Pypy: type object 'c_char' has no attribute 'from_buffer'

[liZe]

Launching WeasyPrint tests with pypy 2.0.2 gives this error:

weasyprint/tests/testing_utils.py:77: in wrapper
>               function(*args, **kwargs)
weasyprint/tests/test_draw.py:2714: in test_radial_gradients
>           save_pixels_to_png(pixels, 10, 16, '/tmp/a.png')
weasyprint/tests/test_draw.py:50: in save_pixels_to_png
>           data=bytearray(pixels), stride=width * 4
/usr/lib64/pypy2.0/site-packages/cairocffi/surfaces.py:646: in __init__
>           address, length = from_buffer(data)
/usr/lib64/pypy2.0/site-packages/cairocffi/surfaces.py:68: in from_buffer
>           return ctypes.addressof(ctypes.c_char.from_buffer(obj)), len(obj)
E           AttributeError: type object 'c_char' has no attribute 'from_buffer'

[SimonSapin]

http://pythonhosted.org/cairocffi/api.html#cairocffi.ImageSurface

Note: Currently only array.array buffers are supported on PyPy.

[SimonSapin]

(I know "it’s documented" is not an excuse, I wrote both of these pieces of code :))

[liZe]

No excuse but good explanation :)

[SimonSapin]

The actual explanation is that PyPy has no "buffer API" like CPython’s. The GC reserves the right to move objects around in memory, and thus you can’t have a stable pointer to inside an object. array works because it has a specific buffer_info() method which PyPy supports (by allocating a static buffer and having the array object point to it.)

[SimonSapin]

@liZe , do you think the docs should be expanded on this?

[liZe]

I've fixed the test in WeasyPrint (and you don't want to look at this commit) (no, really, you don't) (OK, this one).

Adding an explicit message for this exception may be even better: if it looks like an expected behaviour, users will probably read the doc or look for an open feature request, and find the line of documentation.

What about opening an issue? Someone may be interested in fixing it! (Just joking, at least for the second part.)

[SimonSapin]

Ok, leaving this open for now. Patches welcome ;)

[liZe]

Probably fixed with https://bitbucket.org/pypy/pypy/issues/1354/from_buffer-is-not-supported-in-ctypes