如何在python中不使用HMAC库来实现hmac？

import hashlib

def hmac(key, message, digestmod=hashlib.sha256):
    block_size = 64
    if len(key) > block_size:
        key = digestmod(key).digest()
    if len(key) < block_size:
        key = key + b'\x00' * (block_size - len(key))

    inner_key = bytes((x ^ 0x36) for x in key)
    outer_key = bytes((x ^ 0x5C) for x in key)

    inner_hash = digestmod(inner_key + message).digest()
    outer_hash = digestmod(outer_key + inner_hash).digest()

    return outer_hash

# 示例用法
key = b'secret_key'
message = b'hello world'
result = hmac(key, message)
print(result.hex())