Python Dynamic Subclassing

Question

Possible duplicates: Is there a way to create subclasses on-the-fly? Dynamically creating classes - Python

I would like to create a subclass where the only difference is some class variable, but I would like everything else to stay the same. I need to do this dynamically, because I only know the class variable value at run time.

Here is some example code so far. I would like FooBase.foo_var to be "default" but FooBar.foo_var to be "bar." No attempt so far has been successful.

class FooBase(object):
    foo_var = "default"

    def __init__(self, name="anon"):
        self.name = name

    def speak(self):
        print self.name, "reporting for duty"
        print "my foovar is '" +  FooBase.foo_var + "'"


if __name__ == "__main__":
    #FooBase.foo_var = "foo"
    f = FooBase()
    f.speak()

    foobarname = "bar"
    #FooBar = type("FooBar", (FooBase,), {'foo_var': "bar"})
    class FooBar(FooBase): pass
    FooBar.foo_var = "bar"
    fb = FooBar()
    fb.speak()

Many thanks

EDIT so I obviously have a problem with this line:

print "my foovar is '" + FooBase.foo_var + "'"

The accepted answer has self.foo_var in the code. That's what I should be doing. I feel ridiculous now.

Robert Kajic · Accepted Answer · 2013-10-18 00:14:52Z

3

What about this:

def make_class(value):
  class Foo(object):
    foo_var = value

    def speak(self):
      print self.foo_var
  return Foo

FooBar = make_class("bar")
FooQux = make_class("qux")

FooBar().speak()
FooQux().speak()

That said, can't you make the value of foo_var be a instance variable of your class? So that the same class instantiated with different input behaves in different ways, instead of creating a different class for each of those different behaviours.

edited Oct 18, 2013 at 0:14

answered Oct 18, 2013 at 0:08

Robert Kajic

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3 Comments

Aaron Over a year ago

Thanks, I was missing self.foo_var. The reason I preferred to have a class variable instead of an instance variable, is I'll have zillions of instances, but probably 1-3 subclasses.

Robert Kajic Over a year ago

Based on that motivation only, I think you should make foo_var an instance variable. Creating those instances of yours will be just as fast with our without an instance variable. It sounds like you are trying to optimise something without needing too.

Aaron Over a year ago

yes, you're right. I have pressure to optimize this as much as possible. I will try with/without class variable and see if there's any difference (probably not)

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