[Cython] automatically raise MemoryError on exceptional C return values

[Stefan Behnel]

Robert Bradshaw, 09.08.2012 01:45:
> On Tue, Aug 7, 2012 at 2:53 AM, Stefan Behnel wrote:
>> given how ubiquitous manual memory management is in C, I think it would be
>> nice to let Cython generate the exception raising also for C, not only for
>> C++. The difference is this:
>>
>>   cdef extern from "...":
>>       char* make_new_buffer() except NULL as MemoryError
>>       int append_to_buffer(char* buffer, char* value) \
>>                                      except -1 as MemoryError
>>
>>   c_buffer = make_new_buffer()           # raises MemoryError on NULL
>>   append_to_buffer(c_buffer, "testdata") # raises MemoryError on -1
>>   append_to_buffer(c_buffer, "moredata") # raises MemoryError on -1
>>
>> versus this:
>>
>>    cdef extern from "...":
>>        char* make_new_buffer()
>>        int append_to_buffer(char* buffer, char* value)
>>
>>    c_buffer = make_new_buffer()
>>    if c_buffer is NULL:
>>        raise MemoryError()
>>    if append_to_buffer(c_buffer, "testdata") == -1:
>>        raise MemoryError()
>>    if append_to_buffer(c_buffer, "moredata") == -1:
>>        raise MemoryError()
>>
>> I don't think it's necessary to support this for anything but MemoryError,
>> since you'd normally want a formatted error message for everything else.
>>
>> Alternative colours for the bike shed:
>>
>>     char* make_new_buffer() except NULL raise MemoryError
>>
>>     char* make_new_buffer() raise MemoryError for NULL
>>
>>     char* make_new_buffer() raise MemoryError from NULL
>>
>> What do you think?
> 
> I'm +0.5 (with a preference to the "except NULL as MemoryError"
> flavor). Basically, it is new syntax to make up for the lack of proper
> meta programming.

In a way, yes.


> Alternatively (just throwing it out there, not sure
> if it's a good idea), could this be done as a decorator of some sort?
> That's how one would write such a concept in Python. The need to fix
> types does constrain things.

A decorator on the external function declaration, you mean? Something like
this?

    @autoraise(MemoryError)
    char* make_new_buffer() except NULL

Or like this?

    @autoraise(MemoryError, error_value="NULL")
    char* make_new_buffer()

I wouldn't mind. Although there isn't currently a pure Python syntax for
external declarations that would benefit from this ...

Or did you mean that it should be a user implemented decorator? That would
be nice, but even fused types would make a simple case like this rather
code heavy, I guess.

Stefan