FileLoader JSON

JSON file loader for the vcti-fileloader framework — parses with the stdlib json module and attaches the parsed object as a single locked subtree under a caller-supplied parent.

Overview

vcti-fileloader-json is a JSON loader plugin for the vcti-fileloader framework. It implements the Loader protocol: populate(handle, tree, parent) parses a JSON file with the Python standard library json module and attaches the parsed value (a dict, list, scalar, or None) as the .data of a single child under a one-node subtree. The returned subtree is structure-locked and payload-locked.

The package depends only on vcti-fileloader and vcti-tree; the JSON parser is the Python stdlib. JSON files have no native attribute mechanism, so the loader stamps no file_attributes — callers that want to enrich with file paths, tags, or derived metadata should use the before_lock hook on populate.

Installation

pip install vcti-fileloader-json>=1.0.3

In pyproject.toml dependencies

dependencies = [
    "vcti-fileloader-json>=1.0.3",
]

Quick Start

from pathlib import Path

from vcti.fileloader.core import DataNode
from vcti.fileloader.json import JsonLoader
from vcti.tree.core import DictTree

loader = JsonLoader()
tree: DictTree[DataNode] = DictTree(DataNode())

with loader.open(Path("config.json")) as handle:
    subtree_root = loader.populate(handle, tree, tree.root_handle)

# The parsed object lives on the single child's `.data`.
[child_handle] = tree.children(subtree_root)
parsed = tree.payload(child_handle).data
print(parsed)               # e.g. {"name": "demo", "values": [1, 2, 3]}

With a before_lock hook

JSON has no native attribute mechanism, so all attribute stamping happens via the hook. A typical pattern is to attach the file path:

path = Path("config.json")

def stamp_file_path(tree, root):
    tree.payload(root).enricher_attributes["file_path"] = str(path)

with loader.open(path) as handle:
    root = loader.populate(handle, tree, tree.root_handle, before_lock=stamp_file_path)

For rule-driven enrichment, pair the hook with vcti-attribute-enricher.

What the subtree looks like

For any JSON file (whether it parses to a dict, list, scalar, or null), the subtree shape is uniform:

The root's payload is an empty DataNode (data is None, no attributes) — it exists to give callers a single locked anchor regardless of the parsed value's type. The child carries the parsed value as .data.

API

JsonLoader

Helpers

Error Handling

from vcti.fileloader.core import (
    LoadError,
    UnsupportedFormatError,
    TreeAttachmentError,
)

try:
    with loader.open(Path("config.json")) as handle:
        subtree_root = loader.populate(handle, tree, tree.root_handle)
except FileNotFoundError:
    ...
except UnsupportedFormatError:
    # File extension is not .json
    ...
except LoadError:
    # File could not be opened or contained invalid JSON
    ...
except TreeAttachmentError:
    # parent is missing, deleted, or structure-locked in `tree`
    ...

If populate fails partway (an invalid JSON token, or a before_lock exception), the partial subtree is removed before the exception propagates — callers never see a half-built subtree.

What this package does NOT do

• Streaming parses. json.load reads the whole file at once. Use a streaming JSON parser (e.g. ijson) and a custom loader for very large files.
• Schema validation. The loader accepts any well-formed JSON. Validation belongs in a before_lock hook or downstream pass.
• Expand JSON into a multi-node tree. The parsed object lands on a single leaf as a Python object. If you want each JSON key as a separate tree node, that is a separate (future) loader; this one is deliberately one-node.

Dependencies

• vcti-fileloader (>=5.1.0) — Loader protocol, SubtreeBuilder, DataNode (import from vcti.fileloader.core)
• vcti-tree (>=1.0.0) — LockableTree protocol

The JSON parser is json from the Python standard library — no third-party dependency.