[Ethereum] Get method signature from contract rather than manually constructing

Is there a way to get a method signature from the contract object (using Web3.py) rather than manually constructing it? I don't see a function in there that accomplishes this. It seems like something should be built-in.

I have always calculated the method signature manually using Web3.sha3 as seen below, but this requires me to manually type out the function params.

params = 'uint256,uint256,bytes,address[]'
method_signature = Web3.sha3(text=f"functionNameGoesHere({params})")[0:4]

The contract object already knows the ABI JSON, so shouldn't it be able to generate it for me?

It would be nice if I could do this:

function.method_signature

Since I can already do this:

function.abi

Note, I'm NOT trying to build a transaction to broadcast to the network. I'm trying to generate lots of function hashes from contracts given the contract object an ABI. And these contracts will change over time so I don't want to hard code anything.

Edit: I wrote this myself to do the job. It supports regular types as well as tuples. Would still prefer to use something built-in though.

def GetMethodSignature_GivenAbi(methodName, abi):
    params = ''
    for index, input in enumerate(abi['inputs']):
        if index > 0:
            params += ','

        if input['type'] == 'tuple':
            params += '('
            for index2, tupleComponent in enumerate(input['components']):
                if index2 > 0:
                    params += ','

                params += tupleComponent['type']

            params += ')'

        else:
            params += input['type']

    methodSignature = GetMethodSignature(methodName, params)
    return methodSignature

ABI simple example:

{
  'constant': False,
  'inputs': [
    {
      'name': 'name1',
      'type': 'uint256'
    },
    {
      'name': 'name2',
      'type': 'uint256'
    }
  ],
  'name': 'simpleFunction',
  'outputs': [

  ],
  'payable': False,
  'stateMutability': 'nonpayable',
  'type': 'function'
}

ABI tuple example:

{
  'constant': False,
  'inputs': [
    {
      'name': 'name1',
      'type': 'uint256'
    },
    {
      'name': 'name2',
      'type': 'uint256'
    },
    {
      'name': 'name3',
      'type': 'uint256'
    },
    {
      'components': [
        {
          'name': 'makerAddress',
          'type': 'address'
        },
        {
          'name': 'takerAddress',
          'type': 'address'
        },
        {
          'name': 'feeRecipientAddress',
          'type': 'address'
        },
        {
          'name': 'senderAddress',
          'type': 'address'
        },
        {
          'name': 'makerAssetAmount',
          'type': 'uint256'
        },
        {
          'name': 'takerAssetAmount',
          'type': 'uint256'
        },
        {
          'name': 'makerFee',
          'type': 'uint256'
        },
        {
          'name': 'takerFee',
          'type': 'uint256'
        },
        {
          'name': 'expirationTimeSeconds',
          'type': 'uint256'
        },
        {
          'name': 'salt',
          'type': 'uint256'
        },
        {
          'name': 'makerAssetData',
          'type': 'bytes'
        },
        {
          'name': 'takerAssetData',
          'type': 'bytes'
        },
        {
          'name': 'makerFeeAssetData',
          'type': 'bytes'
        },
        {
          'name': 'takerFeeAssetData',
          'type': 'bytes'
        }
      ],
      'name': 'order',
      'type': 'tuple'
    },
    {
      'name': 'name4',
      'type': 'bytes'
    },
    {
      'name': 'name5',
      'type': 'address[]'
    }
  ],
  'name': 'functionName',
  'outputs': [

  ],
  'payable': False,
  'stateMutability': 'nonpayable',
  'type': 'function'
}