Convert Instance Fields into Properties

Question

I would like to convert all instance fields of an object into properties (getter only) in order to make them read only. The fields might be defined by a subclass.

How can I achieve this?

class SomeClass(object):

    def __init__(self, foo, bar):

        self.foo = foo
        self.bar = bar

        convert_all_instance_fields_into_properties(self)   # implementation ?

Answer 1

It is possible to achieve readonly fields using python builtins quite easily:

class X:

    def __init__(self, val):
        self.foo = val

    def __setattr__(self, key, value):
        if not hasattr(self, key):  # only for first set
            super(X, self).__setattr__(key, value)
        else:
            raise ValueError


def main():
    x = X('bar')
    y = X('baz')

    assert x.foo == 'bar'
    assert y.foo == 'baz'

    # raises ValueError
    x.foo = 'Raise an error!'

If you want to specify which fields are readonly

class X:

    readonly = ['foo']

    def __init__(self, val):
        self.foo = val

    def __setattr__(self, key, value):
        if key in self.readonly and not hasattr(self, key):
            super(X, self).__setattr__(key, value)
        else:
            raise ValueError