Writing byte stream as jpeg format to disk

Robert Kern robert.kern at gmail.com
Thu Aug 26 23:10:26 CEST 2010

On 8/26/10 3:47 PM, Navkirat Singh wrote:
> On 27-Aug-2010, at 1:57 AM, MRAB wrote:
>> On 26/08/2010 21:14, Navkirat Singh wrote:
>>> On 27-Aug-2010, at 1:32 AM, Dave Angel wrote:
>>>> Navkirat Singh wrote:
>>>>> Hey guys,
>>>>> I am programming a webserver, I receive a jpeg file with the POST
>>>>> method.The file (.jpeg) is encoded in bytes, I parse the bytes by
>>>>> decoding them to a string. I wanted to know how i could write the
>>>>> file (now a string) as a jpeg image on disk. When I try to encode the
>>>>> same string to a bytes and write them in binary format to disk, the
>>>>> file is not recognized as jpeg. I would be grateful if someone could
>>>>> help me with this.
>>>>> Regards,
>>>>> Nav
>>>> If by "decoding them to a string" you mean converting to Unicode, then
>>>> you've already trashed the data. That's only valid if the bytes had
>>>> been encoded from valid Unicode characters, and then only if you use
>>>> the corresponding decoding technique.
>>>> If you mean some other decoding, then the question is meaningless
>>>> without telling us just what the decoding is, preferably with some code.
>>>> It also might be useful to know what version of Python you're using,
>>>> when you post the code.
>>>> DaveA
>>> Dave,
>>> I am using Python3 and I receive a byte stream with a jpeg attached sent
>>> by the web browser over a socket, which looks like this:
>>> b': image/jpeg\r\nAccept: text/*\r\nReferer:
>>>\r\nAccept-Language: en-us\r\nAccept-Encoding:
>>> gzip, deflate\r\nContent-Length: 91783\r\nConnection:
>>> keep-alive\r\n\r\n\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01\x01\x00\x00\x01\x00\x01\x00\x00\xff\xdb\x00\x84\x00\x03\x02\x02\x03\x02\x02\x03\x03\x03\x03\x04\x03\x03\x04\x05\x08\x05\x05\x04\x04\x05\n\x07\x07\x06\x08\x0c\n\x0c\x0c\x0b\n\x0b\x0b\r\x0e\x12\x10\r\x0e\x11\x0e\x0b\x0b\x10\x16\x10\x11\x13\x14\x15\x15\x15\x0c\x0f
>>>  From the above, I need to:
>>> a) Split the header content from the image content, which comes after
>>> the keep-alive\r\n\r\n part
>>> b) Then write the image content to file for further use as a jpeg.
>> Try:
>>     image = header.split(b'keep-alive\r\n\r\n', 1)[-1]
>>     open(image_path, 'wb').write(image)
>> --
>> http://mail.python.org/mailman/listinfo/python-list
> I think I forgot to mention that the original is a stream of bytes decoded using ISO-8859-1 as utf-8 trhrew errors (lack of knowlegdge again).
> @MRAB - the split() method in python 3 works only on strings and throws an error if I try to use bytes

This is incorrect.

Python 3.1.2 (r312:79360M, Mar 24 2010, 01:33:18)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

 >>> bytes = b'Connection: keep-alive\r\n\r\nbody'
 >>> bytes.split(b'\r\n\r\n', 1)[-1]

FYI: the JPEG data is not in the header. The b'\r\n\r\n' sequence delimits the 
header from the body. Do not rely on "Connection: keep-alive" being the last header.

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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