Reading a real file header

You've read a made-up header, and now it's time for the header of a real file you'll find on your own computer: a PNG image. Every PNG file starts with an eight-byte signature, followed by a series of chunks, and the first one is always a chunk named IHDR that holds the image's dimensions. Every multi-byte number in PNG is big-endian. This is also called network byte order, the usual choice for formats meant to move between different machines.

Each chunk is four consecutive fields: the length of its data in four bytes, then its type as four bytes of text, then its data, then a four-byte checksum. The IHDR data is thirteen bytes long, and its first few fields are what matter here:

Offset  Field
0       PNG signature (eight bytes)
8       chunk data length (four bytes)
12      chunk type IHDR (four bytes)
16      width  (four bytes, big-endian)
20      height (four bytes, big-endian)
24      bit depth (one byte)
25      color type (one byte)

So the width and height sit at offset 16, after the signature, the length and the type. struct.unpack_from(fmt, data, offset) reads from an offset you choose without slicing the data: you pass the offset as its third argument.