I know virtual methods from PHP or Java.
How can they be implemented in Python?
Or have I to define an empty method in an abstract class and override it?
Sure, and you don't even have to define a method in the base class. In Python methods are better than virtual - they're completely dynamic, as the typing in Python is duck typing.
class Dog:
def say(self):
print "hau"
class Cat:
def say(self):
print "meow"
pet = Dog()
pet.say() # prints "hau"
another_pet = Cat()
another_pet.say() # prints "meow"
my_pets = [pet, another_pet]
for a_pet in my_pets:
a_pet.say()
Cat and Dog in Python don't even have to derive from a common base class to allow this behavior - you gain it for free. That said, some programmers prefer to define their class hierarchies in a more rigid way to document it better and impose some strictness of typing. This is also possible - see for example the abc standard module.