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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_forwarding_address(destination_address, api_key, callback_url=None, coin_symbol='btc'):
 """ Give a destination address and return an input address that will automatically forward to the destination address. See get_forwarding_address_details if you also need the forwarding address ID. Note: a blockcypher api_key is required for this method """

    assert api_key, 'api_key required'

    resp_dict = get_forwarding_address_details(
            destination_address=destination_address,
            api_key=api_key,
            callback_url=callback_url,
            coin_symbol=coin_symbol
            )

    return resp_dict['input_address']