List all the classes that currently exist

{
    "type": "Person",
    "firstName": "John",
    "lastName": "Smith",
    "address": {
    	"streetAddress": "21 2nd Street",
    	"city": "New York",
    	"state": "NY",
    	"postalCode": 10021
    },
    "phoneNumbers": [
    	"212 555-1234",
    	"646 555-4567"
    ]
}

if not json_object.has_key('type'):
    raise TypeError('JSON stream is missing a "type" attribute.')

##### THIS IS WHERE I WANT TO CHECK IF THE TYPE BY THAT NAME EXISTS ALREADY ####
# create a type definition and add attributes to it
definition = type(json_object['type'], (object,), {})
for key in json_object.keys():
    setattr(definition, key, None)

# instantiate and populate a test object
tester = definition()
for key, value in json_object.iteritems():
    setattr(tester, key, value)

import types
listOfClasses = [cls for cls in globals().values() if type(cls) == types.ClassType]

json_types = {}
def get_json_type(name):
  try:
    return json_types[name]
  except KeyError:
    json_types[name] = t = type(json_object['type'], (object,), {})
    # any further initialization of t here
    return t

definition = get_json_type(json_object['type'])
tester = definition()
# or: tester = get_json_type(json_object['type'])()

json_types = globals()

Python 2.5.2 (r252:60911, Oct  5 2008, 19:29:17)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> dir()
['__builtins__', '__doc__', '__name__']
>>> class Foo:
...     pass
...
>>> dir()
['Foo', '__builtins__', '__doc__', '__name__']
>>> type(eval('Foo'))
<type 'classobj'>