[Image-SIG] PIL 1.1.6 ImageFile.Parser destroys data for PngImagePlugin

[Nils Olaf de Reus]

I am experiencing inconvenience from the behaviour of ImageFile.py in
PIL 1.1.6 when attempting to read in a PNG image.

The method I see documented to retreive an image through ImageFile is by
invoking close() on the ImageFile.Parser, like this.

>>> import ImageFile
>>> import urllib
>>> 
>>> f = urllib.urlopen('<url-path-to-png-file>')
>>> 
>>> p = ImageFile.Parser()
>>> 
>>> while True:
...     s = f.read(1024)
...     if not s:
...         break
...     p.feed(s)
... 
>>> im = p.close()

The way ImageFile.Parser works, it creates an ImageFile._ParserFile to
act as a file, and calls Image.open() on that. So far so good, and this
results in a valid png image object.. for a very brief moment, because
in the 'finally' section immediately after, it calls close() on that
ImageFile._ParserFile.

The definition of close() in ImageFile._ParserFile is as follows:

    def close(self):
        self.data = self.offset = None

But PngImagePlugin does not copy data on opening - it just keeps a
reference to the source. So this just now destroyed the data on the
object that PngImagePlugin is still referring to as the location of its
IDAT chunks.. not a bright move, because we have now created a condition
where from the point of view of the PngImagePlugin, self.fp is an
ImageFile._ParserFile instance, and self.fp.data is None.

Which means that as soon as we try to do anything with that image,
PngImagePlugin is going to be firing read() instructions at its self.fp,
which in turn blindly trusts that its self.data supports __getitem__()..
and predictably, there is our Traceback:

>>> im.convert('RGB')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/PIL/Image.py", line 653, in
convert
    self.load()
  File "/usr/lib/python2.5/site-packages/PIL/ImageFile.py", line 189, in
load
    s = read(self.decodermaxblock)
  File "/usr/lib/python2.5/site-packages/PIL/PngImagePlugin.py", line
365, in load_read
    return self.fp.read(bytes)
  File "/usr/lib/python2.5/site-packages/PIL/ImageFile.py", line 300, in
read
    data = self.data[pos:pos+bytes]
TypeError: 'NoneType' object is unsubscriptable


I am currently working around the issue by creating my own
ImageFile._ParserFile object that avoids having its data None'd.

>>> import ImageFile
>>> import urllib
>>> 
>>> f = urllib.urlopen('<url-path-to-png-file>')
>>> 
>>> p = ImageFile.Parser()
>>> 
>>> while True:
...     s = f.read(1024)
...     if not s:
...         break
...     p.feed(s)
... 
>>> virtualfile = ImageFile._ParserFile(p.data)
>>> im = Image.open(virtualfile)
>>> p.close()
>>> im.convert('RGB')
<Image.Image instance at 0x2b890f5cff80>


This works great -just need to keep the virtualfile around until I'm
really done with it- but obviously I am not happy having to use what is
supposed to be an internal from ImageFile.

>From where I stand, the sensible thing would be to eliminate these two
lines from ImageFile.Parser.close():

            finally:
                fp.close() # explicitly close the virtual file

..except that because of the comment, I must assume that there is a
reason to be doing this. Could someone please shed light on why it is
necessary to explicitly close fp there?

Kind regards,
  Nils Olaf de Reus