Can a Python class be written such it may be passed to write()?

Question

I'd like to pass an instance of my class to write() and have it written to a file. The underlying data is simply a bytearray.

mine = MyClass()

with open('Test.txt', 'wb') as f:

    f.write(mine)

I tried implementing __bytes__ to no avail. I'm aware of the buffer protocol but I believe it can only be implemented via the C API (though I did see talk of delegation to an underlying object that implemented the protocol).

Answer 1

No, you can't, there are no special methods you can implement that'll make your Python class support the buffer protocol.

Yes, the io.RawIOBase.write() and io.BufferedIOBase.write() methods document that they accept a bytes-like object, but the buffer protocol needed to make something bytes-like is a C-level protocol only. There is an open Python issue to add Python hooks but no progress has been made on this.

The __bytes__ special method is only called if you passed an object to the bytes() callable; .write() does not do this.

So, when writing to a file, only actual bytes-like objects are accepted, everything else must be converted to such an object first. I'd stick with:

with open('Test.txt', 'wb') as f:
    f.write(bytes(mine))

which will call the MyClass.__bytes__() method, provided it is defined, or provide a method on your class that causes it to write itself to a file object:

with open('Test.txt', 'wb') as f:
    mine.dump(f)