lclang

A typed, async-first configuration expression language for Python.

lclang gives applications a disciplined way to express calculated configuration, evaluate it only when needed, and explain where every result came from. It occupies the useful space between static data files and putting all configuration logic in Python.

LCL stands for Lazy Context Language:

• Lazy definitions run only when a requested result depends on them.
• Context comes from a short-lived Frame containing host inputs, lookup hierarchy, cached results, dependency evidence, and owned async work.
• Language means rules are parsed into an immutable syntax tree and run by lclang's own interpreter, rather than hidden in string substitution or application glue.

The package includes the expression language, reusable Modules and Frames, UTF-8 .lclcfg files, dependency analysis, runtime inspection, reviewed standard namespaces, a typed command-line framework, workflow status trees, and a composable business-day calendar system.

Why lclang

Static formats are excellent for static values. They become awkward when a configuration needs derived names, conditional policy, reusable functions, request-specific inputs, or an explanation of why a value has its current form. Plain Python can calculate all of that, but it also makes the boundary between policy and application behavior easy to blur.

lclang provides a strong way of working while leaving room for each application to supply its own values and callables:

• Parse rules once, then reuse them safely across independent runs.
• Keep stable policy separate from request, tenant, command, or environment inputs.
• Evaluate only the dependency path needed for the requested output.
• Share concurrent work and cache value or failure snapshots predictably.
• Inspect definitions, owners, cache states, lookup paths, and dependencies.
• Compose configuration files without losing source and history information.
• Use the same evaluation model in services, batch jobs, CLIs, and tests.
• Stay fully typed with no third-party runtime dependencies.

This makes lclang especially useful for deployment policy, derived service settings, pricing and eligibility rules, command defaults, scheduling logic, and other trusted configuration that has real relationships between values.

Installation

python -m pip install lclang

Start in 60 seconds

Define a reusable Module once. Create a fresh Frame for one calculation, supply the values owned by the application, and ask only for the outputs you need.

import asyncio

import lclang


ORDER = lclang.define_module(
    "order",
    {
        "subtotal": "unit_price * quantity",
        "shipping": "0 if subtotal >= 50 else 5",
        "total": "subtotal + shipping",
        "summary": 'f"{quantity} items: {total:.2f}"',
    },
)


async def main() -> None:
    async with lclang.define_frame(
        ORDER,
        preset={"unit_price": 13, "quantity": 3},
    ) as frame:
        assert await frame.get("subtotal") == 39
        assert await frame.get("total") == 44
        assert await frame.get("summary") == "3 items: 44.00"


asyncio.run(main())

The first lookup multiplies the two host inputs and stores subtotal as a snapshot. The total lookup reuses that snapshot, takes the false branch of the shipping condition because 39 is below 50, and adds 5. The final lookup follows summary -> total -> subtotal; the previously calculated values are reused, and the f-string formats 44 with two decimal places.

Definition order is not evaluation order. Every expression is parsed and validated when the Module is created, while evaluation follows name lookups from the requested result.

For a single expression in synchronous code, use the smaller boundary:

import lclang

total = lclang.evaluate_sync(
    "unit_price * quantity",
    {"unit_price": 6, "quantity": 4},
)
assert total == 24

Do not call evaluate_sync() from a running event loop. Async applications should use Frames or await evaluate().

The Module and Frame model

Most integrations need only two concepts.

Module: what can be calculated?

A Module is an immutable collection of named, unevaluated definitions. It has no per-run cache or lifecycle state, so the same Module can be reused across requests, tenants, commands, and tests.

Frame: what do those definitions mean now?

A Frame combines a Module with concrete host inputs, parent lookup, lazy result snapshots, dependency observations, and owned asynchronous work. It is stateful, belongs to one event loop, and should normally represent one independent run.

expression source -> immutable Module
immutable Module + host inputs -> short-lived Frame -> requested results

Use async with so owned work and async context managers close deterministically. A Frame is safe for concurrent tasks in the same event loop, but not across different loops.

The first await frame.get(name) calculates and stores either a value or an ordinary failure. Concurrent callers share the in-flight work. Later reads get the same snapshot. This is deliberate: a Frame is a reproducible calculation run, not a reactive spreadsheet.

Changing an input does not silently invalidate dependants. Recalculation is explicit and local:

frame.mixin({"unit_price": 10})
await frame.recalculate("subtotal")
await frame.recalculate("total")

If several related inputs change, creating a new Frame is often clearer. Use mixin() and recalculate() when retaining the other snapshots in the current run is intentional.

Feature tour

Expression language

LCL is expression-only, Unicode-aware, and intentionally familiar to Python users. It supports arithmetic, comparisons, short-circuit Boolean logic, conditionals, null coalescing, safe attributes, calls, slices, unpacking, collection displays, comprehensions, generators, f-strings, exceptions, context managers, and sync or async iteration.

profile?.display_name ?? "anonymous"
[item * 2 for item in values if item > 0]
f"{service}: {port}"
(left, right=10) -> left + right
try primary() except ServiceError: fallback()

Arrow functions provide defaults, keyword arguments, lexical closures, and recursive-program support. parse_expression() returns immutable syntax, to_source() renders canonical source, evaluate() is the async evaluation boundary, and evaluate_sync() is the synchronous convenience boundary.

Configuration files with provenance

.lclcfg files move definitions outside Python while preserving file, line, override, and inclusion history.

__LCL_VERSION__: 1

service: "catalog"
port: 8443
host: f"{service}.{environment}"
address: f"{host}:{port}"

The application supplies environment; the file owns the stable relationship between it and the final address. Definitions can refer forward or backward. using declarations expand other .lclcfg files in source order, later definitions win, and complete history remains available for diagnostics.

from pathlib import Path

from lclang.config import evaluate_config, load_config


async def address_for(environment: str) -> str:
    config = await load_config(Path("settings.lclcfg"))
    result = await evaluate_config(
        config,
        "address",
        values={"environment": environment},
    )
    assert isinstance(result, str)
    return result

Loading is asynchronous, UTF-8, bounded by configurable limits, cycle-aware, and concurrency-sharing. It performs no globbing, environment expansion, network access, or evaluation while files are being composed.

Dependency analysis and runtime inspection

lclang can explain calculated configuration instead of treating it as a black box.

• Static analysis classifies eager, conditional, and deferred references.
• Module and Frame graphs support filtered queries and deterministic topological ordering.
• Runtime tracing records the names actually selected during evaluation.
• Dependency snapshots reconcile static possibilities with observed work.
• frame.get_definition(name) returns selected syntax without evaluating it.
• frame.inspect_variable(name) builds a detached tree of owners, lookup paths, cache states, values, failures, and dependency descendants.

tree = frame.inspect_variable("total")
print("\n".join(tree.to_lines()))

Syntax, name, evaluation, circular-dependency, closed-Frame, and configuration failures use specific LclError subclasses. Source-aware errors retain the variable evaluation stack so application boundaries can report useful context.

Async-first evaluation

Resolver values, host callables, call results, iterators, and context-manager protocols may be synchronous or asynchronous. lclang awaits them when needed and keeps cancellation isolated between the owner of a calculation and other tasks waiting for it.

Task-local evaluation limits can bound work. Circular lookups produce a named dependency path. Frame closing finalizes owned resources deterministically. These properties let the same policy work naturally in an async service without making simple synchronous scripts cumbersome.

Reviewed standard namespaces

Canonical Frames include familiar pure builtins plus small, read-only namespaces assembled from reviewed manifests:

• iter.collect and iter.first consume sync or async iterables.
• text.join and text.lines provide deterministic text operations.
• data.lookup and data.merge work with immutable mapping snapshots.
• json.encode and json.decode provide strict JSON conversion.
• parse_ymd and to_ymd convert strict calendar-date integers.
• recursive builds eager fixed-point functions for recursive LCL programs.

They provide no ambient filesystem, process, network, dynamic import, reflection, or mutation capability.

Command-line applications

lclang.cli is a typed async framework for Python scripts whose parameters may come from declared defaults, .lclcfg files, and command-line overrides. A decorated command handler receives one CliContext, including its invocation Frame, as-of date, dry-run flag, and logging context, and returns a deterministic CliResult.

The framework provides immutable invocation values, nested command groups, structured help, full-argument parsing, platform-neutral process entry points, isolated logging, and opt-in internal tracing. Pass --verbose after a selected command to trace expression parsing, value provenance, caching, fallbacks, and evaluation to stderr; enabled file logging receives the same records. Trace values use bounded representations but are not redacted, so verbose output may contain configuration data. The existing -v/--version spelling remains the version command. Precedence rises from preset and command defaults through configuration definitions and command-line overrides to reserved runtime values. Built-in commands inventory available values, parse LCL, and evaluate LCL using the same routing model.

Dry-run remains an explicit handler decision, so the framework never pretends to know whether an application-specific side effect is safe.

Workflow status

lclang.workflow records nested task and step outcomes when one Boolean or exit code cannot explain a run. Scoped steps finalize success automatically, record exception detail without suppressing the exception, and aggregate parent status deterministically. Immutable snapshots preserve child order and can be rendered, logged, or converted to an application's own wire format.

This is useful for import pipelines, release processes, multi-stage commands, and any operation where users need to see what succeeded, failed, was skipped, or never finished.

Business-day calendars

lclang.utils.calendar provides an async-first, three-state calendar algebra. Every date is a business day, holiday, or undefined; undefined means that the calendar has no opinion and enables meaningful composition.

Calendars support union, intersection, subtraction, ordered fallback, reversal, business-only and holiday-only filters, bounded date movement, immutable date mappings, built-in weekday and period-boundary calendars, strict JSON loading, concurrent named-calendar caching, and dependency-aware retirement. Canonical Frames expose the reviewed factories and singletons through the calendars namespace.

The calendar layer works as a Python utility in its own right and also gives LCL configuration a precise vocabulary for settlement dates, processing windows, and scheduling policy.

Choose the smallest entry point

For optional lookup, await frame.get(name, fallback=value) returns the fallback only when the name is absent from the complete Frame hierarchy. It does not hide a definition that exists but fails, and it does not cache the fallback.

When lclang fits

Choose lclang when configuration has meaningful relationships and benefits from lazy evaluation, per-run context, source-aware diagnostics, or dependency evidence. It is a strong fit when rules should be reusable and inspectable but host applications must retain control over inputs and capabilities.

Prefer JSON, TOML, YAML, or plain Python data when the input is already static and has no useful derived relationships. Prefer ordinary Python when the logic is fundamentally application behavior, needs unrestricted object access, or is clearer as a normal function than as configuration policy.

Practical integration rules

1. Parse definitions once and reuse the resulting Module.
2. Create one Frame per independent request, command, tenant, or run.
3. Pass narrow values and purpose-built callables, not broad service objects.
4. Request only the output names the application needs.
5. Treat cached values as snapshots and recalculate explicitly.
6. Use caller-owned Frames as async context managers.
7. Use evaluate_config() when only one file-backed value is required.
8. Catch LclError at the application boundary and preserve its diagnostic context.

Production characteristics

• Stable 1.0 release line.
• Pure Python for Python 3.14 and newer.
• Fully typed and ships a py.typed marker.
• No third-party runtime dependencies.
• Custom lexer, parser, immutable AST, and async interpreter.
• No Python eval, exec, AST compilation, Java, or generated parser runtime.
• 100% branch coverage enforced by the project quality gate.
• Parser differential and property tests, concurrency and dependency stress tests, lifecycle leak checks, and executable documentation.
• Reproducible source distribution and platform-independent wheel validation.

Trust and security

lclang is a trusted configuration language, not a hostile-code sandbox. Its own standard values deliberately omit ambient filesystem, process, network, dynamic-code, and reflection powers. However, expressions can use the values and callables supplied by the host application, so those inputs determine the authority available during evaluation.

Treat LCL source, .lclcfg files, host objects, and host callables as trusted application configuration. Keep the Python boundary narrow and use loading and evaluation limits where bounded work matters.

Requirements and license

• Python 3.14 or newer
• No third-party runtime dependencies
• MIT license