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comp / comp.lang.python / Re: super().__init__() and bytes

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* Re: super().__init__() and bytesRoel Schroeven
`- Re: super().__init__() and bytesStefan Ram

1
Subject: Re: super().__init__() and bytes
From: Roel Schroeven
Newsgroups: comp.lang.python
Date: Tue, 3 Dec 2024 10:01 UTC
References: 1 2
Path: eternal-september.org!news.eternal-september.org!feeder3.eternal-september.org!fu-berlin.de!uni-berlin.de!not-for-mail
From: roel@roelschroeven.net (Roel Schroeven)
Newsgroups: comp.lang.python
Subject: Re: super().__init__() and bytes
Date: Tue, 3 Dec 2024 11:01:00 +0100
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Op 3/12/2024 om 10:41 schreef Roel Schroeven via Python-list:
> [...]
> When I try the same with bytes as base class though, that doesn't work
> (at least in the Python version I'm using, which is CPython 3.11.2
> 64-bit on Windows 10):
>
> class MyBytes(bytes):
>     def __init__(self, data):
>         super().__init__(data)
> print(MyBytes(b'abcdefghijlkmn'))
>
> This results in an exception:
>
> Traceback (most recent call last):
>   File "test_mybytes.py", line 4, in <module>
>     print(MyBytes(b'abcdefghijlkmn'))
>           ^^^^^^^^^^^^^^^^^^^^^^^^^^
>   File "test_mybytes.py", line 3, in __init__
>     super().__init__(data)
> TypeError: object.__init__() takes exactly one argument (the instance
> to initialize)
>
> I'm passing two arguments (data and the implicit self), and apparently
> that's one too many. Let's try without arguments (i.e. only the
> implicit self):
>
> class MyBytes(bytes):
>     def __init__(self, data):
>         super().__init__()
> print(MyBytes(b'abcdefghijlkmn'))
>
> Now it works, and prints b'abcdefghijlkmn'. The same happens with int
> as base class, and presumably a number of other classes.

As a follow-up, it looks like this behavior is because bytes and int are
immutable. When I try with bytesarray instead of bytes, which works
largely the same but is mutable, things do work as I expect. There's a
hint in the documentation of __new__(): "__new__() is intended mainly to
allow subclasses of immutable types (like int, str, or tuple) to
customize instance creation". But that doesn't tell me why using
super().__init__(<custom arguments>) doesn't work for immutable classes.

The documentation for __init__() says " If a base class has an
__init__() method, the derived class’s __init__() method, if any, must
explicitly call it to ensure proper initialization of the base class
part of the instance; for example: super().__init__([args...])". So does
that mean that bytes and int not have an __init__() method? Is there a
link between being immutable and not having __init__()?

--
"Man had always assumed that he was more intelligent than dolphins because
he had achieved so much — the wheel, New York, wars and so on — whilst all
the dolphins had ever done was muck about in the water having a good time.
But conversely, the dolphins had always believed that they were far more
intelligent than man — for precisely the same reasons."
-- Douglas Adams

Subject: Re: super().__init__() and bytes
From: Stefan Ram
Newsgroups: comp.lang.python
Organization: Stefan Ram
Date: Tue, 3 Dec 2024 10:26 UTC
References: 1
Path: eternal-september.org!news.eternal-september.org!feeder3.eternal-september.org!fu-berlin.de!uni-berlin.de!not-for-mail
From: ram@zedat.fu-berlin.de (Stefan Ram)
Newsgroups: comp.lang.python
Subject: Re: super().__init__() and bytes
Date: 3 Dec 2024 10:26:33 GMT
Organization: Stefan Ram
Lines: 67
Expires: 1 Jan 2026 11:59:58 GMT
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Roel Schroeven <roel@roelschroeven.net> wrote or quoted:
>As a follow-up, it looks like this behavior is because bytes and int are
>immutable.

Alright, so you've stumbled onto some pretty gnarly behavior with
Python's immutable types. Let's unpack this burrito:

1) Here's the skinny on how it goes down with immutable types
like "bytes" and "int":

These bad boys do their thing in "__new__", not
"__init__". It's like trying to change a burrito after
it's been wrapped - ain't going to happen.

When you hit "MyBytes( b'abcdefghijlkmn' )", it's like this:

a) Python calls up "MyBytes.__new__".

b) That passes the buck to "bytes.__new__", which
whips up the new "bytes" object.

c) Then "MyBytes.__init__" gets a shot, but the horse
has already left the barn.

2) Wanna customize those args?

For immutable types, you got to override `__new__`.
It's like swapping out the ingredients before the
burrito's wrapped:

class MyBytes( bytes ):
def __new__( cls, data ):
return super().__new__( cls, data * 2 )

print( MyBytes( b'abc' )) # Spits out: b'abcabc'

3) Why "bytes" (and "int", ...) are different beasts:

You nailed it in your follow-up. It's all about being immutable.
These types are coded in C for speed, but the real kicker
is they can't change after creation.

On your other questions:

- Yeah, immutable types like "bytes" and "int" pretty much
ghost "__init__". All the magic happens in "__new__".

- There's definitely a connection between being immutable
and ditching "__init__". It's like setting your GPS
before you start driving - once you're moving, you can't
change the route.

- That "__init__" doc you quoted? It's legit for mutable
types, but for immutable ones, it's like trying to add
avocado to a sealed burrito - not going to work.

To wrap it up:

- Mutable types: Override "__init__" to set things up.

- Immutable types: "__new__" is where the party's at.

This setup lets Python keep its promises about immutability while
still letting you customize through subclassing. It's like having
your artisanal, gluten-free burrito and eating it too.

1

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