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__()?
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