I want to forbid further assignments on some attributes of a class after it was initialized. For instance; no one can explicitly assign any value to 'ssn' (social security number) property after the Person instance 'p' has been initialized. _setattr_ is also being called while assigning the value inside _init_ method, thus it is not what I want. I'd like to restrict only further assignments. How can I achieve that?
class Person(object):
def __init__(self, name, ssn):
self.name = name
self._ssn = ssn
def __setattr__(self, name, value):
if name == '_ssn':
raise AttributeError('Denied.')
else:
object.__setattr__(self, name, value)
>> p = Person('Ozgur', '1234')
>> AttributeError: Denied.
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!
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'
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