Question

I’m writing a class that has a dict containing int to method mappings. However setting the values in this dict results in the dict being populated with unbound functions.

class A:
    def meth_a: ...
    def meth_b: ...
    ...
    map = {1: meth_a, 2: meth_b, ...}
    for int in ...:
        map[int] = meth_x

This doesn’t work for a few reasons:

1. The methods aren’t bound when the class is initialized because they’re not in the class dict?
2. I can’t bind the methods manually using __get__ because the class name isn’t bound to any namespace yet.

So:

1. How can I do this?
2. Do I have to drop out of the class and define the dict after the class has been initialized?
3. Is it really necessary to call __get__ on the methods to bind them?

Update0

The methods will be called like this:

def func(self, int):
    return self.map[int]()

Also regarding the numeric indices/list: Not all indices will be present. I’m not aware that one can do list([1]=a, [2]=b, [1337]=leet) in Python, is there an equivalent? Should I just allocate a arbitrary length list and set specific values? The only interest I have here is in minimizing the lookup time, would it really be that different to the O(1) hash that is {}? I’ve ignored this for now as premature optimization.

Answer 1

I’m not sure exactly why you’re doing what you’re doing, but you certainly can do it right in the class definition; you don’t need __init__.

class A:
    def meth_a(self): pass

    m = {1: meth_a}
    def foo(self, number):
        self.m[number](self)

a = A()
a.foo(1)

An “unbound” instance method simply needs you to pass it an instance of the class manually, and it works fine.

Also, please don’t use int as the name of a variable, either, it’s a builtin too.

A dictionary is absolutely the right type for this kind of thing.

Edit: This will also work for staticmethods and classmethods if you use new-style classes.