How to restrict setting an attribute outside of constructor?

Python does not support private or protected attributes. You need to implement the descriptor protocol instead. The standard library provides decorators to do that succinctly.

Just declare the attribute with two underscores in front of it in the init method. It is called name mangling and prevents the attribute from being accessible via __ssn, although it can still be accessed and modified by _Person__ssn in this case. However, if you do not explicitly define a setter for it will raise an AttributeError.

Of course if someone has an intention to misuse the API that person can if he is very intent. But it will not happen by accident.

import re

class Person:
    """Encapsulates the private data of a person."""

    _PATTERN = re.COMPILE(r'abcd-efgh-ssn')

    def __init__(self, name, ssn):
       """Initializes Person class with input name of person and
       his social security number (ssn).
       """
       # you can add some type and value checking here for defensive programming
       # or validation for the ssn using regex for example that raises an error
       if not self._PATTERN.match(ssn):
           raise ValueError('ssn is not valid')
       self.__name = name
       self.__ssn = snn

    @property
    def name(self):
        return self.__name

    @name.setter
    def name(self, value):
        self.__name = value

    @property
    def ssn(self):
        return self.__ssn

>>> p = Person('aname', 'abcd-efgh-ssn')
>>> p.ssn
'abcd-efgh-ssn'
>>> p.ssn = 'mistake'
AttributeError: 'can't set attribute'

Just pointing out that we could still modify _ssn.

Objects have the special attribute, __dict__ that is a dictionary that maps all instance attributes of the object with their corresponding values. We can add/update/delete instance attributes directly by modifying the __dict__ attribute of an object.

We can still modify _snn like this:

p = Person('Ozgur', '1234')

p.__dict__.get('_ssn') # returns '1234'

p.__dict__['_ssn'] = '4321'

p.__dict__.get('_ssn') # returns '4321'

As we can see, we were still able to change the value of _ssn. By design, there isn't a straight forward way, if any, to circumvent attribute access in Python in all cases.

I'll show a more common way to restrict attribute access using property() as a decorator:

class Person(object):
    def __init__(self, name, ssn):
        self.name = name
        self._ssn = ssn

    @property
    def ssn(self):
        return self._ssn

    @ssn.setter
    def ssn(self, value):
        raise AttributeError('Denied')


>> p = Person('Ozgur', '1234')
>> p.ssn
>> '1234'
>> p.ssn = '4321'
>> AttributeError: Denied

Hope this helps!

The usual way is to use a "private" attribute starting with an underscore, and a read-only property for public access:

import operator

class Person(object):
    def __init__(self, name, ssn):
        self.name = name
        self._ssn = ssn
    ssn = property(operator.attrgetter("_ssn"))

Note that this does not really hinder anybody to change the attribute _ssn, but the leading _ documents that the attribute is private.