Python - Type Checking Instances of Objects

Question

I'm creating a class that emulates numeric types so as to be able to use basic arithmetic operators, like +, -, etc on instances of this class. However, I want to be able to handle the operation in different ways depending on what types the operands are. For instance, if I'm creating a class foo_c with __add__() as a function, then I want to be able to handle addition cases where one operand is of type foo_c and the other is of type int or float or numpy.ndarray (or foo_c).

The solution I want to implement is to have a collection of 'adder' functions to switch between based off of the operand type. The different functions are being assigned to a dictionary, as such:

class foo_c:

   ...

   self.addOps = { int: self.addScalar,
                   float: self.addScalar,
                   foo_c: self.addFoo }

   ...

   self.addScalar(self, sclr):
      ...

   self.addFoo(self, foo):
      ...

   self.__add__(self, operand):
      return self.addOps[type(operand)](operand)

The problem that I'm having is that I can't get the type() function to return the appropriate value to be used as a key for the dictionary. After creating an instance of the class as foo = foo_c(), the built-in function type(foo) returns instanceinstead of foo_c. I assume this is because I'm not creating the object in question; rather I am creating an instance of the object. I've used foo.__class__ as well, but am getting __main__.foo_c as the returned class, which isn't right either...

I don't want to have to use lines of isinstance() checks, so is there a way to get type() to return the class as desired?

user2357112 · Accepted Answer · 2016-09-08 21:55:12Z

1

You forgot to have foo_c inherit from object, so you're getting an old-style class. Make it inherit from object:

class foo_c(object):
    ...

answered Sep 8, 2016 at 21:55

user2357112

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3 Comments

S. Gamgee Over a year ago

So, I gave it a try, and the problem I'm getting now is that the type(foo) is still outputting __main__.foo_c as its type, which it's type should be foo_c without __main__.

user2357112 Over a year ago

@S.Gamgee: No, that's correct. __main__.foo_c is the right class - it's the foo_c class defined in the __main__ module.

S. Gamgee Over a year ago

Ok, I gotcha. I was operating in the IPython console and was executing type(foo) == foo_c() instead of type(foo) == foo_c. I'm good to go.

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