Writing to the bytes()

[telenkor]

I don't understand. There is an object that, in essence, is a file, only in RAM. Why can't I update his Exif? Although a check in the code shows that the original Exif has been erased, and only a single new key remains. But when saving from "preview.data" to disk, everything remains the same.
I tried wrapping it in BytesIO, but it still doesn’t update 😢

import exiv2
import rawpy

with rawpy.imread("/Users/test/Desktop/_DVC0754.NEF") as raw:
        preview = raw.extract_thumb()
        if preview.format == rawpy.ThumbFormat.JPEG:

            # Here we could save this JPEG to disk, but we need to change its Exif first.
            # with open("/Volumes/Disk/output.jpg", 'wb') as file:
            #         file.write(preview.data)

            # Create new key and value "Exif.Image.Software"
            exifData_out = exiv2.ExifData()
            key = exiv2.ExifKey("Exif.Image.Software")
            v = exiv2.Value.create(exiv2.TypeId.asciiString)
            v.read("Some Text XXX»)
            exifData_out.add(key, v)

            # Trying to write to the original Exif.
            image = exiv2.ImageFactory.open(preview.data)
            image.setExifData(exifData_out)
            image.writeMetadata()

            # Here we only check that "image" has taken on the new value "Exif.Image.Software"
            # All metadata has been erased, leaving only the new value. Great!
            image.readMetadata()
            exifData = image.exifData()
            i = exifData.begin()
            end = exifData.end()
            while i != end:
                print('{:44s} {:04x} {:9s} {:3d} {:s}'.format(
                    i.key(), i.tag(), i.typeName(), i.count(), i.toString()))
                next(i)

            # But file is saved with the old value
            with open("/Volumes/Disk/output.jpg", 'wb') as file:
                    file.write(preview.data)

Anticipating the question that Exiv2 is also able to extract previews from a raw image, I will answer this way: in my process there is not only extraction of previews, but also conversion from raw. Therefore, for Exiv2 I leave the work exclusively with Metadata.

[jim-easterbrook]

When you pass preview.data to libexiv2 it (probably) copies the data, or as much of it as it needs. No subsequent operation in the C++ library libexiv2 can affect the contents of the Python object preview.data.

You probably need to call image.io() to get the modified data. See
https://exiv2.org/doc/classExiv2_1_1Image.html#a62345361501892f42c6bbc86514bf5c6

io.mmap() is used to get a Python buffer to access the data without copying.

io = image.io()
with io.mmap() as image_data:
    with open("/Volumes/Disk/output.jpg", 'wb') as file:
        file.write(image_data)
io.munmap()
io.close()

[jim-easterbrook]

PS Instead of this:

            key = exiv2.ExifKey("Exif.Image.Software")
            v = exiv2.Value.create(exiv2.TypeId.asciiString)
            v.read("Some Text XXX»)
            exifData_out.add(key, v)

you could have this:

            exifData_out["Exif.Image.Software"] = "Some Text XXX"

[telenkor]

Thanks for the tip.

with io.mmap() as image_data:
—> ((<class 'TypeError'>, 'exiv2.preview.byte_buffer' object does not support the context manager protocol, <traceback object at 0x111e8c080>))

It worked with this.

            io = image.io()
            with open("/Volumes/Disk/output.jpg", 'wb') as file:
                file.write(io.mmap())
            io.munmap()
            io.close()

[jim-easterbrook]

Ah yes, I'd copied my answer from somewhere where I was using with memoryview(io.mmap()) as image_data:. The memoryview has the context manager.

[jim-easterbrook]

I've added a context manager to Exiv2::BasicIO objects to get easy read-only access to data, so in the next release you will be able to replace

            io = image.io()
            io.open()
            with open("/Volumes/Disk/output.jpg", 'wb') as file:
                file.write(io.mmap())
            io.munmap()
            io.close()

with

            with image.io() as data:
                with open("/Volumes/Disk/output.jpg", 'wb') as file:
                    file.write(data)

I need to expand USAGE.rst to include more detail about this.

[jim-easterbrook]

I decided the context manager was not very intuitive, so I've gone back to providing a Python buffer interface to the Exiv2::BasicIO data (using mmap() etc internally). But now the buffer interface can be writeable, allowing direct modification of the data. Saving data to a file gets even easier:

            with open("/Volumes/Disk/output.jpg", 'wb') as file:
                file.write(image.io())