What is StringIO() used for in this script?

Question

I just started using Django and Python and I'm trying to build a photo app. This script is generating thumbnails and I'd like to do that myself. Unfortunately I don't understand what StringIO() is doing. The Python Docs aren't very helpful to me in that case.

Can someone please explain to me what StringIO() does in this particular case?

From http://djangosnippets.org/snippets/1172/:

def save(self):
    from PIL import Image
    #Original photo
    imgFile = Image.open(self.image.path)

    #Convert to RGB
    if imgFile.mode not in ('L', 'RGB'):
        imgFile = imgFile.convert('RGB')

    #Save a thumbnail for each of the given dimensions
    #The IMAGE_SIZES looks like:
    #IMAGE_SIZES = { 'image_web'      : (300, 348),
    #                'image_large'    : (600, 450),
    #                'image_thumb'    : (200, 200) }
    #each of which corresponds to an ImageField of the same name
    for field_name, size in self.IMAGE_SIZES.iteritems():
        field = getattr(self, field_name)
        working = imgFile.copy()
        working.thumbnail(size, Image.ANTIALIAS)
        fp = StringIO()
        working.save(fp, "JPEG", quality=95)
        cf = ContentFile(fp.getvalue())
        field.save(name=self.image.name, content=cf, save=False);

    #Save instance of Photo
    super(Photo, self).save()

In the future, please post the code in your question. StackOverflow makes it easy and readable. — cheeken, Dec 13 '11 at 21:54
`StringIO` creates in-memory files (as opposed to normal files created with `open` which usually use the file system), and so you're basically doing file operations in memory. — wkl, Dec 13 '11 at 21:57

score 2 · Accepted Answer · answered Dec 13 '11 at 22:06

StringIO is a class which can be used as a file-like object. You can use it exactly as you would a regular file, except instead of the data being written to disk, it will be written to a buffer ( a string buffer ) in memory.

In this script it looks like the image is first saved to a StringIO memory buffer, and after that the value of the string is retrieved and passed to the constructor for ContentFile to create a new instance of ContentFile, which is then passed to the field save function.

I would presume that the reason the script is using StringIO is that the constructor for ContentFile takes in a string, and writting to and then reading a StringIO file is the easiest way to get the image contents represented as a string.

As a side note, I would like to suggest that you look at Django's ImageFile field type, it has been more than enough for my image related needs, and is more clear than going through StringIO and ContentFiles.

thanks, i think i got it now :) i'm actually going to use an image field, but while saving the model i want the image to be taken, resized and put into a second image field as a thumbnail. — JasonTS, Dec 13 '11 at 22:16

bcc32 · Answer 2 · 2011-12-13T22:13:03.073

0

StringIO provides the ability to read and write to a string just like one would write to a file. This could make the coding more convenient, easier, or both.

It also lets you edit strings, unlike regular python strings.

edited Dec 13 '11 at 22:13

answered Dec 13 '11 at 21:57

bcc32

328
1
7
17

i got that from the documentation, but why is it used here? i don't get the concept at all. what is actually saved in fp ? the path or the binary file ? why is stringio used here instead of a normal string ? – JasonTS Dec 13 '11 at 22:05
@user: The image is saved. However it's not saved to a file, but into the stringIO, i.e. after the call to save the stringIO will contain the contents of the image. The reason that the author didn't use a string is that a) you can't write to a string (strings are read-only), so there's no way the save method could write the contents of the image into a string argument and b) if save gets a string as its first argument, it interprets that as a filename. – sepp2k Dec 13 '11 at 22:09
@JasonTS: It's useful as a buffer, where you can do whatever you want to the string, unlike regular python strings. – bcc32 Dec 13 '11 at 22:12

What is StringIO() used for in this script?

2 Answers2