Forbid to add attributes to user-defined class in Python

Question

When you create a user-defined class, you can, by default, dinamically add attributes to it just like in the next example:

# User defined class
class Test:
    def __init__(self):
        self.first_att = 5

test = Test()
test.second_att = 11
print(test.__dict__)

{'first_att': 5, 'second_att': 11}

But built-in classes don't allow such a thing:

# Built-in class
str_example = 'Test string'

str_example.added_att = 5

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'added_att'

How can I modify my class Test so it doesn't allow it also? Thanks in advance

Answer 1

You can define in your class the allowed attributes by adding __slots__.
In there put the names of the attributes you want. All other attributes won't be accepted.
For example try this:

# User defined class
class Test:
    __slots__ = ("fitst_att", "your_att_name")

    def __init__(self):
        self.first_att = 5

test = Test()
test.your_att_name = 11
print(test.__dict__)
# test.second_att = 11  ##this will rais an error

Answer 2

You can achieve that by overwriting the __setattr__ method. As a very simple example for your case

class Test:
    def __init__(self):
        self.first_att = 5

    def __setattr__(self, key, value):
        if key not in ("first_att",):  # allowed attributes
            raise AttributeError(f"Not allowed to set attribute {key}")
        super().__setattr__(key, value)

Note that you have to add an exception list here that holds the names of the attributes you want to be able to assign values to.