How to create object having read-only attributes dynamically

Question

I want to create object which is having read-only attributes. And it need to be initialize dynamically.

Here is situation I want.

readOnlyObject = ReadOnlyClass({'name': 'Tom', 'age': 24})

print(readOnlyObject.name)
>> 'Tom'
print(readOnlyObject.age)
>> 24
readOnlyObject.age = 14
>> AttributeError: can't set attribute
  

I found a way using property function,
but I think property function only works on attributes that is pre-declared.

Here is my code that property doesn't work.

class ReadOnlyClass:
  _preDeclaredVar = "Read-Only!"
  preDeclaredVar = property(lambda self: self._preDeclaredVar)

  def __init__(self, data: dict):
    for attr in data:
      setattr(self, '_' + attr, data[attr])
      setattr(self, attr, property(lambda self: getattr(self, '_' + attr)))



readOnlyObject = ReadOnlyClass({'name': 'Tom', 'age': 24})

print(readOnlyObject.preDeclaredVar)
>> "Read-Only!"

readOnlyObject.preDeclaredVar = "Can write?"
>> AttributeError: can't set attribute '

print(readOnlyObject.name)
>> <property object at 0x016C62A0>  # I think it is weird.. property func only work on pre-declared variable?
  

what happened?

I want to know is there a way to create read-only object dynamically.

Answer 1

Consider starting with __setattr__:

>>> class ReadOnlyClass:
...     def __init__(self, **kwargs):
...         self.__dict__.update(kwargs)
...     
...     def __setattr__(self, key, value):
...         raise AttributeError("can't set attribute")
...
>>> readonly_object = ReadOnlyClass(name='Tom', age=24)
>>> readonly_object.name
'Tom'
>>> readonly_object.age
24
>>> readonly_object.age = 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __setattr__
AttributeError: can't set attribute

However, this may not fully meet your expectations. You can still set the attributes through __dict__:

>>> readonly_object.__dict__['age'] = 10
>>> readonly_object.age
10

Answer 2

You can use Named tuples:

>>> import collections
>>> def ReadOnlyClass(data):
...     class_ = collections.namedtuple('ReadOnlyClass', data.keys())
...     return class_(**data)
... 
>>> readOnlyObject = ReadOnlyClass({'name': 'Tom', 'age': 24})
>>> print(readOnlyObject.name)
Tom
>>> print(readOnlyObject.age)
24
>>> readOnlyObject.age = 14
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute