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Python

Posted by boi
boi
14,242 Points

Trouble understanding some concepts of OOP and max recursion error. Python

Hello, this is my code;

class Polo:
    def __init__(self,value):
        self.value = value


    def __str__(self):
        return str(self.value)

when I run this code and create an instance like;

age = Polo(30)

age.value
#output 30

print(age)
#output 30

I don't understand why the instance "age" has the same value as "age.value".

Isn't "age" an instance of "Polo" class? and "value" an attribute? aren't these supposed to be two different things? but why does an instance "age" gives the same output as "age.value"?

2nd question: MAX RECURSION ERROR

I understand that "max recursion error" occurs when a function keeps executing itself till a condition is met, looking at the code below;

class Polo:
    def __init__(self,value):
        self.value = value


    def __str__(self):
        return str(self) # Removed (.value)

when I replaced "return str(self)" instead of "return str(self.value)" a max recursion error occurs, but what I don't understand is that why the function;

def __str__(self):
        return str(self) # Removed (.value)

keeps executing itself, shouldn't it just give an error instead of executing itself?

I would really appreciate your detailed answers, Thank you!

1 Answer

Steven Parker
Steven Parker
243,201 Points
Steven Parker
Steven Parker
243,201 Points

When you print the object itself, the system has no idea how to do that so it looks for the "__str__" method and runs that. Your class then return the "value" expressed as a string for printing.

And for question 2, what kind of error were you expecting? When you write "str(self)" the system calls "__str__" again to convert the class into a string, so it is essentially calling itself.

boi
boi
14,242 Points

I somewhat got the idea of the answers, however, can the code below be possible;

class Polo:
    def __init__(self,value):
        self.value = value


    def __str__(self):
        return str("{} is an instance of {}".format(self,self.__class__))

I need to print out the name of my instance and the class it belongs too, but I get a recursion error, or maybe there is another way to get what I want?

Steven Parker
Steven Parker
243,201 Points
Steven Parker
Steven Parker
243,201 Points

An instance doesn't know it's own name, besides it could have more than one. So leave off "self" to avoid recursion:

        return "an instance of {}".format(self.__class__)