用可选参数将python类子类化

Question

I am new to Python. I have this code:

class SomeClass(OtherClass):
    name = "whatever"
    task = "nothing"

Now, I want to create a child class such that, I am able to instantiate it like this:

child = ChildClass(task = "sometask")
child.name #=> "whatever"
child.task #=> "sometask"
child2 = ChildClass()
child2.task #=> "nothing"

How can I do that?

Answer 1

First, you need to note that you are creating class variables in SomeClass , not instance variables

class SomeClass(object):
    def __init__(self, name = "whatever", task = "nothing"):
        self.name = name
        self.task = task

Now we have designed a class, which accepts two keyword arguments with default values. So, if you don't pass values to any of them, by default whatever will be assigned to name and nothing will be assigned to task .

class ChildClass(SomeClass):
    def __init__(self, name = "whatever", task = "nothing"):
        super(ChildClass, self).__init__(name, task)

child1 = ChildClass(task = "sometask")
print child1.name, child1.task
# whatever sometask
child2 = ChildClass()
print child2.name, child2.task
# whatever nothing

Answer 2

You need to overwrite its initializer (sort of like a constructor in other languages):

>>> class SomeClass:
...   name='whatever'
...   task='nothing'
...
>>> class ChildClass(SomeClass):
...   def __init__(self, name=None, task=None):
...     if name is not None:
...       self.name = name
...     if task is not None:
...       self.task = task
...
>>> child = ChildClass(task='sometask')
>>> child.name
'whatever'
>>> child.task
'sometask'
>>> child2 = ChildClass()
>>> child2.task
'nothing'