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