class Bear:
    pass

class Fish:
    pass

things = [Bear(), Bear(), Fish(), Bear(), Fish()]

def getPopulation(species):
    count = 0
    for thing in things:
        if isinstance(thing, species):
            count = count + 1
    return count

def getPopulation2(species):
    return sum([1 for thing in things if isinstance(thing, species)])

def getPopulation3(species):
    uncheckedThings = things[:]
    count = 0
    while len(uncheckedThings) > 0:
        thing = uncheckedThings.pop()
        if isinstance(thing, species):
            count = count + 1
    return count

def getPopulation4(species):
    return getPopulationIn(species, things)

def getPopulationIn(species, listOfThings):
    if listOfThings == []:
        return 0
    elif len(listOfThings) == 1:
        if isinstance(listOfThings[0], species):
            return 1
        else:
            return 0
    else:
        half = len(listOfThings) // 2
        leftSubPopulation = getPopulationIn(species, listOfThings[:half])
        rightSubPopulation = getPopulationIn(species, listOfThings[half:])
        return leftSubPopulation + rightSubPopulation