential for security—it ensures you explicitly authorize smart contracts to handle your tokens.

Executing the Deposit

Once approved, you can deposit collateral:

lending_pool.deposit(erc20_address, amount, account.address, 0, {"from": account})

This function call specifies:

The token address (WETH).
The amount to deposit.
The recipient address (your account).
A referral code (deprecated, so set to 0).

Upon success, you receive aTokens in return. These aTokens represent your deposited collateral and accrue interest over time. For example, depositing WETH yields aWETH, which increases in value as the protocol generates yield from lending activities.

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Borrowing Against Collateral

With collateral deposited, you can now borrow other assets. Borrowing in DeFi is often used for:

Short-selling without friction.
Accessing liquidity without closing existing positions.

Checking Borrowing Capacity

First, check how much you can borrow:

borrowable_eth, total_debt_eth = get_borrowable_data(lending_pool, account)

The get_borrowable_data function calls getUserAccountData on the lending pool:

def get_borrowable_data(lending_pool, account):
    (
        total_collateral_eth,
        total_debt_eth,
        available_borrow_eth,
        current_liquidation_threshold,
        tlv,
        health_factor,
    ) = lending_pool.getUserAccountData(account.address)
    available_borrow_eth = Web3.fromWei(available_borrow_eth, "ether")
    total_collateral_eth = Web3.fromWei(total_collateral_eth, "ether")
    total_debt_eth = Web3.fromWei(total_debt_eth, "ether")
    print(f"You have {total_collateral_eth} worth of ETH deposited.")
    print(f"You have {total_debt_eth} worth of ETH borrowed.")
    print(f"You can borrow {available_borrow_eth} worth of ETH.")
    return (float(available_borrow_eth), float(total_debt_eth))

This function returns key metrics like:

Total collateral value.
Total debt.
Available borrow amount.
Health factor.

Understanding Health Factor

The health factor is critical—it measures the safety of your loan. If it drops below 1, your position may be liquidated. For example:

Depositing 1 ETH and borrowing 0.5 ETH with a liquidation threshold of 50% gives a health factor of 2.
Borrowing 2 ETH against 1 ETH collateral with a 100% threshold results in a health factor of 0.5, triggering liquidation.

Liquidation occurs when a liquidator repays part of your debt in exchange for discounted collateral. This mechanism ensures the protocol remains solvent.

Executing the Borrow

After assessing your capacity, you can borrow assets:

borrowable_eth, total_debt_eth = get_borrowable_data(lending_pool, account)
erc20_eth_price = get_asset_price()
amount_erc20_to_borrow = (1 / erc20_eth_price) * (borrowable_eth * 0.95)
borrow_erc20(lending_pool, amount_erc20_to_borrow, account)

Here, we calculate the borrow amount based on the current asset price and available borrowing power. The borrow_erc20 function handles the actual borrowing transaction.

Frequently Asked Questions

What is the purpose of aTokens?

aTokens are interest-bearing tokens minted when you deposit collateral into a lending pool. They represent your share of the pool and increase in value over time as interest accrues. When you withdraw, aTokens are burned in exchange for the underlying asset.

How does liquidation work in DeFi lending?

If your health factor falls below 1, your position becomes undercollateralized. Liquidators can repay your debt and claim your collateral at a discount. This protects the protocol from insolvency and incentivizes users to maintain healthy positions.

Why approve token transfers before depositing?

Approval is a security feature of ERC-20 tokens. It ensures that smart contracts can only spend tokens after explicit user consent. This prevents unauthorized access to your funds.

Can I borrow without collateral?

No. DeFi lending requires overcollateralization to mitigate risk. You must deposit collateral worth more than the borrowed amount to open a loan position.

What is a health factor?

The health factor is a numerical representation of your loan’s safety. It is calculated as the ratio of your collateral value to your borrowed value, adjusted by the liquidation threshold. A higher health factor indicates a safer position.

How do I avoid liquidation?

Monitor your health factor regularly and maintain sufficient collateral. Avoid borrowing close to your limit, and consider market volatility—price swings can reduce your collateral value unexpectedly.

Conclusion

Building a DeFi lending application involves multiple steps: depositing collateral, approving transactions, and borrowing assets. Each step requires careful attention to smart contract interactions and financial metrics like health factors.

By leveraging Python and Web3 tools, developers can create robust applications that interact seamlessly with blockchain protocols. Whether you’re building for personal use or a larger audience, understanding these fundamentals is key to success in the DeFi space.

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