Property setter with multiple values

Question

I have a property setter which generates a unique id by taking two strings and hashing it:

@id.setter
def id(self,value1,value2):
    self._id = sha512(value1+value2)

I have two questions:

1. Is it allowed (considering good python coding practices) to code this way
2. How do I pass two values to the setter?

Answer 1

How do I pass two values to the setter?

You can pass an iterable(tuple, list) to the setter, for example:

class A(object):
    def __init__(self, val):
        self.idx = val

    @property    
    def idx(self):
        return self._idx

    @idx.setter
    def idx(self, val):
        try:
            value1, value2 = val
        except ValueError:
            raise ValueError("Pass an iterable with two items")
        else:
            """ This will run only if no exception was raised """
            self._idx = sha512(value1+value2)

Demo:

>>> a = A(['foo', 'bar'])     #pass a list
>>> b = A(('spam', 'eggs'))   #pass a tuple
>>> a.idx
<sha512 HASH object @ 0xa57e688>
>>> a.idx = ('python', 'org')  #works
>>> b.idx = ('python',)         #fails
Traceback (most recent call last):
    ...
    raise ValueError("Pass an iterable with two items")
ValueError: Pass an iterable with two items

Answer 2

The setter can only take one value, so use a tuple: (value1, value2).

@id.setter
def id(self,value):          
    self._id = sha512(str(value))

...

self.id = (value1, value2)

(You didn't post what sha512 is. I'm assuming you are using hashlib.sha512, and sha512 is calling the update method, which requires a string as input.)

Answer 3

I have some doubts regarding the coding practices here. As a user of an API, I'd always assume that whatever I set as a property, I can get it back from the same property. In addition to that, having something called id mutable looks suspicious.

As for passing two values, how about:

@id.setter
def id(self, vals):
    value1, value2 = vals
    self._id = sha512(value1+value2)

Then assign a tuple:

myobj.id = (val1, val2)