From Newsgroup: comp.lang.python

Roel Schroeven <roel@roelschroeven.net> wrote:

As a follow-up, it looks like this behavior is because bytes and int are immutable.

Yes.

But that doesn't tell me why using super().__init__(<custom arguments>) doesn't work for immutable classes.

bytes.__init__ does work, but it's just an inherited object.__init__, which does nothing, and takes no parameters.
__init__ cannot change the value of the bytes object; the value is set by bytes.__new__ and cannot change after that.

Best not to define an __init__ method at all, just use __new__.

Something like:

class BytesSubclass(bytes):
def __new__(cls, whatever, arguments, you, like):
bytesvalue = compute(whatever, arguments, you, like)
ob = bytes.__new__(cls, bytesvalue)
ob.some_other_att = compute_something_else(whatever, arguments, you, like)
return ob

regards,
Anders

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From Newsgroup: comp.lang.python

Op 3/12/2024 om 13:55 schreef Anders Munch via Python-list:

Roel Schroeven <roel@roelschroeven.net> wrote:

As a follow-up, it looks like this behavior is because bytes and int are immutable.

Yes.

OK.

But that doesn't tell me why using super().__init__(<custom arguments>) doesn't work for immutable classes.

bytes.__init__ does work, but it's just an inherited object.__init__, which does nothing, and takes no parameters.
__init__ cannot change the value of the bytes object; the value is set by bytes.__new__ and cannot change after that.

I see now why __init__, being a regular method, can't change an object's
value (or attributes in general) if that object is immutable. I'm not
sure why I didn't think of that before.

It's not entirely clear to me though how bytes.__new__ *can* set an
object's value. Isn't __new__ also a regular function? Are these
immutable classes special cases in the language that can't be recreated
in the same way with user-defined classes? Not that that's something I
want to do, and it's also not terribly important to me, but I'm trying
to better understand what's going on.

Best not to define an __init__ method at all, just use __new__.

Something like:

class BytesSubclass(bytes):
def __new__(cls, whatever, arguments, you, like):
bytesvalue = compute(whatever, arguments, you, like)
ob = bytes.__new__(cls, bytesvalue)
ob.some_other_att = compute_something_else(whatever, arguments, you, like)
return ob

Thanks, that works perfectly. That's also more important than
understanding all the nitty-gritty details (I feel a basic understanding
is important, but not necessarily always all the low-level details).
--
"There is no cause so noble that it will not attract fuggheads."
-- Larry Niven

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From Newsgroup: comp.lang.python

On 4/12/24 3:24 am, Roel Schroeven wrote:

It's not entirely clear to me though how bytes.__new__ *can* set an
object's value. Isn't __new__ also a regular function?

Yes, but the __new__ methods of the builtin immutable objects (int,
str, bytes, etc.) are implemented in C, and so are able to do things
that Python methods cannot.
--
Greg

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From Newsgroup: comp.lang.python

Op 4/12/2024 om 0:14 schreef Greg Ewing via Python-list:

On 4/12/24 3:24 am, Roel Schroeven wrote:

It's not entirely clear to me though how bytes.__new__ *can* set an
object's value. Isn't __new__ also a regular function?

Yes, but the __new__ methods of the builtin immutable objects (int,
str, bytes, etc.) are implemented in C, and so are able to do things
that Python methods cannot.

Aha, yes, that's what I already suspected, but I wasn't sure. Thanks for confirming that.

All clear now. Thanks to Anders and Greg for explaining this to me.

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