如何在Python中正确序列化和反序列化分页大小？

Question

在我的Python应用程序中，我对Cassandra数据库进行查询。我试图通过cassandra-driver包实现分页。从下面的代码中可以看到，paging_state返回bytes数据类型。我可以将此值转换为string数据类型。然后我将str_paging_state变量的值发送给客户机。如果此客户端再次向我发送str_paging_state，我希望在查询中使用它

这部分代码的作用是：

query = "select * from users where user_type = 'clients';"
statement = SimpleStatement(query, fetch_size=10)
results = session.execute(statement)

paging_state = results.paging_state
print(type(paging_state)) # <class 'bytes'>

str_paging_state = str(paging_state)
print(str_paging_state) # "b'\\x00C\\x00\\x00\\x00\\x02\\x00\\x00\\x00\\x03_hk\\x00\\x00\\x00\\x11P]5C#\\x8bGD~\\x8b\\xc7g\\xda\\xe5rH\\xb0\\x00\\x00\\x00\\x03_rk\\x00\\x00\\x00\\x18\\xee\\x14\\xf7\\x83\\x84\\x00tTmw[\\x00\\xec\\xdb\\x9b\\xa9\\xfd\\x00\\xb9\\xff\\xff\\xff\\xff\\xfe\\x01\\x00'"

这部分代码引发错误：

results = session.execute(
    statement,
    paging_state=bytes(str_paging_state.encode())
)

错误：

[ERROR] NoHostAvailable: ('Unable to complete the operation against any hosts')
Traceback (most recent call last):
  File "/var/task/lambda_function.py", line 51, in lambda_handler
    results = cassandra_connection.execute(statement, paging_state=bytes(paging_state.encode()))
  File "/opt/python/lib/python3.8/site-packages/cassandra/cluster.py", line 2618, in execute
    return self.execute_async(query, parameters, trace, custom_payload, timeout, execution_profile, paging_state, host, execute_as).result()
  File "/opt/python/lib/python3.8/site-packages/cassandra/cluster.py", line 4877, in result
    raise self._final_exceptionEND RequestId: 4b7bf588-a2d2-45e5-ad7e-8611f1704313

在Java documentation中，我找到了.fromString方法，该方法从先前使用toString()生成的字符串创建PagingState对象。不幸的是，我没有在Python中找到此方法的等效项

我还尝试使用codecs包对paging_state进行解码和编码

str_paging_state = codecs.decode(paging_state, encoding='utf-8', errors='ignore')
# "\u0000C\u0000\u0000\u0000\u0002\u0000\u0000\u0000\u0003_hk\u0000\u0000\u0000\u0011P]5C#GD~grH\u0000\u0000\u0000\u0003_rk\u0000\u0000\u0000\u0018\u0014\u0000tTmw[\u0000ۛ\u0000\u0001\u0000"

# Raise error
results = session.execute(statement, paging_state=codecs.encode(str_paging_state, encoding='utf-8', errors='ignore'))

在这种情况下，我看到下一个错误：

[ERROR] ProtocolException: <Error from server: code=000a [Protocol error] message="Invalid value for the paging state">
Traceback (most recent call last):
  File "/var/task/lambda_function.py", line 50, in lambda_handler
    results = cassandra_connection.execute(
  File "/opt/python/lib/python3.8/site-packages/cassandra/cluster.py", line 2618, in execute
    return self.execute_async(query, parameters, trace, custom_payload, timeout, execution_profile, paging_state, host, execute_as).result()
  File "/opt/python/lib/python3.8/site-packages/cassandra/cluster.py", line 4877, in result
    raise self._final_exceptionEND RequestId: 979f098a-a566-4904-821a-2ce06522d909

在我的例子中，protocol version是4

cluster = Cluster(..., protocol_version=4)

我将感谢任何帮助

Answer 1

只需将二进制数据转换为十六进制字符串或base64-使用^{} module即可。例如，对于第一种情况的函数hexlify/unhexlify（或者在Python 3中使用二进制数据的.hex方法），对于base64-使用函数b2a_base64/a2b_base64