Raising an exception on updating a 'constant' attribute in python

Question

As python does not have concept of constants, would it be possible to raise an exception if an \'constant\' attribute is updated? How?

class MyClass():
            


        

Answer 1

You can use a metaclass to achieve this:

class ImmutableConstants(type):
    def __init__(cls, name, bases, dct):
        type.__init__(cls, name, bases, dct)

        old_setattr = cls.__setattr__
        def __setattr__(self, key, value):
            cls.assert_attribute_mutable(key)
            old_setattr(self, key, value)
        cls.__setattr__ = __setattr__

    def __setattr__(self, key, value):
        self.assert_attribute_mutable(key)
        type.__setattr__(self, key, value)

    def assert_attribute_mutable(self, name):
        if name.isupper():
            raise AttributeError('Attribute %s is constant' % name)

class Foo(object):
    __metaclass__ = ImmutableConstants
    CONST = 5
    class_var = 'foobar'

Foo.class_var = 'new value'
Foo.CONST = 42 # raises

But are you sure this is a real issue? Are you really accidentally setting constants all over the place? You can find most of these pretty easily with a grep -r '\.[A-Z][A-Z0-9_]*\s*=' src/.