Raise ValueError for io.StringIO in Image.open

[radarhere]

Resolves #4097

The conclusion of discussion in the issue is that StringIO can't be used to pass bytes into Image.open.

import io
from PIL import Image
im = Image.open(io.StringIO("'\x89\x50\x4E\x47\x0D"))

OSError: cannot identify image file <_io.StringIO object at 0x104ecdd70>

A ValueError with a clearer message with now be raised in this situation, explaining that byte data is required instead of StringIO.

[kdschlosser]

question.... am I able to use this when calling image.open

image.open(b'\x65\x32\x86\x54\xD2\xA1')

the byte data is nothing just for example.

or does it have to explicitly be.

image.open(io.BytesIO(b'\x65\x32\x86\x54\xD2\xA1'))

[radarhere]

@kdschlosser the second one

Image.open(io.BytesIO(b'\x65\x32\x86\x54\xD2\xA1'))

The first code sample can be used for a path -

Image.open(b"Tests/images/hopper.jpg")

[kdschlosser]

the reason I bring that up is because of the error description..
"StringIO cannot be used to open an image. Binary data must be used instead"

byte literal and bytearray fall into the binary data category.. BytesIO kind of does but fits into the stream or file like object more so then being binary data..

This would be a better description "StringIO is not supported, please use BytesIO instead."
with the description above it provides the exact solution that works. There is no possibility of a user misunderstanding what needs to be used as a replacement for StringIO.

just a suggestion....