Construct (python library) 

Construct is a powerful declarative parser (and builder) for binary data.

The library provides both simple, atomic constructs (such as integers of 
various sizes), as well as composite ones which allow you form hierarchical 
structures of increasing complexity. Construct features bit and byte 
granularity, easy debugging and testing, an easy-to-extend subclass system, 
and lots of primitive constructs to make your work easier:

    -Fields: raw bytes or numerical types
    -Structs and Sequences: combine simpler constructs into more complex ones
    -Adapters: change how data is represented
    -Arrays/Ranges: duplicate constructs
    -Meta-constructs: use the context (history) to compute the size of data
    -If/Switch: branch the computational path based on the context
    -On-demand (lazy) parsing: read only what you require
    -Pointers: jump from here to there in the data stream
