How to pass a method as argument?

[dn]

On 30/09/2021 18.11, Anil Anvesh wrote:
> I want to write a python calculator program that has different methods to add, subtract, multiply which takes 2 parameters. I need to have an execute method when passed with 3 parameters, should call respective method and perform the operation. How can I achieve that?
> 
> 
> 
> class calc():
>     def __init__(self,a,b):
>         self.a=a
>         self.b=b
>         
>     def ex(self,fun):
>         self.fun=fun
>         if fun=="add":
>             self.add()
>     
>     def add(self):
>         return self.a+self.b
>     def sub(self):
>         return self.a-self.b
>     def mul(self):
>         return self.a*self.b
>     def div (self):
>         return self.a/self.b
>     def execu(
> obj1=calc()
> obj1.execu("add",1,,2)
>

This is a common course-assignment.

NB the code-examples below are incomplete. You may need to research
these techniques further, and thus learn how 'it' all fits-together
(I set 'homework' rather than doing it!)


There are several techniques which can be employed and/or combined here.
1 if-elif 'ladder'
2 dict[ionary] as select/case construct
3 functions as 'first-class objects'

The basic problem is to take a 'command' which is formatted as a
str[ing], and decide which of several options to choose:

1 if-elif is a simple and easy-to-read solution:

if command == "add":
    do_this...
elif command == "sub":
    do_that...
...
else:
    # oops: "I'm afraid I can't do that, Dave"
    # don't forget to add a 'catch-all' to handle user-error!

2 using a dict is shorter and avoids some of the 'boiler-plate'
repetition which causes many of us to rebel against using the above. We
use the command received from the user as a key into the dict.

3 a function and/or its name is as much data as the 'variable' "command"
or indeed the str-constant "add" - once it has been defined! Thus it is
possible to treat functions like anything else:

data_as_string = "my string"
# can be likened to:-
def my_function(): pass
data_as_function = my_function

Note that (like most else), the function must be defined before it can
be 'used'!

Returning to the stated-problem:

def add( etc ): ...

then using the if-elif 'ladder' above as a framework create a dict which
'links' the input command (as str) to the applicable function:

calculator = { "add": add,
               "sub": sub,
# indeed we can become rather more 'creative'
               "+" " add,
               ...
               }

Thereafter, we can apply the dict to solve the problem:

calculator.get( command,
                "Error message/advice, eg mentioning add, or + etc"
                )


NB it has been left to you to perfect the technique so that the value(s)
to be calculated are properly communicated to the chosen function.

PS you may find the Python-Tutor Discussion List helpful
-- 
Regards,
=dn