Assuming you wrote them just like the Firaxis code you'd call them the same way, for instance:
pCity.getOilTotalConsumption() or pCity.setOilTotalConsumption (x)
That would use the city 'pCity' which should be available depending on where you're calling it. If you don't know 'pCity' you can get it easily enough, for instance here is an example of iterating over the list of the current player's cities:
Code:
kTriggeredData = argsList[0]
player = gc.getPlayer(kTriggeredData.ePlayer)
(loopCity, iter) = player.firstCity(false)
while(loopCity):
if (loopCity.getPopulation() >= 5):
if (loopCity.canConstruct(iBuilding, false, false, true)):
loopCity.setNumRealBuilding(iBuilding, 1)
(loopCity, iter) = player.nextCity(iter, false)
So kTriggeredData is the args passed to the function, ePlayer (enum, not object) is one of those args which you convert to the player 'object' so you can use it to call the player methods like firstCity() which returns a city 'object' which can than be used to call city methods on itself.
The exact process may change depending on your desired effect but the process should be similar. So you'll have an enum passed to your python function (ePlayer) which you can use to find the object using the GC functions (player = gc.getPlayer(ePlayer)) and form there you can grab additional objects associated with that object (city = player.firstCity()) and then call your own methods on the final object (city.getOilTotalConsumption()).
Remember, python ALWAYS passes the object 'self' so if you're working in a python function called by a city you don't need to specify the object, just call the method directly like 'getOilTotalConsumption()' and you're done.
Fun isn't it? Have I mentioned that I hate python?
Anyway, you can probably hook into your own methods by inserting your codes in an existing python function that is already working with a specific city object, or at the very least a player object. Look in the CvRandomEventInterface.py file for some great examples of iterating over players & cities (that's where my example came from).