Write to buffer or return bytes

[flekschas]

This is more a question or feature request / idea rather than an issue.

If I am not mistaken it's currently not possible to write to a buffer or return the written bytes somehow right? What do you think about expanding the interface to support something like:

buf = BytesIO()
imsave(buf, my_fancy_array, formatstr='png')

If this is out of scope never mind. I am just curious.

[luispedro]

This would be a good feature, but it would require more effort than I am currently willing to invest.

[flekschas]

I understand. I'll see if I can get something to work when I have some free time.

[luispedro]

Here are some pointers:

1. You should probably do a derived class from byte_sink (see
   

   imread/imread/lib/base.h

   Line 63 in b2d5569

   class byte_sink : virtual public seekable {

   ) which writes to a buffer in memory.

2. Then, in the imsave_may_multi function, (

   imread/imread/_imread.cpp

   Line 286 in b2d5569

   const int fd = ::open(filename, O_CREAT|O_RDWR|O_BINARY|O_TRUNC, 0644);

   ), you use it instead of the current fd_source_sink (depending on the input, naturally).

3. Finally, convert it to a memory object and return it to Python.

A few things to watch out for:

• some writers need to seek in the output they've written
• writing happens without the GIL so avoid calling Python API functions.

HTH
Luis

[fish2000]

@flekschas as per the pointers offered by @luispedro – I have some things that may help you:

• A subclass of the byte_source type that operates on the Py_buffer structure from PEP 3118 and supports seeking: header and source
• Context-manager-esque RAII structures for GIL state-management and GIL-aware PyFileObject*-based I/O

… if you’re curious, I also have an example of a method returning a PyObject* containing the bytes that result from calling ImageFormat::write(…), and a Python C-API module method in which it gets used.

I, too, HTH. Salud,

-Alex