How do I make all instance variables of a class readonly?

Question

Let's say I have a class like the following:

class Test:
    def __init__(self, some_data_source):
        self.a = 5
        self.b = some_data_source['c']

I want to make all instance variables of this class read-only and will use it as follows:

data_source = {
    'c': 'some data'
}

x = Test(data_source)

# Should be illegal i.e result in an exception
x.a = 5
x.b = None

# This should be legal
print(x.a)

Initially, I thought about using properties, but then I realized that in order to add them dynamically, I would need to add these attributes after the class is defined (e.g Test.attribute = property(...)). This doesn't work because I want to define these properties inside the class (specifically in __init__).

How else can I make all of my instance variables read-only for a class?

Answer 1

check with hasattr if the variable is exists and if it is raise error that you can't set new value to this variable and check it in __setattr__

class Test:
    def __init__(self, some_data_source):
        self.a = 5
        self.b = some_data_source['c']

    def __setattr__(self, name, value):
        if hasattr(self, name):
            raise ValueError('cannot set %s to %s' %(value, name))
        self.__dict__[name]=value


data_source = {
    'c': 'some data'
}

x = Test(data_source)
x.b='raise' # this will raise error