In [1]: inventory = {}

In [2]: inventory['bmx'] = []

In [3]: inventory
Out[3]: {'bmx': []}

In [4]: inventory['bmx']
Out[4]: []

In [5]: inventory['bmx'].append(bike1)
KeyboardInterrupt

In [5]: class Bike(object):
   ...:     def __init__(self, name):
   ...:         self.name = name
   ...:

In [6]: bike = Bike('bmx')

In [7]: bike
Out[7]: <__main__.Bike at 0x10b18f290>

In [8]: bike.name
Out[8]: 'bmx'

In [9]: inventory
Out[9]: {'bmx': []}

In [10]: inventory[bike.name].append(bike)

In [11]: print inventory
{'bmx': [<__main__.Bike object at 0x10b18f290>]}

In [12]: bike2 = Bike('bmx')

In [13]: inventory[bike.name].append(bike)

In [14]: print inventory
{'bmx': [<__main__.Bike object at 0x10b18f290>, <__main__.Bike object at 0x10b18f290>]}

In [15]: print inventory['bmx']
[<__main__.Bike object at 0x10b18f290>, <__main__.Bike object at 0x10b18f290>]

In [16]: print len(inventory['bmx']) # stock