在 python 运行时创建对象

Question

how do i create object-instances on runtime in python?

say i have 2 classes:

class MyClassA(object):
    def __init__(self, prop):
        self.prop = prop
        self.name = "CLASS A"

    def println(self):
        print self.name


class MyClassB(object):
    def __init__(self, prop):
        self.prop = prop
        self.name = "CLASS B"

    def println(self):
        print self.name

and a dict

{('a': MyClassA), ('b': MyClassB)}

how can i create dynamic an instance of one of my two classes, depending of i choose 'a' or 'b'.

kind of this:

somefunc(str):
    if 'a': return new MyClassA
    if 'b': return new MyClassB

to get "CLASS B" on calling: somefunc('a').println

but in a more elegant and dynamic way (say i add more classes to the dict on runtime)

Answer 1

You might create a dispatcher, which is a dictionary with your keys mapping to classes.

dispatch = {
    "a": MyClassA,
    "b": MyClassB,
}

instance = dispatch[which_one]() # Notice the second pair of parens here!

Answer 2

You create a class instance by calling the class. Your class dict {('a': MyClassA), ('b': MyClassB)} returns classes; so you need only call the class:

classes['a']()

But I get the sense you want something more specific. Here's a subclass of dict that, when called with a key, looks up the associated item and calls it:

>>> class ClassMap(dict):
...     def __call__(self, key, *args, **kwargs):
...         return self.__getitem__(key)(*args, **kwargs)
... 
>>> c = ClassMap()
>>> c['a'] = A
>>> c['b'] = B
>>> c('a')
<__main__.A object at 0x1004cc7d0>