Python: Check if uploaded file is jpg

No need to use and install the PIL lybrary for this, there is the imghdr standard module exactly fited for this sort of usage.

See http://docs.python.org/library/imghdr.html

import imghdr

image_type = imghdr.what(filename)
if not image_type:
    print "error"
else:
    print image_type

As you have an image from a stream you may use the stream option probably like this :

image_type = imghdr.what(filename, incomming_image)

---

Actualy this works for me in Pylons (even if i have not finished everything) : in the Mako template :

${h.form(h.url_for(action="save_image"), multipart=True)}
Upload file: ${h.file("upload_file")} <br />
${h.submit("Submit", "Submit")}
${h.end_form()}

in the upload controler :

def save_image(self):
    upload_file = request.POST["upload_file"]
    image_type = imghdr.what(upload_file.filename, upload_file.value)
    if not image_type:
        return "error"
    else:
        return image_type

If you need more than looking at extension, one way would be to read the JPEG header, and check that it matches valid data. The format for this is:

Start Marker  | JFIF Marker | Header Length | Identifier
0xff, 0xd8    | 0xff, 0xe0  |    2-bytes    | "JFIF\0"

so a quick recogniser would be:

def is_jpg(filename):
    data = open(filename,'rb').read(11)
    if data[:4] != '\xff\xd8\xff\xe0': return False
    if data[6:] != 'JFIF\0': return False
    return True

However this won't catch any bad data in the body. If you want a more robust check, you could try loading it with PIL. eg:

from PIL import Image
def is_jpg(filename):
    try:
        i=Image.open(filename)
        return i.format =='JPEG'
    except IOError:
        return False