gh-143715: deprecate incomplete initialization of struct.Struct()

[skirpichev]

• Struct.__new__() will require a mandatory argument (format)
• Calls of __init__() method on initialized Struct are deprecated

• Issue: Make struct.Struct() really immutable #143715

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📚 Documentation preview 📚: https://cpython-previews--143659.org.readthedocs.build/

• Issue: Make struct.Struct() really immutable #143715

[skirpichev]

The evil plan is to remove custom Struct.__init__() method and move all initialization logic to the Struct.__new__(). Something, that was done by #94532 before.

[serhiy-storchaka]

I think this needs a separate issue for discussion. Please open a new issue, and refer #78724 and #112358 (maybe even with citations).

We need to discuss what should be the end goal after the end of the deprecation period, and how deprecations canl help to transit the user code.

[skirpichev]

CC @meadori per experts index.

[skirpichev]

And what about the following case?

class MyStruct(struct.Struct):
    def __init__(self, arg):
        super().__init__('>h')

my_struct = MyStruct('<h')
my_struct.pack(12345)

There should emit a FutureWarning, because the current and the future code produce different results. MyStruct(5) should emit a DeprecationWarning, because it works in the current code, but will be error in future. ``MyStruct('>h')` should work without warnings.

@serhiy-storchaka, this seems too complex for me.

Look, we are going to that state: #94532. (I did a working patch to play with in skirpichev#17.) That means, eventually the Struct's __init__() will be no-op (and accept any arguments).

Thus, we should warn users on this pattern: explicit call of the __init__() on an object of some Struct subclass. Just in all above examples. Does make sense for you?

CC @vstinner

severe code changes

[vstinner]

The overall change LGTM, but I have a few more minor comments.

[vstinner]

LGTM.

@serhiy-storchaka: Do you want to review this change?

[serhiy-storchaka]

I tried to fix corner cases, but since all code and tests were rewritten, I created a separate PR #145580.

[skirpichev]

Ah, I forgot that format is positional or keyword argument. Will adapt code a bit.