Handling class properties with unknown default value in Python

Question

I've a class like following:

class Invoice
    def __init__(self, invoice_id):
        self.invoice_id = invoice_id
        self._amount = None

    @property
    def amount(self):
        return self._amount

    @amount.setter
    def amount(self, amount):
        self._amount = amount

In the above example, whenever I would try to get invoice amount without setting its value, I would get a None, like following:

invoice = Invoice(invoice_id='asdf234')
invoice.amount
>> None

But in this situation, None is not the correct default value for amount. I should be able to differentiate between amount value as None vs amount value not been set at all. So question is following:

How do we handle cases when class property doesn't have a right default value ? In the above example if I remove self._amount = None from init, I would get AttributeError for self._amount and self.amount would return a valid value only after I call invoice.amount = 5. Is this the right way to handle it ? But this also leads to inconsistency in object state as application would be changing instance properties at runtime.
I've kept amount as a property for invoice for better understanding and readability of Invoice and its attributes. Should class properties only be used when we're aware of its init / default values ?

Why is there an @property at all if it acts exactly like a field? Anyway, some relevant questions to decide what kind of approach you want are: 1) what does an invoice with an amount of None mean? 2) is an invoice whose amount has never been set supposed to exist, or does its existence always indicate a programming error? — Ry-
– Ry- ♦, Commented Nov 1, 2018 at 17:46
Let's say my application is listening an external source which is sending me Invoice id and its amount in two different events. For first event I create an invoice with no amount, make some decisions based on if amount is set or not. On second event, a new instance would have the amount value too. Provider can send a None value too for amount, application service would do certain other operations in that case. — pkumar
– pkumar, Commented Nov 1, 2018 at 17:55
Then don't create the Invoice object until you have both the ID and its amount. — kindall
– kindall, Commented Nov 1, 2018 at 17:57
But I still need to call different services which takes invoice instance as input and perform certain actions on first event when just the invoice id is received. — pkumar
– pkumar, Commented Nov 1, 2018 at 18:05
Those services won't work if the invoice has an amount on it? — kindall
– kindall, Commented Nov 1, 2018 at 20:15

kindall · Accepted Answer · 2018-11-01 17:52:54Z

None is conventionally used for the absence of a value, but sometimes you need it to actually be an allowable value. If you want to be able to distinguish between None and an un-set value, simply define your own singleton for this purpose.

class Undefined:
    __str__ = __repr__ = lambda self: "Undefined"

Undefined = Undefined()

class Invoice
    def __init__(self, invoice_id):
        self.invoice_id = invoice_id
        self._amount = Undefined

    @property
    def amount(self):
        return self._amount

    @amount.setter
    def amount(self, amount):
        if amount is Undefined:
            raise ValueError("amount must be an actual value")
        self._amount = amount

Of course, you may now need to test for Undefined in other methods to make sure they're not being used before the instance is properly initialized. A better approach might be to set the attribute during initialization and require its value to be passed in to __init__(). That way, you avoid having an Invoice in an invalid (incompletely initialized) state. Someone could still set _amount to an invalid value, but they'd simply get the trouble they were asking for. We're all adults here.

juanpa.arrivillaga · Accepted Answer · 2018-11-01 17:47:50Z

2

Don't use a property in this case. Your getters and setters don't do anything except return and set the value. Just use a normal attribute. If you later want to control access, you can just add that property later without changing the interface of your class!

As for dealing with non-set values, just create your own object to represent a value that has never been set. It can be as simple as:

>>> NOT_SET = object()
>>> class Invoice:
...     def __init__(self, invoice_id):
...         self.invoice_id = invoice_id
...         self.amount = NOT_SET
...
>>> inv = Invoice(42)
>>> if inv.amount is NOT_SET:
...     inv.amount = 1
...

You could also use an enum if you want better support for typing.

answered Nov 1, 2018 at 17:47

juanpa.arrivillaga

97.6k14 gold badges141 silver badges190 bronze badges

Comments

mVChr · Accepted Answer · 2018-11-01 17:49:27Z

1

You can make a static class and use that to determine whether a value has been set at all.

class AmountNotSet(object):
    pass


class Invoice(object):
    def __init__(self, invoice_id):
        self.invoice_id = invoice_id
        self._amount = AmountNotSet
    # ...etc...

Then you can check whether the invoice is set or not like so:

invoice1 = Invoice(1)
invoice2 = Invoice(2)
invoice2.amount = None

invoice1.amount is AmountNotSet  # => True
invoice2.amount is None          # => True

answered Nov 1, 2018 at 17:49

mVChr

50.3k11 gold badges111 silver badges105 bronze badges

Collectives™ on Stack Overflow

Handling class properties with unknown default value in Python

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related