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Python

Anthony Grodowski
Anthony Grodowski
4,902 Points
Posted by Anthony Grodowski
Anthony Grodowski
4,902 Points

Why can't I use just self in __mul__ and __rmul__?

I've been discovering the usage of self and self.attribute and I thought I get it but...

class NumString:
    def __init__(self, value):
        self.value = str(value)

    def __str__(self):
         return self.value

    def __int__(self):
        return int(self.value)

    def __float__(self):
        return float(self.value)

    def __add__(self, other):
        if '.' in self.value:
            return float(self) + other
        return int(self) + other

    def __radd__(self, other):
        return self + other

    def __iadd__(self, other):
        self.value = self + other
        return self.value

    def __mul__(self, other):
        return self * other

    def __rmul__(self, other):
        return self * other

Why does it throw and error like this, when for example trying to multiply the instance times 3.2: 

File "numstring.py", line 23, in __mul__                                               
    return self * other                                                                  
  [Previous line repeated 329 more times]                                                
RecursionError: maximum recursion depth exceeded while calling a Python object

Code that gives me output as wanted is:

def __mul__(self, other):
    return int(self.value) * other

def __rmul__(self, other):
    return int(self.value) * other

but I don't understand why previous one gets into some sort of loop... TIA

1 Answer

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

When you define "__mul__", you are replacing the standard code for how to handle the " ‍*‍ " operator. So when you use that operator inside the definition, then the definition is depending on itself which causes the recursion.

But by using the " ‍*‍ " operator on an int value, you are invoking the multiply operation for int instead of the one for your own type (that you are redefining). So no recursion.

Anthony Grodowski
Anthony Grodowski
4,902 Points
Anthony Grodowski
Anthony Grodowski
4,902 Points

What do you mean by saying "definition"?

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

The function (or method) "definition" is the code you create starting with the "def" keyword.

Anthony Grodowski
Anthony Grodowski
4,902 Points
Anthony Grodowski
Anthony Grodowski
4,902 Points

Thanks Steven! So putting int() causes redirection from __mul__ to __int__, which doesn't cause recursion. But I don't understand what means that "the definition is depending on itself which causes the recursion". How does it depend on itself?

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

The * operator calls the __mul__ method to get the result. So if you use it inside the definition of __mul__ (on items of the same type), then it is calling (depending on) itself. But if you use a * to multiply items of a different type (like int), then it calls the __mul__ method that belongs to int instead.

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

Anthony Grodowski — Glad to help. You can mark a question solved by choosing a "best answer".
And happy coding!