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Python Object-Oriented Python Advanced Objects Multiplication

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 68,064 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

Thankyou :) All good! This makes sense!