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Python Object-Oriented Python Advanced Objects Emulating Built-ins

Posted by annecalija
annecalija
9,031 Points

In numstring.py, why is the implementation of __radd__ different from __add__?

Instead of 

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

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

why not this?

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

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

2 Answers

Marcin Lubke
Marcin Lubke
13,253 Points
Marcin Lubke
Marcin Lubke
13,253 Points

If you do it like in the first example, than yes - __radd__ will call __add__ method "internally". By typing float(self) + other you call __add__ method and by typing other + float(self) you call __radd__ method.

You could also try to do it another way - to write the whole code in __radd__ method, and inside __add__ call the second method by using other + float(self).

annecalija
annecalija
9,031 Points

Hi Marcin,

Thank you so much! This gets clearer to me now.

Marcin Lubke
Marcin Lubke
13,253 Points
Marcin Lubke
Marcin Lubke
13,253 Points

As I understand, you can do it both ways, at least in this case.

Here, your __radd__ method simply returns __add__ method:

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

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

Here you just using another code for __radd__ method:

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

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

Both ways are ok, but following DRY concept (don't repeat yourself) the first method should be better, it's a little shorter :)

annecalija
annecalija
9,031 Points

Hi Marcin,

Thanks for answering.

I am fully aware that both cases will work and yes, following DRY is recommended. My bad, I didn't explain clearly on what I really wanted to know from my question.

However, my confusion starts from why adding floats work on the __radd__ when it didn't even address adding floats like the one in __add__. Is there a connection between how add and radd works (i.e. radd is calling add internally, idk)??

Could you kindly help me with this confusion? Thanks again in advance!