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

Instead of writing imperative code to parse a piece of data, you declaratively define a data structure that describes your data. As this data structure is not code, you can use it in one direction to parse data into Pythonic objects, and in the other direction, convert ("build") objects into 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

A PascalString is a string prefixed by its length:

>>> from construct import *
>>>
>>> PascalString = Struct("PascalString",
...     UBInt8("length"),
...     Bytes("data", lambda ctx: ctx.length),
... )
>>>
>>> PascalString.parse("\x05helloXXX")
Container({'length': 5, 'data': 'hello'})
>>> PascalString.build(Container(length = 6, data = "foobar"))
'\x06foobar'

Instead of specifying the length manually, let's use an adapter:

>>> PascalString2 = ExprAdapter(PascalString, 
...     encoder = lambda obj, ctx: Container(length = len(obj), data = obj), 
...     decoder = lambda obj, ctx: obj.data
... )
>>> PascalString2.parse("\x05hello")
'hello'
>>> PascalString2.build("i'm a long string")
"\x11i'm a long string"

See more examples of file formats and network protocols in the repository.

Construct's homepage is http://construct.readthedocs.org, where you can find all kinds of docs and resources. The library itself is developed on github; please use github issues to report bugs, and github pull-requests to send in patches. For general discussion or questions, please use the new discussion group.

Construct should run on any Python 2.6-3.5 or pypy/pypy3 implementation.