How to create an image from a string in python

Question

I'm currently having trouble creating an image from a binary string of data in my Python program. I receive the binary data via a socket but when I try the methods I read about on here like this:

buff = StringIO.StringIO() #buffer where image is stored
#Then I concatenate data by doing a 
buff.write(data) #the data from the socket
im = Image.open(buff)

I get an exception to the effect of "image type not recognized". I know that I am receiving the data correctly because if I write the image to a file and then open a file it works:

buff = StringIO.StringIO() #buffer where image is stored
buff.write(data) #data is from the socket
output = open("tmp.jpg", 'wb')
output.write(buff)
output.close()
im = Image.open("tmp.jpg")
im.show()

I figure I am probably doing something wrong in using the StringIO class but I'm not sure

Answer 1

I suspect that you're not seek-ing back to the beginning of the buffer before you pass the StringIO object to PIL. Here's some code the demonstrates the problem and solution:

>>> buff = StringIO.StringIO()
>>> buff.write(open('map.png', 'rb').read())
>>> 
>>> #seek back to the beginning so the whole thing will be read by PIL
>>> buff.seek(0)
>>>
>>> Image.open(buff)
<PngImagePlugin.PngImageFile instance at 0x00BD7DC8>
>>> 
>>> #that worked.. but if we try again:
>>> Image.open(buff)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\python25\lib\site-packages\pil-1.1.6-py2.5-win32.egg\Image.py", line 1916, in open
    raise IOError("cannot identify image file")
IOError: cannot identify image file

Make sure you call buff.seek(0) before reading any StringIO objects. Otherwise you'll be reading from the end of the buffer, which will look like an empty file and is likely causing the error you're seeing.