getting error "TypeError: object of type 'function' has no len()" when using netmiko to ssh to a Cisco device

Question

I have been playing with network automation and just trying to write a script to configure multiple devices using the netmiko python library. However I keep getting the above error. It looks like the script can connect to the devices, but then the error arises when trying to open one of the files using the dictionary.

from netmiko import ConnectHandler
import getpass




#prompt user for username
username = input('Enter username: ')

##username method to return the username
def credentials_username():
    cred = username
    return cred


#prompt user for password
p = getpass.getpass('Enter password: ')

##password method to return the password
def credentials_password():
    password = p
    return password

#Prompt to enter device file
devices_file = input('Enter devices inventory file name: ')

with open(devices_file) as hosts:
    addresses = hosts.readlines()

#Prompt to enter configuration file
config_file = input('Enter device configuration filename: ')

#The configuration files method
def configuration_file():
    with open(config_file) as file:
        conf_lines = file.readlines()
    return conf_lines





##define device dictionary
def device_list():
    ios_device_info = dict()
    for device in addresses:
        ios_device_info['ip'] = device
        ios_device_info['device_type'] = "cisco_ios"
        ios_device_info['username'] = credentials_username
        ios_device_info['password'] = credentials_password
        net_connect = ConnectHandler(**ios_device_info)
        net_connect.send_config_set(configuration_file)



device_list()