Python, How to extend Decimal class to add helpful methods -

i extend decimal class add helpful methods it, specially handling money.

the problem when go this:

from decimal import decimal class newdecimal(decimal):     def new_str(self):         return "${}".format(self)  d1 = newdecimal(1) print d1.new_str() # prints '$1'  d2 = newdecimal(2) d3 = newdecimal(3) d5 = d2 + d3 print d5.new_str()   #exception happens here 

it throws exception:

attributeerror: 'decimal' object has no attribute 'new_str' 

this because of way decimal arithmetic, returns new decimal object, literally calling decimal(new value) @ end of computation.
no workaround other reimplementing arithmetic?

you don't want have method printing decimal objects in alternate way. top-level function or monkeypatched method whole lot simpler, , cleaner. or, alternatively, money class has decimal member delegates arithmetic to.

but want doable.

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to make newdecimal(1) + newdecimal(2) return newdecimal(3), can override __add__:

def __add__(self, rhs):     return newdecimal(super().__add__(rhs)) 

and of course you'll want override __iadd__ well. , don't forget mul , other numeric special methods.

but still won't decimal(2) + newdecimal(3). make work, need define newdecimal.__radd__. need ensure newdecimal.__radd__ called instead of decimal.__add__, when you're using inheritance, that's easy, because python has rule make easy:

note: if right operand’s type subclass of left operand’s type , subclass provides reflected method operation, method called before left operand’s non-reflected method. behavior allows subclasses override ancestors’ operations.

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you may want read section implementing arithmetic operations in numbers module docs, , the implementation of fractions.fraction (which intended serve sample code creating new numeric types, why docs link directly source). life easier fraction's because can fall decimal every operation , convert (since newdecimal doesn't have different numeric behavior decimal), it's worth seeing issues, , understanding ones , aren't relevant , why.