Lazy Rebuild with non-progressive lambdas

[ev1313]

Hi,

currently something like this is not possible:

d = Struct(
"foo" / Int32ul,
"bar" / Rebuild(Int32ul, lambda ctx: ctx.baz),
"baz" / Rebuild(Int32ul, lambda ctx: ctx.foo),

You'd need to create custom Padding + Pointer for this behaviour.

The above can probably made possible by modifying Rebuild, Container and ListContainer:
Rebuild will not evaluate the lambda directly, but instead will set the lambda as value in the Container.

When any value is accessed in the Container (in getattr) which is callable, the lambda gets called with the context. Something like this in getattr:

    def __getattr__(self, name):
        try:
            if name in self.__slots__:
                ret = object.__getattribute__(self, name)
                if callable(ret):
                    return ret(self)
                return ret
            else:
                ret = self[name]
                if callable(ret):
                    return ret(self)
                return ret
        except KeyError:
            raise AttributeError(name)

This would especially be helpful in reconstructing offsets.

[JayFoxRox]

Probably meant to be closed by #1030

[arekbulski]

This PR is not going to happen because I dont understand what it does...