How to define and instantiate a derived class at once in python?

Question

I have a base class that I want to derive and instantiate together. I can do that in java like:

BaseClass derivedClassInstance = new BaseClass() {
    @override
    void someBaseClassMethod() { // my statements}
};

In python I can derive and and instantiate a base class like:

class DerivedClass(BaseClass):
    def some_base_class_method():
        # my statements

derived_class_instance = DerivedClass()

I need to sub-class single instances of some objects with minor changes. Deriving and assigning them separately seems like overkill.

Is there a Java-like one-liner way to derive and instantiate a class on the fly? Or is there a more concise way to do what I did in python?

Consider using a tool other than inheritance for this. Maybe whatever behavior you're implementing with method overrides, you could instead pass to the parent as callbacks or something. — user2357112
– user2357112, Commented Sep 1, 2016 at 19:54
If BaseClass is a Template Method you won't be able to pass a callback or lambda. — vz0
– vz0, Commented Sep 1, 2016 at 19:56
@user2357112 thanks for the suggestion. I am not familiar with callbacks in python. I'll look into that. vz0's answer was a sufficient deterrent. — hazrmard
– hazrmard, Commented Sep 1, 2016 at 21:14

vz0 · Accepted Answer · 2016-09-01 20:05:30Z

2

In general you won't see this kind of code, because is difficult to read and understand. I really suggest you find some alternative and avoid what comes next. Having said that, you can create a class and an instance in one single line, like this:

>>> class BaseClass(object):
...     def f1(self, x):
...             return 2
...     def f2(self, y):
...             return self.f1(y) + y
... 
>>> 
>>> W = BaseClass()
>>> W.f2(2)
4
>>> X = type('DerivedClass', (BaseClass,), {'f1': (lambda self, x: (x + x))})()
>>> X.f2(2)
6

answered Sep 1, 2016 at 20:05

vz0

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1 Comment

hazrmard Over a year ago

o_0 Consider me warned.

galaxyan · Accepted Answer · 2016-09-01 20:04:09Z

0

I think you are looking for metaclass programming.

class Base(object):
   def test(self):
      print 'hit'
a =type( 'Derived',(Base,),{})()
a.test()
hit

answered Sep 1, 2016 at 20:04

galaxyan

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