Signs inputs in provided transaction object for which unspents
    are provided and can be signed by the private key.

    :param private_key: Private key
    :type private_key: ``PrivateKey`` or ``MultiSig``
    :param tx: Transaction object
    :type tx: ``TxObj``
    :param unspents: For inputs to be signed their corresponding Unspent objects
                     must be provided.
    :type unspents: ``list`` of :class:`~bit.network.meta.Unspent`
    :returns: The signed transaction as hex.
    :rtype: ``str``

def sign_tx(private_key, tx, *, unspents):
    """Signs inputs in provided transaction object for which unspents
    are provided and can be signed by the private key.

    :param private_key: Private key
    :type private_key: ``PrivateKey`` or ``MultiSig``
    :param tx: Transaction object
    :type tx: ``TxObj``
    :param unspents: For inputs to be signed their corresponding Unspent objects
                     must be provided.
    :type unspents: ``list`` of :class:`~bit.network.meta.Unspent`
    :returns: The signed transaction as hex.
    :rtype: ``str``
    """

    # input_dict contains those unspents that can be signed by private_key,
    # providing additional information for segwit-inputs (the amount to spend)
    input_dict = {}
    try:
        for unspent in unspents:
            if not private_key.can_sign_unspent(unspent):
                continue
            tx_input = hex_to_bytes(unspent.txid)[::-1] + \
                unspent.txindex.to_bytes(4, byteorder='little')
            input_dict[tx_input] = unspent.to_dict()
    except TypeError:
        raise TypeError('Please provide as unspents at least all inputs to '
                        'be signed with the function call in a list.')

    # Determine input indices to sign from input_dict (allows for transaction batching)
    sign_inputs = [j for j, i in enumerate(tx.TxIn) if i.txid+i.txindex in input_dict]

    segwit_tx = TxObj.is_segwit(tx)
    public_key = private_key.public_key
    public_key_push = script_push(len(public_key))
    hash_type = HASH_TYPE

    # Make input parameters for preimage calculation
    inputs_parameters = []
    for i in sign_inputs:
        # Create transaction object for preimage calculation
        tx_input = tx.TxIn[i].txid + tx.TxIn[i].txindex
        segwit_input = input_dict[tx_input]['segwit']
        tx.TxIn[i].segwit_input = segwit_input
        # For partially signed transaction we must extract the signatures:
        input_script_field = tx.TxIn[i].script_sig

        script_code = private_key.scriptcode
        script_code_len = int_to_varint(len(script_code))

        # Use scriptCode for preimage calculation of transaction object:
        tx.TxIn[i].script_sig = script_code
        tx.TxIn[i].script_sig_len = script_code_len

        if segwit_input:
            try:
                tx.TxIn[i].script_sig += input_dict[tx_input]['amount']\
                                         .to_bytes(8, byteorder='little')
                # For partially signed transaction we must extract the
                # signatures:
                input_script_field = tx.TxIn[i].witness
            except AttributeError:
                raise ValueError(
                    'Cannot sign a segwit input when the input\'s amount is '
                    'unknown. Maybe no network connection or the input is '
                    'already spent? Then please provide all inputs to sign as '
                    '`Unspent` objects to the function call.')

        inputs_parameters.append([i, hash_type, segwit_input])
    preimages = calculate_preimages(tx, inputs_parameters)

    # Calculate signature scripts:
    for hash, (i, _, segwit_input) in zip(preimages, inputs_parameters):
        signature = private_key.sign(hash) + b'\x01'

        # ------------------------------------------------------------------
        if (private_key.instance == 'MultiSig' or
                private_key.instance == 'MultiSigTestnet'):
            # P2(W)SH input

            script_blob = b''
            sigs = {}
            # Initial number of witness items (OP_0 + one signature + redeemscript).
            witness_count = 3
            if input_script_field:
                sig_list = get_signatures_from_script(input_script_field)
                # Bitcoin Core convention: Every missing signature is denoted
                # by 0x00. Only used for already partially-signed scriptSigs:
                script_blob += b'\x00' * (private_key.m - len(sig_list)-1)
                # Total number of witness items when partially or fully signed:
                witness_count = private_key.m + 2
                # For a partially signed input make a dictionary containing
                # all the provided signatures with public-keys as keys:
                for sig in sig_list:
                    for pub in private_key.public_keys:
                        if verify_sig(sig[:-1], hash, pub):
                            # If we already found a valid signature for pubkey
                            # we just overwrite it and don't care.
                            sigs[pub] = sig
                if len(sigs) >= private_key.m:
                    raise ValueError('Transaction is already signed with '
                                     'sufficiently needed signatures.')

            sigs[public_key] = signature

            witness = b''
            # Sort ingthe signatures according to the public-key list:
            for pub in private_key.public_keys:
                if pub in sigs:
                    sig = sigs[pub]
                    length = int_to_varint(len(sig)) if segwit_input else \
                        script_push(len(sig))
                    witness += length + sig

            script_sig = b'\x22' + private_key.segwit_scriptcode

            witness = (int_to_varint(witness_count) if segwit_input else b'') \
                + b'\x00' + witness + script_blob
            witness += (int_to_varint(len(private_key.redeemscript)) if
                segwit_input else script_push(len(private_key.redeemscript))) \
                + private_key.redeemscript

            script_sig = script_sig if segwit_input else witness
            witness = witness if segwit_input else b'\x00' if segwit_tx else b''

        # ------------------------------------------------------------------
        else:
            # P2(W)PKH input

            script_sig = b'\x16' + private_key.segwit_scriptcode

            witness = (
                      (b'\x02' if segwit_input else b'') +  # witness counter
                      len(signature).to_bytes(1, byteorder='little') +
                      signature +
                      public_key_push +
                      public_key
                     )

            script_sig = script_sig if segwit_input else witness
            witness = witness if segwit_input else b'\x00' if segwit_tx else b''

        # Providing the signature(s) to the input
        tx.TxIn[i].script_sig = script_sig
        tx.TxIn[i].script_sig_len = int_to_varint(len(script_sig))
        tx.TxIn[i].witness = witness

    return tx.to_hex()