Python: Check if uploaded file is jpg

Question

How can I check if a file uploaded by a user is a real jpg file in Python (Google App Engine)?

This is how far I got by now:

Script receives image via HTML Form Post and is processed by the following code

...
incomming_image = self.request.get("img")
image = db.Blob(incomming_image)
...

I found mimetypes.guess_type, but it does not work for me.

Answer 1

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