Extending Construct **

Adapters

Adapters are the standard way to extend and customize the library. Adapters operate at the object level (unlike constructs, which operate at the stream level), and are thus easy to write and more flexible. For more info see, the adapter tutorial.

In order to write custom adapters, implement _encode and _decode:

class MyAdapter(Adapter):
    def _encode(self, obj, context):
        # called at building time to return a modified version of obj
        # reverse version of _decode
        pass

    def _decode(self, obj, context):
        # called at parsing time to return a modified version of obj
        # reverse version of _encode
        pass

Constructs

Generally speaking, you should not write constructs by yourself:

  • It’s a craft that requires skills and understanding of the internals of the library (which change over time).

  • Adapters should really be all you need and are much more simpler to implement.

  • To make things faster, try using psyco, or write your code in pyrex. The python-level classes are as fast as it gets, assuming generality.

The only reason you might want to write a construct is to achieve something that’s not currently possible. This might be a construct that computes/corrects the checksum of data… the reason there’s no such construct yet is because I couldn’t find an elegant way to do that (although Buffered or Union may be a good place to start).

There are two kinds of constructs: raw construct and subconstructs.

Raw constructs

Deriving directly of class Construct, raw construct can do as they wish by implementing _parse, _build, and _sizeof:

class MyConstruct(Construct):
    def _parse(self, stream, context):
        # read from the stream (usually not directly)
        # return object
        pass

    def _build(self, obj, stream, context):
        # write obj to the stream (usually not directly)
        # no return value is necessary
        pass

    def _sizeof(self, context):
        # return computed size, or raise SizeofError if not possible
        pass

Subconstructs

Deriving of class Subconstruct, subconstructs wrap an inner construct, inheriting it’s properties (name, flags, etc.). In their _parse and _build methods, they will call self.subcon._parse or self.subcon._build respectively. Most subconstruct do not need to override _sizeof.

class MySubconstruct(Subconstruct):
    def _parse(self, stream, context):
        obj = self.subcon._parse(stream, context)
        # do something with obj
        # return object

    def _build(self, obj, stream, context):
        # do something with obj
        self.subcon._build(obj, stream, context)
        # no return value is necessary