samredai commented on code in PR #4685:
URL: https://github.com/apache/iceberg/pull/4685#discussion_r868742221
##########
python/src/iceberg/schema.py:
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@@ -262,10 +267,45 @@ def primitive(self, primitive: PrimitiveType) -> T:
... # pragma: no cover
+@dataclass(init=True, eq=True)
+class Accessor:
+ """An accessor for a specific position in a container that implements the
StructProtocol"""
+
+ position: int
+ inner: Optional["Accessor"] = None
+
+ def __str__(self):
+ return f"Accessor(position={self.position},inner={self.inner})"
+
+ def __repr__(self):
+ return self.__str__()
+
+ def with_inner(self, accessor: "Accessor") -> "Accessor":
+ parent = copy.deepcopy(self)
+ acc = parent
+ while acc.inner:
+ acc = acc.inner
+ acc.inner = accessor
+ return parent
+
+ def get(self, container: StructProtocol) -> Any:
+ """Returns the value at self.position in `container`
+
+ Args:
+ container(StructProtocol): A container to access at position
`self.position`
+
+ Returns:
+ Any: The value at position `self.position` in the container
+ """
+ return container.get(self.position)
+
+
@singledispatch
def visit(obj, visitor: SchemaVisitor[T]) -> T:
"""A generic function for applying a schema visitor to any point within a
schema
+ The function traverses the schema in pre-order fashion
Review Comment:
I agree that pre-order feels more intuitive, especially since the index gets
built in ascending field ID order. That being said, it's not worth it if there
are gotchas that we're missing here.
> ...and I think makes the visitor you're building here more complicated
because it can't rely on traversal to create the child accessors it needs.
This is a great point--I'm noticing now that the visit method for
NestedField here is becoming responsible for much than just visiting the
NestedField. I couldn't think of a post-order solution for creating the wrapped
accessor after the inner accessor is visited, but I completely missed what
we're doing there in `_IndexByName`. I think mimicking that is a good idea and
keeping something like a `self._inner` property on the
`_BuildPositionAccessors` class.
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