struct.unpack failed

[DreamingInsanity]

On the line that's erroring (line 9), you are using struct.unpack. You are passing in "bytearray". In python bytearray is a type that an object can be.
It seems you want to unpack the header of the file. In that case you would want to read the file into a byte array and pass that in instead.
This should work:

b_array = bytearray(f.read())
_ignored, n_images, image_columns, image_rows = struct.unpack('>IIII', b_array)

EDIT: I did a quick test. For me, running that code gave me an error saying it wanted a 16 byte buffer. I had to do this instead:

_ignored, n_images, image_columns, image_rows = struct.unpack('>IIII', bytearray(a.read()[:16]))

You may need that, you may not.