To define a subclass in Python, you can use the following syntax:

class ParentClass():
    def __init__(self):
        self.some_attribute = 5

    def some_function(self, x):
        return self.some_attribute + x

class SubClass(ParentClass):
    some_new_attribute = self.some_attribute + 7

    def some_new_function(self):
        return self.some_function(some_new_attribute)

An instance of the class SubClass will inherit the methods and attributes of ParentClass.