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Python Object-Oriented Python Advanced Objects Multiplication

Steven Hender
Steven Hender
5,830 Points
Posted by Steven Hender
Steven Hender
5,830 Points

In Python I have a problems re-assigning '__imul__' I call 'NumString(3) *= 2' and it returns Syntax Error

I can get the add and multiplication to run just fine with mul and rmul , it is the imul that has stumped me on this question.

The Exercise Question reads: Challenge Task 2 of 2

Now wrap it up by adding in imul, which does in-place multiplication. Be sure to update self.value!

My Answer: / problem code reads: 

class NumString:

    def __mul__(self, other):
        if '.' in self.value:
            return float(self) * other
        return int(self) * other

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

    def __imul__(self, other): 
         self.value = self.value * other   
         return self.value

The returned error is:

>>> from numstring import NumString
>>> NumString(3) *= 2
  File "<stdin>", line 1
SyntaxError: can't assign to function call
>>>

Can anyone help? Thankyou :)

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

    def __mul__(self, other):
        if '.' in self.value:
            return float(self) * other
        return int(self) * other

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

    def __imul__(self, other):
        self.value = self * other
        return self.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

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

The function call NumString(3) used to create the instance cannot be on the left side of a iterative statement. Break it in two statements give you:

# Using simple multiply works
>>> NumString(3)
<NumString object at 0x7f5df4f58a20>
>>> NumString(3) * 3
9
# but iterative multiply fails because this:
>>> NumString(3) *= 3
# is the same as
>>> NumString(3) = NumString(3) * 3

# Breaking it in two parts works:
>>> ns = NumString(3)
>>> ns
<imul.NumString object at 0x7f5df4f58a20>
>>> print(ns)
3
>>> ns *= 3
>>> ns
'333'

The result is '333' because a string times an integer n returns the string repeated n times. If you wish to preserve the class, you will need to figure out how to multiply as numbers, then reassign self.value as a string. Lastly, remember to return self

Post back if you need more help. Good luck!!

Steven Hender
Steven Hender
5,830 Points
Steven Hender
Steven Hender
5,830 Points

Thankyou :) All good! This makes sense!