I have created a class foo
as below:
class foo():
def __new__(cls, a, b, c, add=True):
return cls.sum(a, b, c) if add else cls.subtract(a, b, c)
def sum(a, b, c):
return a + b + c
def subtract(a, b, c):
return c - b - a
print(foo(1, 2, 3, True))
This program returns the required result as 6
. However, I needed to get the clarity of a few concepts:
sum
and subtract
are instance methods, how can they be called without instantiating an object as in the above example ie, print(...)
? I have observed many python APIs and frameworks returning an object or a value through class instantiation.
I am trying to understand the core concepts of OOPs in python please help.
The way you have it now, sum
and subtract
are indeed instance methods.
>>> foo_obj = object.__new__(foo) # This will actually create a foo object to demonstrate
>>> foo_obj.sum
<bound method foo.sum of <__main__.foo object at 0x0000000000000000>>
>>> type(foo_obj.sum)
<class 'method'>
But that's just because when you access them through an instance, Python dynamically creates a method (Basically just binds the first argument to the object, usually self
)
But, you can access the wrapped function through the class:
>>> foo_obj.sum.__func__
<function foo.sum at 0x0000000000000001>
>>> foo.sum
<function foo.sum at 0x0000000000000001>
>>> foo_obj.sum.__func__ is foo.sum
True
So in your __new__
function, it won't bind the first argument, and they call the underlying function instead of making them an instance method.
To fix the warnings, you can make them classmethod
s or staticmethod
s. But it is generally bad practice to not return an object that is an instance of the class from the __new__
. If you really wanted to use OOP, either subclass int
or make a wrapper, so you can have:
>>> class Foo:
__slots__ = 'value',
def __init__(self, a, b, c, add=True):
self.value = self.sum(a, b, c) if add else self.subtract(a, b, c)
@staticmethod
def sum(a, b, c):
return a + b + c
@staticmethod
def subtract(a, b, c):
return c - b - a
>>> foo = Foo(1, 2, 3, True)
>>> foo
<__main__.foo object at 0x0000000000000002>
>>> foo.value
6
or
>>> class Foo(int):
__slots__ = ()
def __new__(cls, a, b, c, add=True):
value = cls.sum(a, b, c) if add else cls.subtract(a, b, c)
return super().__new__(cls, value)
@staticmethod
def sum(a, b, c):
return a + b + c
@staticmethod
def subtract(a, b, c):
return c - b - a
>>> foo = Foo(1, 2, 3, True)
>>> foo
6
>>> type(foo)
<class '__main__.Foo'>
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