`Block.get()` raises unhelpful `KeyError` when path traverses through a CID link

[sumanjeet0012]

When Block.get() encounters a CID (link) while traversing a path, it tries to use the CID as a dict/list and raises a generic KeyError. It should either stop at the link boundary and return the CID, or raise a more informative error indicating that the path crosses a block boundary.

Problem

In dag/block.py:

def get(self, path: str) -> Any:
    segments = [s for s in path.split("/") if s]
    current: Any = self._value
    for seg in segments:
        if isinstance(current, dict):
            current = current[seg]
        elif isinstance(current, list):
            current = current[int(seg)]
        else:
            raise KeyError(f"Cannot traverse into {type(current).__name__} with {seg!r}")
    return current

When a value is a CID:

block = Block.encode(
    value={"link": some_cid, "data": "hello"},
    codec=dag_cbor.codec,
)

# This raises: KeyError: "Cannot traverse into CIDv1 with 'nested'"
block.get("link/nested")

The error message doesn't explain that the path crosses a block boundary and that the caller needs to load the linked block.

Proposed Solution

Add explicit CID handling:

def get(self, path: str) -> Any:
    segments = [s for s in path.split("/") if s]
    current: Any = self._value
    for i, seg in enumerate(segments):
        if is_cid(current):
            remaining = "/".join(segments[i:])
            raise LinkBoundaryError(
                f"Path crosses block boundary at CID {current}. "
                f"Remaining path: {remaining!r}. "
                f"Load the linked block to continue traversal."
            )
        if isinstance(current, dict):
            current = current[seg]
        elif isinstance(current, list):
            current = current[int(seg)]
        else:
            raise KeyError(f"Cannot traverse into {type(current).__name__} with {seg!r}")
    return current

Add a new exception class:

class LinkBoundaryError(Exception):
    """Raised when path traversal crosses a block boundary (CID link)."""
    pass

Related

• File: dag/block.py