如何在python中不使用HMAC库来实现hmac？

import hashlib

def custom_hmac(key, message, hash_algorithm):
    # 确保密钥长度不超过哈希函数的块大小
    block_size = {
        hashlib.sha256: 64,
        hashlib.sha1: 64,
        # 根据需要添加其他哈希算法的块大小
    }[hash_algorithm]
    if len(key) > block_size:
        key = hash_algorithm(key.encode()).digest()

    # 如果密钥长度小于块大小，则用0填充
    key += b'\x00' * (block_size - len(key))

    # 定义内、外填充
    inner_pad = bytes(x ^ 0x36 for x in key)
    outer_pad = bytes(x ^ 0x5C for x in key)

    # 计算内部哈希
    inner_hash = hash_algorithm()
    inner_hash.update(inner_pad + message.encode())
    inner_digest = inner_hash.digest()

    # 计算外部哈希
    outer_hash = hash_algorithm()
    outer_hash.update(outer_pad + inner_digest)
    return outer_hash.hexdigest()

# 示例用法
key = "secret_key"
message = "Hello, World!"
hash_algorithm = hashlib.sha256

result = custom_hmac(key, message, hash_algorithm)
print(result)