Python Web3调用OneSplit合约Swap WETH转USDC报错：SafeERC20底层调用失败

问题场景 #

调用OneSplit合约（地址0xC586BeF4a0992C495Cf22e1aeEE4E446CECDee0E）执行WETH→USDC兑换时，代码如下：

transaction = onesplit_abi_contract.functions.swap(
            '0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2',  #WETH TOKEN
            '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48',  #USDC TOKEN
            1000000000000000000,
            0,
            [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 95, 0, 0, 0, 0, 0, 0, 0, 0],
            0,
            ).buildTransaction({
                'from': wallet_address,
                'nonce': w3.eth.get_transaction_count(wallet_address),
                'gas': 190000,
                'gasPrice': w3.eth.gas_price
            })
signed_trx = w3.eth.account.signTransaction(transaction, private_key)
trx_hash = w3.eth.sendRawTransaction(signed_trx.rawTransaction)

始终收到错误：Fail with error 'SafeERC20: low-level call failed'

但将WETH替换为ETH地址0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE，并在buildTransaction中添加'value': 1000000000000000000后，交易可正常执行：

transaction = onesplit_abi_contract.functions.swap(
            '0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE',  #ETH TOKEN
            '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48',  #USDC TOKEN
            1000000000000000000,
            0,
            [100, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
            0,
            ).buildTransaction({
                'from': wallet_address,
                'nonce': w3.eth.get_transaction_count(wallet_address),
                'gas': 190000,
                'gasPrice': w3.eth.gas_price,
                'value': 1000000000000000000
            })
signed_trx = w3.eth.account.signTransaction(transaction, private_key)
trx_hash = w3.eth.sendRawTransaction(signed_trx.rawTransaction)

核心原因 #

ETH是以太坊原生代币，交易时直接通过value字段转入合约即可；而WETH是ERC20标准代币，合约无法直接划转用户钱包里的WETH，必须先由用户调用WETH合约的approve方法，授权OneSplit合约拥有划转指定额度WETH的权限。当前代码跳过了授权步骤，导致合约执行WETH划转时触发SafeERC20调用失败。

解决方案 #

1. 先执行WETH授权操作 #

调用WETH合约的approve方法，授权OneSplit合约可以划转你的WETH：

# WETH合约地址与简化ABI（仅包含approve方法）
weth_address = '0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2'
weth_abi = [{
    "constant": False,
    "inputs": [{"name": "_spender", "type": "address"}, {"name": "_value", "type": "uint256"}],
    "name": "approve",
    "outputs": [{"name": "", "type": "bool"}],
    "payable": False,
    "stateMutability": "nonpayable",
    "type": "function"
}]

weth_contract = w3.eth.contract(address=weth_address, abi=weth_abi)

# 构建授权交易：授权OneSplit合约划转1WETH（可设为2**256-1实现无限授权，仅对可信合约使用）
approve_tx = weth_contract.functions.approve(
    '0xC586BeF4a0992C495Cf22e1aeEE4E446CECDee0E',  # OneSplit合约地址
    1000000000000000000  # 授权金额与swap的金额一致
).buildTransaction({
    'from': wallet_address,
    'nonce': w3.eth.get_transaction_count(wallet_address),
    'gas': 50000,  # approve交易所需gas足够即可
    'gasPrice': w3.eth.gas_price
})

# 签名并发送授权交易
signed_approve_tx = w3.eth.account.signTransaction(approve_tx, private_key)
approve_tx_hash = w3.eth.sendRawTransaction(signed_approve_tx.rawTransaction)

# 等待授权交易上链，确保授权生效
w3.eth.wait_for_transaction_receipt(approve_tx_hash)

2. 执行原Swap交易 #

授权完成后，再执行你原来的Swap交易，注意更新nonce（会自动递增，无需手动计算），同时可适当调高gas值（WETH兑换的gas消耗可能高于ETH）：

transaction = onesplit_abi_contract.functions.swap(
            '0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2',  #WETH TOKEN
            '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48',  #USDC TOKEN
            1000000000000000000,
            0,
            [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 95, 0, 0, 0, 0, 0, 0, 0, 0],
            0,
            ).buildTransaction({
                'from': wallet_address,
                'nonce': w3.eth.get_transaction_count(wallet_address),
                'gas': 250000,  # 适当调高gas避免不足
                'gasPrice': w3.eth.gas_price
            })
signed_trx = w3.eth.account.signTransaction(transaction, private_key)
trx_hash = w3.eth.sendRawTransaction(signed_trx.rawTransaction)

额外检查点 #

• 确认钱包内WETH余额≥1e18（即1WETH）
• 若使用无限授权（2**256 - 1），仅对可信合约操作，避免资产风险
• 若仍报错，可进一步调高gas值，或检查OneSplit合约是否支持当前链上的WETH-USDC兑换对

内容的提问来源于stack exchange，提问作者Edoardo Calderoni