On Tue, 26 Apr 2016 06:25 pm, Marko Rauhamaa wrote: > Ben Finney <ben+pyt...@benfinney.id.au>: > >> Gary Herron <gher...@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.
I'm afraid that's a failure of your imagination then. The obvious reason for overriding __new__ is to construct an immutable instance. You have to override __new__, because by the time it returns the instance is immutable and you can no longer initialise 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. No object can guarantee that any method is called. Python doesn't make an exception for __new__ or __init__. I'm not sure why you would want it to. > 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. What makes you think it needs to be called? > It can only work if _base.Executor does not specify an __init__. That's not the only way. For example, if the __init__ doesn't nothing, just like object.__init__. You can actually pass anything you like to object.__init__, and nothing happens: object.__init__([]) -- Steven -- https://mail.python.org/mailman/listinfo/python-list
