Pythonic switching using functions inside a class

Question

I am attempting to use a dict to control function selection from within a class, however, I get this following TypeError:

TypeError: A() missing 1 required positional argument: 'self'

Example code:

class Tt:
    def __init__(self):
        pass
    def A(self):
        print("a")
    def B(self):
        print("b")
    def C(self):
        print("c")

    a_diction={
         '0' : A,
         '1' : B,
         '2' : C
    }

    def ad(self,data):
        self.a_diction[data]()

How should I construct this switch in ad to provide self to both a_diction and to the called method A, B and C?

Kevin · Accepted Answer · 2014-10-07 15:39:09Z

3

The error message indicates that you're missing an argument. I suggest adding an argument.

self.a_diction[data](self)

answered Oct 7, 2014 at 15:39

Kevin

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3 Comments

user3007395 Over a year ago

yes, this works.... but why did it work? logically it should have been self.(self.a_diction[data])() .. which is wrong i know.. but passing self as an argument here works.. why so?

mfitzp Over a year ago

@user3007395 can you post your actual code in the question? The code that is there as it stands will not work, even with this addition.

Kevin Over a year ago

It works because someObject.method() can also be called using the syntax someClass.method(someObject). Similarly, self.thing() and thing(self) both work.

Alok-- · Accepted Answer · 2014-10-07 16:46:11Z

1

Could you do the following instead?

class Tt:
    def __init__(self):
        self.a_diction = {
             0: self.A,
             1: self.B,
             2: self.C
        }

    def A(self):
        print("a")

    def B(self):
        print("b")

    def C(self):
        print("c")

    def ad(self,data):
        self.a_diction[data]()

I.e., make a_diction an instance attribute and initialize it in __init__? Then you can choose bound methods as values in the dictionary instead of unbound methods.

answered Oct 7, 2014 at 16:46

Alok--

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