Anchor requires key presence in container

[ius]

When building a Struct which contains an Anchor, not providing the anchor's label as a key in the Container instance yields an AttributeError.

#!/usr/bin/python2
from construct import *

struct = Struct(None, Anchor('foo_anchor'))
struct.build(Container())

Running the testcase yields: AttributeError: 'Container' object has no attribute 'foo_anchor'

As an anchor's value is solely determined by it's position in the construct, it shouldn't have to be specified in the Container?

[nnemkin]

Same thing happens with ConstAdapter and related macros.

There probably should be a flag that informs Struct to pass None if the field is not present.
This flag would also make it possible to create constructs that have a default build value and computed build value.

[MostAwesomeDude]

Hm. I haven't messed with Anchor much, but I'll look into this.