Why does the following not work?

class Army:   

    units = []

    def add_units(self, unit_type, amount, units=units):
        for x in range(amount):
            units.append(unit_type())

I inspected it and verified that it correctly creates the class and correctly adds units. Yet the code doesn't check out. If I instead define the class like this:

class Army:

    def __init__(self):
        self.units = []

    def add_units(self, unit_type, amount):
        for x in range(amount):
            self.units.append(unit_type())

Then it works. I don't understand why the first piece of code does not. What's the difference between these two?