[issue11427] ctypes from_buffer no longer accepts bytes

[benrg]

benrg <benrudiak at gmail.com> added the comment:

I am still interested in this for the same reason I was interested in this in the first place; nothing has changed. I guess I will reiterate, and try to expand.

The problem is that ctypes tries to enforce const correctness (inconsistently), but it has no way to declare its objects as const, and assumes they are all non-const. This is the worst possible combination. It would make sense to have no notion of const and never try to enforce it (like K&R C), and it would make sense to have constness and try to enforce it (like C++). But the present situation is like a compiler that treats string literals as const char[], and doesn't allow casting away const, but doesn't allow the use of the const keyword in user code, so there's no way to pass a string literal as a function argument because the necessary type can't be expressed. Instead you have to copy the literal to a char array and pass that.

Calling C functions is inherently unsafe. Calling C functions could always corrupt the Python heap or otherwise screw around with state that the Python environment thinks that it owns. All I want is some means to assert that my code is not going to do that, as a way of getting around the limited type system that can't provide those sorts of guarantees. More broadly, what I want from ctypes is a way to do the sorts of things that I can do in C. If I can call foo(&bar + 17) in C, I want to be able to make that call in Python.

I wasn't using (c_type*N).from_buffer because I wanted to. I was using it after wasting literally hours trying to find some other way to get ctypes to agree to pass a pointer into the buffer of a Python object to an external function (which was not going to alter it, I promise). This should be easy; instead it's nearly impossible. I don't want to wrestle with random un-overridable attempts to enforce correctness when calling a language where that can never be enforced. I just want to call my C function.

I'm pretty sure that I verified that this code worked in 3.1.3 before opening this bug, but it's been a while. I could try to reproduce it, but I think this functionality should be present regardless. You can call it a feature request instead of a regression if you want.

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resolution: invalid -> 
status: closed -> open

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11427>
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