Use is when you want to check against an object's identity (e.g. checking to see if var is None). Use == when you want to check equality (e.g. Is var equal to 3?).
You can have custom classes where my_var == None will return True
e.g:
class Negator(object):
def __eq__(self,other):
return not other
thing = Negator()
print thing == None #True
print thing is None #False
is checks for object identity. There is only 1 object None, so when you do my_var is None, you're checking whether they actually are the same object (not just equivalent objects)
In other words, == is a check for equivalence (which is defined from object to object) whereas is checks for object identity:
lst = [1,2,3]
lst == lst[:] # This is True since the lists are "equivalent"
lst is lst[:] # This is False since they're actually different objects