需求
最近我们需要在BSC上实现代币的自动化兑换，比如自动把BNB兑换成USDT，自动把USDT兑换成CAKE等其它代币，同时也要监视价格，在价格合适的时候再兑换代币。而PancakeSwap正是BSC上最大的去中心化平台，我们已经学会了手动在PancakeSwap上进行代币兑换，那么如何使用Python对这个操作进行自动化呢？
快速学习法
本文从新手的角度，讲解如何在面对一个从来没接触过的新事物时，快速上手做出东西来。
我们在学习一些新知识，新编程语言，新框架的时候，往往会陷入一个误区，就是总想一来就要把东西从头到尾的啃透，在网上找了一大堆资料、书籍、视频，然后正襟危坐的从第一页开始学起，过不了多久，就会越来越枯燥，越来越疲惫，从而半途而费。
多年来我总结出了一种快速学习法，就是先借助搜索引擎看个大概，对目标事物有个基本了解，然后就直接动手开始操练，刚开始肯定是写的磕磕绊绊，但是不要紧，边写边查边改，慢慢摸索，很快就能写出个勉强能用的东西，接下来再慢慢改进，不断优化，最终改成一份能放入产品的代码，此时基本用法已经学到个七七八八了，再越到类似需求则可以融会贯通，快速做出东西。
调用合约
要实现在PancakeSwap上的自动化需求，首先我们了解到，PancakeSwap的底层是一套智能合约。那么第一步我们的目标就是学会使用Python调用智能合约。
通过一番搜索我们找到了一个名为【web3.py】的库，正是做这个事情的。
初略阅读一下【web3.py】的文档，我们直接看调用合约的部分：
https://web3py.readthedocs.io/en/v5/contracts.html#contractcaller
文档中有大量的示例可以参考：
https://web3py.readthedocs.io/en/v5/examples.html
接下来马上动手操练，写一个调用合约的代码，比如调用BUSD的合约，查询我们的BUSD余额。
我们在BSC浏览器上找到BUSD的合约地址：
https://bscscan.com/address/0xe9e7CEA3DedcA5984780Bafc599bD69ADd087D56#readContract
在下方的【Contract】选项卡，我们可以看到该合约的方法，其中【Read Contract】是只读方法，用于查询一些数据，不需要签名即可调用，【Write Contract】是写入方法，需要签名授权才可以调用，我们可以直接在这边页面手动测试，比如上图中，在【balanceOf】方法中输入我们的钱包地址，点击【Query】按钮就可以查出我们的余额，由于USDT有18位小数精度，所以这个数值需要除以 10^18，就可以得到人类可读的余额约为 35.026 BUSD。其实这是一个标准的ERC20代币合约，在BSC上也称为【BEP20 Token】 标准，有着统一的ABI接口，但先不用了解那么多，我们直接上手，使用Python的【web3.py】库，调用这个合约的【balanceOf】方法：
import web3from decimal import Decimalimport jsonclient = web3.Web3(web3.HTTPProvider("https://bsc-dataseed1.binance.org"))address_busd = "0xe9e7CEA3DedcA5984780Bafc599bD69ADd087D56"abi_busd = json.loads(r'[{"inputs":[],"payable":false,"stateMutability":"nonpayable","type":"constructor"},{"anonymous":false,"inputs":[{"indexed":true,"internalType":"address","name":"owner","type":"address"},{"indexed":true,"internalType":"address","name":"spender","type":"address"},{"indexed":false,"internalType":"uint256","name":"value","type":"uint256"}],"name":"Approval","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"internalType":"address","name":"previousOwner","type":"address"},{"indexed":true,"internalType":"address","name":"newOwner","type":"address"}],"name":"OwnershipTransferred","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"internalType":"address","name":"from","type":"address"},{"indexed":true,"internalType":"address","name":"to","type":"address"},{"indexed":false,"internalType":"uint256","name":"value","type":"uint256"}],"name":"Transfer","type":"event"},{"constant":true,"inputs":[],"name":"_decimals","outputs":[{"internalType":"uint8","name":"","type":"uint8"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"_name","outputs":[{"internalType":"string","name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"_symbol","outputs":[{"internalType":"string","name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"internalType":"address","name":"owner","type":"address"},{"internalType":"address","name":"spender","type":"address"}],"name":"allowance","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"internalType":"address","name":"spender","type":"address"},{"internalType":"uint256","name":"amount","type":"uint256"}],"name":"approve","outputs":[{"internalType":"bool","name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"internalType":"address","name":"account","type":"address"}],"name":"balanceOf","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"internalType":"uint256","name":"amount","type":"uint256"}],"name":"burn","outputs":[{"internalType":"bool","name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"internalType":"uint8","name":"","type":"uint8"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"internalType":"address","name":"spender","type":"address"},{"internalType":"uint256","name":"subtractedValue","type":"uint256"}],"name":"decreaseAllowance","outputs":[{"internalType":"bool","name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"getOwner","outputs":[{"internalType":"address","name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"internalType":"address","name":"spender","type":"address"},{"internalType":"uint256","name":"addedValue","type":"uint256"}],"name":"increaseAllowance","outputs":[{"internalType":"bool","name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"internalType":"uint256","name":"amount","type":"uint256"}],"name":"mint","outputs":[{"internalType":"bool","name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"name","outputs":[{"internalType":"string","name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"owner","outputs":[{"internalType":"address","name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"renounceOwnership","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"internalType":"string","name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"internalType":"address","name":"recipient","type":"address"},{"internalType":"uint256","name":"amount","type":"uint256"}],"name":"transfer","outputs":[{"internalType":"bool","name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"internalType":"address","name":"sender","type":"address"},{"internalType":"address","name":"recipient","type":"address"},{"internalType":"uint256","name":"amount","type":"uint256"}],"name":"transferFrom","outputs":[{"internalType":"bool","name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"internalType":"address","name":"newOwner","type":"address"}],"name":"transferOwnership","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"}]')def main():contract = client.eth.contract(address = address_busd, abi = abi_busd)balance = contract.caller.balanceOf("0xeaC7d998684F50b7A492EA68F27633a117Be201d")balance = Decimal(balance) / (10 ** 18)print(balance)if __name__ == '__main__':main()执行结果：
果然成功调用合约拿到了余额数字
合约ABI
代码其中有个参数是abi_busd，即BUSD这个合约的ABI，这个ABI从哪里来呢，依然是在BSC浏览器里面查询合约地址就可以找到：
https://bscscan.com/address/0xe9e7CEA3DedcA5984780Bafc599bD69ADd087D56#code
在【Contract】的【Code】里可以找到它的ABI，是一个JSON，复制出来即可。
ABI我们可以理解为对该合约所有公开方法的描述，它声明了函数名，函数参数个数、类型，返回值，这样在调用该合约的时候才能正确的序列化参数以及获取返回值，不难理解为什么要有这么个东西，想想在Windows上的C++开发中，很多第三方闭源库都是提供一个DLL文件和一个 .h头文件和一个 .lib 导入库，.h里的函数定义就如同这些JSON ABI，描述每个函数的函数名和参数，我们才能正确调用，至于C# 的DLL为什么可以直接调用而不需要接口描述文件，那是因为它的接口描述信息已经写到了DLL中，可以直接从DLL文件读取。
那么这个合约的ABI文件，除了从BSC浏览器中手动复制，有没有办法从网络上获取呢？当然是有的，【bscscan】提供了很多API其中就包括获取合约ABI的API，文档在此：
https://docs.bscscan.com/api-endpoints/contracts
import web3from web3.middleware import geth_poa_middlewarefrom decimal import Decimalimport requestsclient = web3.Web3(web3.HTTPProvider("https://bsc-dataseed1.binance.org"))# 连接公共节点需要注入此中间件client.middleware_onion.inject(geth_poa_middleware, layer=0)# 我们的钱包地址address_wallet = "0xeaC7d998684F50b7A492EA68F27633a117Be201d"# 私钥从环境变量获取private_key = os.getenv("key")# BUSD合约地址address_busd = "0xe9e7CEA3DedcA5984780Bafc599bD69ADd087D56"# pancakeswap合约地址address_pancakeswap = "0x10ED43C718714eb63d5aA57B78B54704E256024E"# 获取合约ABIdef fetch_abi(address):url = "https://api.bscscan.com/api"params = {"module": "contract","action": "getabi","address": address,"apikey": "6HQZV977GIY8FNT36XN1JJSE12IP2IYVTQ",}resp = requests.get(url, params = params).json()return resp["result"]# 获取账户ERC20代币余额def erc20_balance(address_erc20: str, wallet: str) -> Decimal:abi_busd = fetch_abi(address_erc20)contract = client.eth.contract(address=address_erc20, abi=abi_busd)# 先获取该代币精度decimals = contract.functions.decimals().call()# 获取代币余额balance = contract.functions.balanceOf(wallet).call()balance = Decimal(balance) / (10 ** decimals)return balancedef main():balance = erc20_balance(address_busd, address_wallet)print(balance)if __name__ == '__main__':main()于是我们写了一个Python函数用于从网上获取ABI，这样在调用不同合约的时候，就不用一一手动复制ABI了。
注意上面的 apikey 是 bscscan.com 的 apikey ，我们可以在这里注册账号免费申请：https://bscscan.com/apis
免费版的 apikey 一天可以调用10万次 bscscan API，足够使用了。
不过这种从网上获取ABI的方式，在项目实践中最好获取一次然后缓存到本地，可以存到数据库也可以存到文本文件，总之每调用一次合约函数，就要发送HTTP请求获取一次ABI，是不太明智的选择，当然我们现在是学习阶段，这样简单写一写也无妨。
授权
接下来，我们需要调用 PancakeSwap 合约，实现代币的兑换，以BUSD兑换CAKE为例，最简单快速的方法，就是直接去 PancakeSwap 网页上手动发起一笔交易，待交易成功后，到BSC浏览器里查看链上的交易记录，看它调用了哪个合约的哪个方法，传递了什么参数，然后使用Python写代码，照葫芦画瓢即可。
首先，在 PancakeSwap 第一次交易某种代币时，需要授权，这个授权实际上是调用 BUSD 合约的【approve】方法，将我们账户的BUSD授权给 PancakeSwap 的合约进行支配，这样在接下来的兑换交易中，PancakeSwap 的合约才有权从我们账户中提取我们的BUSD代币，然后缓存CAKE代币转给我们。这个授权该如何用Python去实现呢？最简单的方法是直接提交交易，然后到BSC浏览器查询这笔交易：
https://bscscan.com/tx/0x6bdb716c91b4446dc9f66305578569241eb6b3ee058d943730a9ff3645f7994f#eventlog
在 Logs 这里我们可以看到这笔授权交易调用了
0xe9e7cea3dedca5984780bafc599bd69add087d56 合约（BUSD合约）的 Approval 方法
在【Overview】中我们可以看到传递的两个参数值是什么
再到BSC浏览器中查看BUSD的合约方法：
确实有【approve】这个方法，我们也可以直接在这里输入一样的参数进行手动测试，以验证结果。网上搜索资料，我们可以了解到这两个参数的意思，【spender】就是授权给谁，这里是授权给PancakeSwap的合约地址，【amount】是授权金额，图片里的这个值就是uint256的最大值，最终结果就等同于【无限制】。
接下来我们用Python代码实现这个授权操作：
import web3from web3.middleware import geth_poa_middlewarefrom decimal import Decimalimport requestsclient = web3.Web3(web3.HTTPProvider("https://bsc-dataseed1.binance.org"))# 连接公共节点需要注入此中间件client.middleware_onion.inject(geth_poa_middleware, layer=0)# 我们的钱包地址address_wallet = "0xeaC7d998684F50b7A492EA68F27633a117Be201d"# 私钥从环境变量获取private_key = os.getenv("key")# BUSD合约地址address_busd = "0xe9e7CEA3DedcA5984780Bafc599bD69ADd087D56"# pancakeswap合约地址address_pancakeswap = "0x10ED43C718714eb63d5aA57B78B54704E256024E"# 获取合约ABIdef fetch_abi(address):url = "https://api.bscscan.com/api"params = {"module": "contract","action": "getabi","address": address,"apikey": "6HQZV977GIY8FNT36XN1JJSE12IP2IYVTQ",}resp = requests.get(url, params = params).json()return resp["result"]# 授权pancakeswap合约使用我们账户的BUSD代币def approve_busd_to_pancakeswap(wallet: str):abi_busd = fetch_abi(address_busd)contract = client.eth.contract(address=address_busd, abi=abi_busd)txn = contract.functions.approve(address_pancakeswap, 2 ** 256 - 1).build_transaction({"chainId": client.eth.chain_id,"from": wallet,"nonce": client.eth.get_transaction_count(wallet),"gasPrice": client.eth.gas_price})signed_txn = client.eth.account.sign_transaction(txn, private_key)txn_hash = client.eth.send_raw_transaction(signed_txn.rawTransaction)txn_hash = client.toHex(txn_hash)return txn_hashdef main():txn_hash = approve_busd_to_pancakeswap(address_wallet)print(txn_hash)if __name__ == '__main__':main()查询价格
授权完成后，在进行兑换交易之前，我们需要先查询兑换价格，这样才能确定多少代币可以兑换多少另一种代币，我们以BUSD购买CAKE为例，先查询一个BUSD可以买多少个CAKE。
我们先在BSC浏览器中找到 PancakeSwap 的合约地址：
https://bscscan.com/address/0x10ed43c718714eb63d5aa57b78b54704e256024e#readContract
同理，我们在下方的【Contract】【Read Contract】中可以测试该合约的所有可读方法，其中【getAmountsOut】方法就是用于查询多少A代币可以兑换多少B代币的函数，我们可以直接在这里手动测试，其中【amountIn】参数就是输入多少A代币，【path】参数就是兑换路径，这是一个数组，这里我们填入BUSD的合约地址和CAKE的合约地址即可，表示直接从BUSD兑换成CAKE，因为这两个代币之间有一个很大的流动性池子，可以直接兑换。有的时候，要兑换的两种代币之间没有池子，则需要将A先换成B再换成C，这个时候【path】就有三个元素，分别是A/B/C代币的合约地址。
比如要把HIGH兑换成CAKE，由于HIGH和CAKE之间没有池子，所以需要先讲HIGH换成BUSD再换成CAKE，那么【path】就需要三个元素，分别是HIGH/BUSD/CAKE的合约地址。
弄清楚大致原理后，我们就可以着手写Python代码如下：
import web3from web3.middleware import geth_poa_middlewarefrom decimal import Decimalimport requestsclient = web3.Web3(web3.HTTPProvider("https://bsc-dataseed1.binance.org"))# 连接公共节点需要注入此中间件client.middleware_onion.inject(geth_poa_middleware, layer=0)# 我们的钱包地址address_wallet = "0xeaC7d998684F50b7A492EA68F27633a117Be201d"# BUSD合约地址address_busd = "0xe9e7CEA3DedcA5984780Bafc599bD69ADd087D56"# CAKE合约地址address_cake = "0x0E09FaBB73Bd3Ade0a17ECC321fD13a19e81cE82"# pancakeswap合约地址address_pancakeswap = "0x10ED43C718714eb63d5aA57B78B54704E256024E"# 获取合约ABIdef fetch_abi(address):url = "https://api.bscscan.com/api"params = {"module": "contract","action": "getabi","address": address,"apikey": "6HQZV977GIY8FNT36XN1JJSE12IP2IYVTQ",}resp = requests.get(url, params = params).json()return resp["result"]# 查询CAKE与BUSD兑换价格def query_cake_price() -> Decimal:abi_pancakeswap = fetch_abi(address_pancakeswap)contract = client.eth.contract(address=address_pancakeswap, abi=abi_pancakeswap)price = contract.functions.getAmountsOut(1 * 10 ** 18, [address_busd, address_cake]).call()return Decimal(price[1]) / (10 ** 18)def main():price = query_cake_price()print(price)if __name__ == '__main__':main()执行结果：
可以看到，执行结果为，1个BUSD可以换约0.2535个CAKE
需要注意的是，函数中的 10 ** 18，因为我们知道 BUSD 和 CAKE 的精度都是18位小数，所以我们在这里直接写死了，在实际项目中需要根据你的情况更改。
兑换交易
接下来最后一步就是进行兑换交易了，我们可以先手动兑换一笔，然后查看交易记录：
https://bscscan.com/tx/0x71f5d27d08803ca834b685c86b7f8a2b72039b468fc6600256a18e8ba0c7a173
我们可以看到它调用了 PancakeSwap 合约的 【swapExactTokensForTokens】方法，【Decode Input Data】后可以看到传递的参数是什么：
有同学就要问了，你怎么知道 PancakeSwap 合约的这些方法和参数是什么含义呢？包括上面的【getAmountsOut】方法，答案就在PancakeSwap的文档中，解释得很清楚了：
https://docs.pancakeswap.finance/code/smart-contracts/pancakeswap-exchange/v2/router-v2
接下来我们用 Python 代码实现兑换操作：
import web3from web3.middleware import geth_poa_middlewarefrom decimal import Decimalimport requestsfrom datetime import datetime, timedeltaclient = web3.Web3(web3.HTTPProvider("https://bsc-dataseed1.binance.org"))# 连接公共节点需要注入此中间件client.middleware_onion.inject(geth_poa_middleware, layer=0)# 我们的钱包地址address_wallet = "0xeaC7d998684F50b7A492EA68F27633a117Be201d"# 私钥从环境变量获取private_key = os.getenv("key")# BUSD合约地址address_busd = "0xe9e7CEA3DedcA5984780Bafc599bD69ADd087D56"# CAKE合约地址address_cake = "0x0E09FaBB73Bd3Ade0a17ECC321fD13a19e81cE82"# pancakeswap合约地址address_pancakeswap = "0x10ED43C718714eb63d5aA57B78B54704E256024E"# 获取合约ABIdef fetch_abi(address):url = "https://api.bscscan.com/api"params = {"module": "contract","action": "getabi","address": address,"apikey": "6HQZV977GIY8FNT36XN1JJSE12IP2IYVTQ",}resp = requests.get(url, params = params).json()return resp["result"]# 将BUSD兑换为CAKEdef swap_busd_to_cake(wallet: str, amount_busd: Decimal):abi_pancakeswap = fetch_abi(address_pancakeswap)contract = client.eth.contract(address=address_pancakeswap, abi=abi_pancakeswap)price_cake = contract.functions.getAmountsOut(1 * 10 ** 18, [address_busd, address_cake]).call()price_cake = Decimal(price_cake[1]) / (10 ** 18)# 滑点0.5% 交易手续费 0.25% ，则最小换得的cake数量为理论换得的cake数量 乘 99.25 %amount_cake_min = int(price_cake * amount_busd * Decimal("0.9925") * 10 ** 18)amount_busd = int(amount_busd * 10 ** 18)# deadline为交易必须在此时间前确认，否则交易失败，我们在pancakeswap网页上手动操作的时候默认是20分钟，所以照着它来deadline = datetime.now() + timedelta(minutes = 20)txn = contract.functions.swapExactTokensForTokens(amount_busd, amount_cake_min, [address_busd, address_cake], wallet, int(deadline.timestamp())).build_transaction({"chainId": client.eth.chain_id,"from": wallet,"nonce": client.eth.get_transaction_count(wallet),"gasPrice": client.eth.gas_price})signed_txn = client.eth.account.sign_transaction(txn, private_key)txn_hash = client.eth.send_raw_transaction(signed_txn.rawTransaction)txn_hash = client.toHex(txn_hash)return txn_hashdef main():txn_hash = swap_busd_to_cake(address_wallet, Decimal("1.0"))print(txn_hash)if __name__ == '__main__':main()执行以后，交易成功，perfect ！
https://bscscan.com/tx/0xbb7de1e37f332bd8114ba8d58cf3a13226d76194dc8cc08f3e7dac9ba00df4a0
完整示例代码
【python_pancakeswap】https://github.com/encoderlee/python_pancakeswap