Walk a Visitor to Leaves on a Graph in Python

walk.py

from collections.abc import MutableMapping, MutableSequence
from typing import Callable, Iterable, Union

MutableCollection = Union[MutableMapping, MutableSequence]
Visitor = Callable[[str], str]  # we only support string scalars for now


def visit(value: str) -> str:
    """Visit a string scalar, and mutate it if needed before returning it."""
    if len(value) > 4:  # example criteria
        return value[::-1]  # example implementation
    return value


def walk(graph: MutableCollection, visitor: Visitor) -> None:
    """Walk a updatable graph (nested collection supporting __getitem__ and __setitem__)
    and visit the leaves in place.
    """
    nodes: Iterable = tuple()
    if isinstance(graph, MutableMapping):
        nodes = graph.items()
    elif isinstance(graph, MutableSequence):
        nodes = enumerate(graph)
    else:
        raise ValueError(
            f"I don't understand what {graph} is. It may not be updatable in place?"
        )
    for key, value in nodes:
        if isinstance(value, str):
            graph[key] = visitor(value)
        else:
            walk(value, visitor=visitor)


if __name__ == "__main__":
    graph = {
        "name": "sean",
        "age": "too old",
        "job": [{"description": "it's complicated"}],
        "hobbies": ["linux", "food", "aws", "data structures"]
    }
    walk(graph, visitor)
    print(graph)