Getting Kivy to update Async image with Clock - python-2.7

Question

I have several screens run by ScreenManager, and on one I have an image that is loaded with an image taken from a URL (for weather). My issue is, I can't seem to quite get the URL & image updated periodically:
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import ObjectProperty, StringProperty
from kivy.clock import Clock
import pyowm
class Weather(Screen):
addr = StringProperty()
def wx_forecast(self):
owm = pyowm.OWM('******************')
observation = owm.weather_at_place('London,uk')
w = observation.get_weather()
i = w.get_weather_icon_name()
addr = str("http://openweathermap.org/img/w/" + i + ".png")
print(addr)
return addr
def update(self):
Clock.schedule_interval(self.ids.wxlabel.source, 45)
#def update(self):
#self.wx_forecast()
#Clock.schedule_interval(lambda dt: self.ids.wxlabel, 30)
class ScreenManagement(ScreenManager):
pass
smsettings = Builder.load_string( """
ScreenManagement:
Weather:
<Weather>:
name: 'weather'
FloatLayout:
BoxLayout:
AsyncImage:
id: wxlabel
source: root.wx_forecast()
BoxLayout:
size_hint_y: None
height: 30
Button:
text: 'Change Location'
on_press:
Button:
text: 'More Details'
on_release:
""" )
class HomeUtilities(App):
def build(self):
return smsettings
if __name__=='__main__':
HomeUtilities().run()
Where the def update is where I am having issues; I have tried calling the function wx_forecast again, even trying another function outside the Weather class, but all it says is str: has no attribute 'wx_forecast'
So I believe I am on the right lines? Or I could be completely mistaken, but nothing I try seems to work. Even putting self.ids.wxlabel.source.sunny.png brings up str: has no attribute image but the file is there.