jsonc-edit

A Python interface to Microsoft's jsonc-parser, designed for source-preserving JSONC edits.

What it is

jsonc-edit is a lightweight Python package that provides surgical source-editing capabilities for JSONC (JSON with Comments) files. Instead of providing standard JSON parsing and dumping, it wraps Microsoft's world-class AST-aware jsonc-parser library under the hood, giving you exact, developer-grade source manipulation in Python.

Why it exists

When modifying configuration files (like tsconfig.json or .vscode/settings.json), the standard programmatic approach is highly destructive:

parse source string → modify Python dict in memory → dump back to string

This naive approach immediately strips all // line comments, /* block comments */, trailing commas, and completely mangles the developer's original indentation and formatting choices.

jsonc-edit uses a fundamentally different approach:

semantic JSONC edit → generate precise text offset edits → apply edits to raw string

By analyzing the AST and applying surgical character replacements, jsonc-edit perfectly preserves all human-authored elements in the file, including neighboring comments and specific array formatting.

Scope

jsonc-edit is intended to be a reusable, generalized JSONC source-editing library. It is strictly scoped to this purpose.

It is not:

• A tsconfig.json manager.
• A framework-specific plugin or library.
• A full JSONC configuration framework that validates schemas or paths.

Installation

pip install jsonc-edit

Runtime Prerequisites

Because jsonc-edit leverages the official Microsoft parser to ensure exact semantic correctness, it runs a tiny invisible Node.js daemon under the hood to process the edits.

You must have node and npm installed and available on your system path. During its first execution, jsonc-edit will automatically install the necessary pinned parser package into an isolated user-level cache (~/.jsonc-edit/versions/). It does not pollute your project's node_modules.

Basic Example

from jsonc_edit import modify, apply_edits

source = """{
  // My custom aliases
  "compilerOptions": {
    "paths": {
      "@/*": ["./src/*"]
    }
  }
}"""

# Generate the specific string manipulations needed
edits = modify(
    source,
    ["compilerOptions", "paths", "@example"],
    ["./types.d.ts"],
)

# Apply them to the raw source code
result = apply_edits(source, edits)

print(result)

The output will safely insert the new alias while completely respecting the existing // My custom aliases comment and the surrounding whitespace preferences.

Important Semantics

Because jsonc-edit uses Microsoft's parser engine, you inherit its intelligent formatting behavior natively:

• Comments: // and /* */ comments anywhere in the file are perfectly preserved.
• Trailing Commas: Existing trailing commas on unaffected properties remain untouched.
• Formatting: You can optionally pass options={"formattingOptions": {"insertSpaces": True, "tabSize": 4}} to modify() to ensure any newly scaffolded objects match your project's preferred spacing.
• Existing Values: Modifying a deeply nested property that does not exist yet (e.g., ["compilerOptions", "paths", "new"] on an empty {} file) will correctly and cleanly scaffold out the missing compilerOptions and paths objects for you.
• Idempotency: Attempting to set an array or object that is already functionally identical may still result in edits if the underlying parser chooses to reformat the specific block to match uniform JSON spacing.
• Errors: Invalid operations or syntax crashes will natively bubble up into Python as a JsoncParseError. Missing Node/NPM environments will raise a RuntimeBootstrapError.

Architecture

jsonc-edit uses a persistent, line-buffered Node.js daemon managed directly by Python. This means:

1. It does not spawn a new Node process for every edit. It boots once.
2. It communicates blisteringly fast via newline-delimited JSON over stdin/stdout.
3. It cleanly shuts down alongside the Python interpreter via atexit hooks.

You do not need to manage this daemon yourself. It operates entirely transparently behind the modify and apply_edits functions. If you need explicit deterministic lifecycle management, a context manager is available:

from jsonc_edit import edit_session, modify

with edit_session():
    edits = modify("{}", ["a"], 1)