How to write PNG image to string with the PIL?

Question

You can use the BytesIO class to get a wrapper around strings that behaves like a file. The BytesIO object provides the same interface as a file, but saves the contents just in memory:

import io

with io.BytesIO() as output:
    image.save(output, format="GIF")
    contents = output.getvalue()

You have to explicitly specify the output format with the format parameter, otherwise PIL will raise an error when trying to automatically detect it.

If you loaded the image from a file it has a format parameter that contains the original file format, so in this case you can use format=image.format.

In old Python 2 versions before introduction of the io module you would have used the StringIO module instead.

For Python3 it is required to use BytesIO:

from io import BytesIO
from PIL import Image, ImageDraw

image = Image.new("RGB", (300, 50))
draw = ImageDraw.Draw(image)
draw.text((0, 0), "This text is drawn on image")

byte_io = BytesIO()

image.save(byte_io, 'PNG')

Read more: http://fadeit.dk/blog/post/python3-flask-pil-in-memory-image

sth's solution didn't work for me
because in ...

Imaging/PIL/Image.pyc line 1423 -> raise KeyError(ext) # unknown extension

It was trying to detect the format from the extension in the filename , which doesn't exist in StringIO case

You can bypass the format detection by setting the format yourself in a parameter

import StringIO
output = StringIO.StringIO()
format = 'PNG' # or 'JPEG' or whatever you want
image.save(output, format)
contents = output.getvalue()
output.close()

save() can take a file-like object as well as a path, so you can use an in-memory buffer like a StringIO:

buf = StringIO.StringIO()
im.save(buf, format='JPEG')
jpeg = buf.getvalue()

With modern (as of mid-2017 Python 3.5 and Pillow 4.0):

StringIO no longer seems to work as it used to. The BytesIO class is the proper way to handle this. Pillow's save function expects a string as the first argument, and surprisingly doesn't see StringIO as such. The following is similar to older StringIO solutions, but with BytesIO in its place.

from io import BytesIO
from PIL import Image

image = Image.open("a_file.png")
faux_file = BytesIO()
image.save(faux_file, 'png')

When you say "I'd like to have number of such images stored in dictionary", it's not clear if this is an in-memory structure or not.

You don't need to do any of this to meek an image in memory. Just keep the image object in your dictionary.

If you're going to write your dictionary to a file, you might want to look at im.tostring() method and the Image.fromstring() function

http://effbot.org/imagingbook/image.htm

im.tostring() => string

Returns a string containing pixel data, using the standard "raw" encoder.

Image.fromstring(mode, size, data) => image

Creates an image memory from pixel data in a string, using the standard "raw" decoder.

The "format" (.jpeg, .png, etc.) only matters on disk when you are exchanging the files. If you're not exchanging files, format doesn't matter.