How can I prevent or alter access to class variables in Python

Question

Is there a way to prevent or alter access to class variables in Python as one can via overriding __setattr__ for instance variables?

Note that this question is mistitled and actually refers to instance variables, not class variables.

Based on reading multiple posts about the (apparent) deathtrap that is __slots__, I'd prefer not to go that route (and I haven't looked into it enough to know if it does what I'm asking).

Example:

class A(object):

    foo = "don't change me"

    def __setattr__(self, name, value):
        raise ValueError

if __name__ == '__main__':
a1 = A()
print a1.foo # don't change me
print A.foo  # don't change me

A.foo = 'bar' # ideally throw an exception or something here
print A.foo   # bar
a2 = A()
print a1.foo  # bar
print a2.foo  # bar

Answer 1

The ActiveState solution that Pynt references makes instances of ROClass have read-only attributes. Your question seems to ask if the class itself can have read-only attributes.

Here is one way, based on Raymond Hettinger's comment:

#!/usr/bin/env python
def readonly(value):
    return property(lambda self: value)

class ROType(type):
    CLASS_PROPERTY = readonly(1)

class Foo(object):
    __metaclass__=ROType

print(Foo.CLASS_PROPERTY)
# 1

Foo.CLASS_PROPERTY=2
# AttributeError: can't set attribute

The idea is this: Consider first Raymond Hettinger's solution:

class Bar(object):
    CLASS_PROPERTY = property(lambda self: 1)
bar=Bar()
bar.CLASS_PROPERTY=2

It shows a relatively simple way to give bar a read-only property.

Notice that you have to add the CLASS_PROPERTY = property(lambda self: 1) line to the definition of the class of bar, not to bar itself.

So, if you want the class Foo to have a read-only property, then the parent class of Foo has to have CLASS_PROPERTY = property(lambda self: 1) defined.

The parent class of a class is a metaclass. Hence we define ROType as the metaclass:

class ROType(type):
    CLASS_PROPERTY = readonly(1)

Then we make Foo's parent class be ROType:

class Foo(object):
    __metaclass__=ROType