import types

if type(a) is types.DictType:
    do_something()
if type(b) in types.StringTypes():
    do_something_else()

if isinstance(a, dict):
    do_something()
if isinstance(b, str) or isinstance(b, unicode):
    do_something_else()

if isinstance(x, basestring)
  return treatasscalar(x)
try:
  return treatasiter(iter(x))
except TypeError:
  return treatasscalar(x)

if isinstance(b, [str, unicode]):
    do_something_else()

if isinstance(b, basestring):
    do_something_else()

class Vehicle:
    pass

class Truck(Vehicle):
    pass

isinstance(Vehicle(), Vehicle)  # returns True
type(Vehicle()) == Vehicle      # returns True
isinstance(Truck(), Vehicle)    # returns True
type(Truck()) == Vehicle        # returns False, and this probably won't be what you want.