Python 3: Calling a class function inside of __init__

Question

I have a little question about python 3.

I want to create a class, which is using a function from within of that class. Just like:

class Plus:
 def __init__(self, x, y):
 self.x = x
 self.y = y
 self.test()
 def test(self):
 return self.x + self.y

now I am doing something like

a = Plus(5,6) 
print(a)

and python is giving me

<__main__.Plus object at 0x000000000295F748>

and not 11 as I want it. I know that I can get 11 by

a = Plus(5, 6).test()
print(a)

but that's not what I want. I want to call the class and getting the result without adding .test() to it.

Can you help me?

Answer 1

I would go for:

class Plus:
 def __init__(self, x, y):
 self.x = x
 self.y = y
 self.test()
 def test(self):
 res = self.x + self.y
 self.__repr__ = lambda:str(res)
 return res
>>> Plus(5,5)
10
>>> a = Plus(5,5)
>>> a
10
>>> a.test()
10

This way you are not recomputing the sum each time you call print, its updated when you call the test method.

• Thomas Orozco: This won't work as-is; __repr__ must be callable, which a str instance isn't.
• lvc: This is an anti-pattern: it will only work in Python 2, and then only when Plus is an old style class. In new style classes, special methods like __repr__ are not looked up on the instance, only on the class. A better pattern would be to capture res as an attribute and implement an __repr__ method as return self.res.

Answer 2

You'd need to define a __str__ method for your Plus class:

class Plus:
 def __init__(self, x, y):
 self.x = x
 self.y = y
 def test(self):
 return self.x + self.y
 def __str__(self):
 return str(self.test())

• Thomas Orozco: @Henrik __str__ should return a string, though.
• Henrik: this is exactly my point. if you want c = a + b, and a is double, b is double, you'd expect cto be a double .. not a string otherwise what is the result of Plus(a,b) + Plus(a,b) ?
• Thomas Orozco: @Henrik __str__ only affects how the object is represented when you call print(...) on it (or str(...)). It has no bearing whatsoever on the return value of Plus(a,b) + Plus(a,b) — that would be controlled by the __add__ method. If you don't add an __add__ method, the above code throws TypeError:: unsupported operand type(s) for +: 'instance' and 'instance'.
• Netwave: @Henrik, you are missunderstanding the problem, Thomas is explaining it properly
• Henrik: While this is true, you don't see a problem with this design? - if I can use a class just like a function, meaning that if I print it to test the value, and it actually shows a value.. I would expect that actually was function returning a value. Everything else makes it difficult to read the code without inspecting the objects.

Answer 3

now I am doing something like

a = Plus(5,6) 
print(a)

and python is giving me

<__main__.Plus object at 0x000000000295F748>

and not 11 as I want it. I know that I can get 11 by

a = Plus(5, 6).test()
print(a)

but that's not what I want. I want to call the class and getting the result without adding .test() to it.

I am not sure what do you mean by 'and not 11 as I want it'. If you want Plus(5, 6) to actually return 11 (int instance), you should make Plus a function that returns the sum. Alternatively you can override __new__ method and hook upon object creation -- but this is a bad idea.

What are you trying to achieve?

I doubt, that by 'and not 11 as I want it' you want something special to be printed (formatted, represented). If so, override __str__ or __unicode__ or __repr__ method.

Answer 4

Edit: ignore this answer, it is a comment on a misinterpretation of the question

This is just wrong. when you instantiate an object, you'd expect to get a reference to that object.

if you just want a global function returning a number, why even bother to make a class with an init?

in python you shouldn't want static class's like in C# for encapsulation. instead name the module something, and use that for encapsulation.

• Netwave: you are getting the reference anyway, its just the representation of the object what he wants to be printed as the number
• Henrik: well that's not my interpretation of that question.. I've now re-read it, and still believes the question concerns getting the number 11
• Netwave: he want that calling print(a) will return the result of the sum instead of the object memory stuff that python usually prints
• lvc: If you no longer think your answer is helpful, note that you can delete it rather than editing 'ignore this answer' into it.
• Henrik: @lvc. Thanks, I'll try politely to let you know that I am aware. I was in doubt and decided to re-read meta.stackexchange.com/questions/5221/… and decided that while I misunderstood the question, and is a bit embarrassed about it, I still feel the comments to the question only makes sense if the answer remains here and gets downvoted