Private fields with __#

Let’s reconsider our Counter class.

Oh no! An update to the value field—outside the class definition itself—caused an exception to be raised inside a class method. That’s not good. If you only call the constructor and tick, you’ll never have any problems… but if someone else messes with your fields, it could be hard to figure out what’s gone wrong.

What we need is encapsulation: we need to be able to protect some of the internals of our objects from messed with, so that outside code doesn’t mess up our invariants (like, e.g., Counter.value is an int).

Python’s approach to encapsulation is a lightweight compromise. Private fields should have names starting with two underscores. Programs should then avoid using those names, which are considered private and internal.

The compromise here is that you can circumvent things. Just naming c.__value raises an AttributeError. If we instead refer to c._Counter__value, then we can see the internal state. In some languages, you can really and truly make things private. That’s hard to do in Python. But it’s easy to make things hard to get to in a way that looks weird—which is enough to discourage people from doing it without making it impossible. (Python is a very pragmatic language and is full of compromises like this.)

You’ll notice that __init__ has a similar name—it starts with two underscores! The theme here is that things starting with __ aren’t for outside consumption.