def __init__(self):

[Marko Rauhamaa]

Ben Finney <ben+python at benfinney.id.au>:

> Gary Herron <gherron at digipen.edu> writes:
>
>>    The __init__ method is the constructor for instances of a class.
>>    It is not required, but the situations in which a constructor is
>>    not needed are few and unusual.
>
> That's needlessly confusing: ‘__init__’ is not a constructor because
> it does not construct the instance. The ‘__new__’ method is the
> constructor for a class (and returns the new instance).

I have never ever had a temptation to specify a __new__ method. I can't
imagine a *beneficial* case of overriding it.

> The ‘__init__’ method requests the already-constructed instance to
> initialise itself (and returns None).

It is a serious practical problem that an object can't guarantee that
its __init__ has been called.

Check out some of the stdlib source code for example:

========================================================================
class ThreadPoolExecutor(_base.Executor):
    def __init__(self, max_workers):
        """Initializes a new ThreadPoolExecutor instance.

        Args:
            max_workers: The maximum number of threads that can be used to
                execute the given calls.
        """
        self._max_workers = max_workers
        self._work_queue = queue.Queue()
        self._threads = set()
        self._shutdown = False
        self._shutdown_lock = threading.Lock()
========================================================================

Notice how _base.Executor.__init__(self) does not get called. It can
only work if _base.Executor does not specify an __init__. That's an
assumption you really couldn't make with a clear conscience even if you
were the author and maintainer of both modules
(concurrent.futures.thread and concurrent.futures._base).


Marko