Reading-in A Binary Jpeg-header (in Python)

I would like to read in a JPEG-Header and analyze it. According to Wikipedia, the header consists of a sequences of markers. Each Marker starts with FF xx, where xx is a specific M

Solution 1:

You seem overly worried about how your binary data is represented on your console. Don't worry about that.

The default built-in string-based representation that print(..) applies to a bytes object is just "printable ASCII characters as such (except a few exceptions), all others as an escaped hex sequence". The exceptions are semi-special characters such as \, ", and ', which could mess up the string representation. But this alternative representation does not change the values in any way!

>>>a = bytes([1,2,4,92,34,39])>>>a
b'\x01\x02\x04\\"\''
>>>a[0]
1

See how the entire object is printed 'as if' it's a string, but its individual elements are still perfectly normal bytes?

If you have a byte array and you don't like the appearance of this default, then you can write your own. But – for clarity – this still doesn't have anything to do with parsing a file.

>>>binary_data = open('iaijiedc.jpg','rb').read(20)>>>binary_data
b'\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01\x02\x01\x00H\x00H\x00\x00'
>>>''.join(['%02x%02x ' % (binary_data[2*i],binary_data[2*i+1]) for i inrange(len(binary_data)>>1)])
'ffd8 ffe0 0010 4a46 4946 0001 0201 0048 0048 0000 '

Why does python not return me nice chunks of 2 bytes (in hex-format)?

Because you don't ask it to. You are asking for a sequence of bytes, and that's what you get. If you want chunks of two-bytes, transform it after reading.

The code above only prints the data; to create a new list that contains 2-byte words, loop over it and convert each 2 bytes or use unpack (there are actually several ways):

>>> wd = [unpack('>H', binary_data[x:x+2])[0] for x inrange(0,len(binary_data),2)]
>>> wd
[65496, 65504, 16, 19014, 18758, 1, 513, 72, 72, 0]
>>> [hex(x) for x in wd]
['0xffd8', '0xffe0', '0x10', '0x4a46', '0x4946', '0x1', '0x201', '0x48', '0x48', '0x0']

I'm using the little-endian specifier < and unsigned short H in unpack, because (I assume) these are the conventional ways to represent JPEG 2-byte codes. Check the documentation if you want to derive from this.