CODEWITHSUNDEEP
pythonclassobjectinstancetypechecking
S. Gamgee
S. Gamgee

Reputation: 521

Python - Type Checking Instances of Objects

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?

Upvotes: 0

Views: 38

Answers (1)

user2357112
user2357112

Reputation: 280181

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):
    ...

Upvotes: 1

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