Initializing Python class

Question

I am newbie in object-oriented design.

I try to incorporate a python function into my init . (Function works, as I asked earlier).

Here is my code:

import struct
import urllib2
import StringIO


class wallpaper:
    def __init__(self, url):
        self.url = url
        self.content_type = ''
        self.height = 0
        self.width = 0
        image = urllib2.urlopen(self.url)
        data = str(image.read(2))
        if  data.startswith('\377\330'):
            self.content_type = 'image/jpeg'
            jpeg = StringIO.StringIO(data)
            jpeg.read(2)
            b = jpeg.read(1)
            try:
                while (b and ord(b) != 0xDA):
                    while (ord(b) != 0xFF): b = jpeg.read(1)
                    while (ord(b) == 0xFF): b = jpeg.read(1)
                    if (ord(b) >= 0xC0 and ord(b) <= 0xC3):
                        jpeg.read(3)
                        h, w = struct.unpack(">HH", jpeg.read(4))
                        break
                    else:
                        jpeg.read(int(struct.unpack(">H", jpeg.read(2))[0])-2)
                        b = jpeg.read(1)
                    self.width = int(w)
                    self.height = int(h)
            except struct.error:
                pass
            except ValueError:
                pass


x = wallpaper('http://i.imgur.com/rapwX.jpg')
print x.url, "\t", x.content_type,"\t", x.height,"\t", x.width

The problem is that I cannot initialize height and width properties (they are equal 0).

Where is the problem?

Edit:

I've found problem, here is solution. Thanks for your hints.

One more question: is my approach proper as far as OOP rules are concerned? Should I compute width/height in init or create other method (eg. size, or so)?

Instead of playing with the binary data your self, you can use the (Python Image Library)[http://www.pythonware.com/products/pil/] to handle all the low-level stuff. — Katriel, Sep 18 '11 at 14:51

score 3 · Answer 1 · answered Sep 18 '11 at 14:50

3

You're only reading 2 bytes into data.

answered Sep 18 '11 at 14:50

Ignacio Vazquez-Abrams

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But that works well - you don't have to download the whole image to get its dimensions. – grotos Sep 18 '11 at 15:06
Do you actually believe that? Have you tried reading the code? – Ignacio Vazquez-Abrams Sep 18 '11 at 15:09
I did only some testing with large images (about 4MB) - this script is as responsive as with smaller ones. I know this is not the best way to check it, but I don't have other tools to measure precisely. – grotos Sep 18 '11 at 15:59

score 1 · Answer 2 · answered Sep 18 '11 at 14:53

It will be difficult to determine the problem if you simply ignore exceptions:

    except struct.error:
        pass
    except ValueError:
        pass

At least you should print the error messages:

    except struct.error as exc:
        print str(exc)
    except ValueError as exc:
        print str(exc)

This way you should be able to debug your code and thus solve your problem.

Of course, I assume that the indentation is correct in your actual code.

While this is true, the problem is not that an exception is being raised. — Ignacio Vazquez-Abrams, Sep 18 '11 at 14:54

score 1 · Answer 3 · answered Sep 18 '11 at 14:56

If this condition

ord(b) >= 0xC0 and ord(b) <= 0xC3

is true at the first iteration of the outer while , you break out of the loop before assigning self.width and self.height. Maybe you meant to indent the two assignments one level less than they are in your code?

Initializing Python class

3 Answers3