Validation of read-only attributes [duplicate]

Question

I would like to have some validation inside of a property setter in a class. This property has to be set during object creation and then it needs to be read-only from the outside.

I found the following solution and I wonder if there is a more straightforward or Pythonic way to do it. (I am aware that by calling user._username from the outside it is still possible to set this property, but "we are all consenting adults" etc. - the important thing is that user.username should not have a setter).

class User:
    def __init__(self, username):
        self._usernname = username

    @property
    def username(self):
        return self.__username

    @property
    def _username(self):
        return self.__username

    @_username.setter
    def _username(self, value):
        if not value or len(value) < 3:
            raise ValueError('Invalid username')
        self.__username = value

Edit: In addition to the comments here, options for validation in the constructor can be found here: numpy performance and random numbers.

just check the value to store into _username with one underscore in the constructor, and remove the _username property altogether... — Antti Haapala
– Antti Haapala, Commented Dec 21, 2017 at 19:50
Do all the initialisation/validation in the __init__ method. The additional _username property looks very artificial and serves no real purpose. — ekhumoro
– ekhumoro, Commented Dec 21, 2017 at 19:52
Thanks @AnttiHaapala, but then if there is no _username property, where can I validate this value? — Samu Balogh
– Samu Balogh, Commented Dec 21, 2017 at 19:55
@SamuBalogh. Just move the setter validation code into __init__ and adjust accordingly. Get rid of the two _username property functions. — ekhumoro
– ekhumoro, Commented Dec 21, 2017 at 20:00
@SamuBalogh, The _username attribute is internal and should have no outward-facing api. It's only ever set once in __init__, so there's no point in creating a property for it. — ekhumoro
– ekhumoro, Commented Dec 21, 2017 at 20:05

smarie · Accepted Answer · 2019-10-17 12:35:28Z

Your example is interesting because you both have a need for read-only (that is why you create a read-only property facade username) and for validation (that is why you create a property also for the internal attribute _username, with a setter).

You can do the same efficiently with pyfields:

from pyfields import field

class User(object):
    username = field(read_only=True, 
                     validators={'should contain more than 2 characters': lambda s: len(s) > 2})

u = User()
u.username = "earthling"
print("Hello %s !\n" % u.username)
print(vars(u))
print()
u.username = "earthling2"  # <-- raises error

yields

Hello earthling !

{'_username': 'earthling'}

pyfields.core.ReadOnlyFieldError: 
   Read-only field '<...>.User.username' has already been initialized on 
   instance <<...>.User object at 0x0000022895908EF0> and cannot be modified anymore.

It is basically similar to what you were doing manually, except that it relies on descriptors rather than properties (descriptor is the lower-level protocol used by python behind the scenes in properties). Also, it only relies on a single "nesting" level: the internal attribute _username is a plain old attribute as ekhumoro's comment above suggests.

See pyfields documentation for details.

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