datalogic-py

Part of datalogic-rs — one engine, every runtime.

Python bindings for datalogic-rs, a fast Rust implementation of JSONLogic. Same rules, same semantics as the Rust crate, with the compile-once / evaluate-many pattern exposed natively — compile a rule once and evaluate it against thousands of data inputs without re-parsing. Every binding runs the same core and passes the same 1,532-case conformance battery (53 suites).

For the cross-runtime overview and the API-tier model every binding implements, see the repo README.

New in v5. datalogic-py is new — there is no v4 Python package. If you were calling the v4 Rust crate or the v4 @goplasmatic/datalogic WASM package, the engine's v4 → v5 changes are catalogued in MIGRATION.md.

Install

pip install datalogic-py

Pre-built wheels are published for:

Python 3.10 and newer are supported via PEP 384 stable ABI (abi3) — one wheel per platform covers every CPython 3.10+ release.

Naming: pip install datalogic-py (PyPI distribution name) → import datalogic_py (Python module name). Python modules can't contain hyphens, so the underscore form is the import.

Quick start

from datalogic_py import apply

result = apply(
    {"if": [{">": [{"var": "score"}, 50]}, "pass", "fail"]},
    {"score": 75},
)
# -> "pass"

API reference

The Python binding mirrors the Rust engine's API tier model.

One-shot — apply(rule, data)

from datalogic_py import apply

apply({"+": [1, 2, 3]}, {})                                # 6
apply({"var": "user.age"}, {"user": {"age": 25}})          # 25
apply({"and": [{">": [{"var": "x"}, 0]}, True]}, {"x": 5}) # True

Both arguments accept Python dict / list values, converted by a direct walk between Python objects and the engine's arena values (no JSON text, no intermediate tree — 2.5-3.5× faster than the pythonize-based conversion earlier builds used, and faster than a json.dumps → evaluate_str → json.loads round-trip at every payload size we measure). Payload size still matters: conversion work scales with node count, so an 8 KB dict costs ~20 µs of walk on top of the evaluation. If your data is already JSON text, call the *_str entry points (Rule.evaluate_str, Session.evaluate_str) and skip conversion; if the same payload is evaluated repeatedly, parse it once into a DataHandle and skip the per-call cost entirely. For payloads with types the walk doesn't cover, see Type conversion below.

Engine — Engine().eval(rule, data)

Construct an Engine when you need templating mode or any non-default configuration:

from datalogic_py import Engine

engine = Engine()                          # default config
engine.eval({"==": [1, 1]}, {})            # True

# Templating mode — multi-key objects become output templates
templating_engine = Engine(templating=True)
templating_engine.eval(
    {"name": {"var": "user.name"}, "ok": {">": [{"var": "score"}, 50]}},
    {"user": {"name": "Ada"}, "score": 99},
)
# {"name": "Ada", "ok": True}

Compile once — Engine().compile(rule) → Rule.evaluate(data)

Compile the rule once when you'll evaluate it against many data inputs.

from datalogic_py import Engine

engine = Engine()
rule = engine.compile({"if": [{">": [{"var": "score"}, 50]}, "pass", "fail"]})

for payload in batch:
    result = rule.evaluate(payload)         # accepts a dict
    fast   = rule.evaluate_str(json_text)   # accepts a JSON string (skips dict conversion)

Rule is thread-safe — clone the reference into worker threads and evaluate concurrently. The Rust eval call releases the GIL, so a multi-threaded server gains real parallelism.

Session — hot loops

For batches where you want to amortise arena reset across iterations, open a Session. The arena is reset between iterations automatically.

from datalogic_py import Engine

engine = Engine()
rule = engine.compile({"+": [{"var": "x"}, 1]})

with engine.session() as sess:
    for payload in batch:
        result = sess.evaluate(rule, payload)

Session is the per-thread workhorse — open one per worker thread. The arena that makes it fast can't be shared across threads (the same way a database connection is per-task in a connection-pool model); Engine and Rule are both thread-safe, so share those.

Data handles, typed results, and batch evaluation

The ABI v2 tiers. A DataHandle is an immutable, pre-parsed JSON document: parse a payload once and every evaluation against it skips JSON parsing (and dict conversion) entirely. Handles are engine-independent (one handle can feed rules compiled by different engines), safe to share across threads for reads, and not consumed by evaluation — the native memory is released when the handle is garbage-collected.

from datalogic_py import DataHandle

data = DataHandle('{"age": 25, "status": "active"}')  # raises ParseError on bad JSON
data.allocated_bytes                    # bytes held by the handle's arena

rule.evaluate_data(data)                # thread-safe, like rule.evaluate
rule.evaluate_data_str(data)            # same, JSON str out
sess.evaluate_data(rule, data)          # hot path: session arena + no parse
sess.evaluate_data_str(rule, data)

For predicates and scalar results, the typed session evaluations skip the result conversion too:

ok = sess.evaluate_bool(rule, data)     # strict JSON boolean
n  = sess.evaluate_int(rule, data)      # exact integer result
f  = sess.evaluate_float(rule, data)    # any JSON number
t  = sess.evaluate_truthy(rule, data)   # JSONLogic truthiness, never mismatches

evaluate_bool, evaluate_int, and evaluate_float raise EvaluateError with error_type == "TypeMismatch" when the rule evaluates fine but the result is not of the requested type. evaluate_truthy coerces any result through the engine's configured truthiness rules (the same coercion if/and/or apply).

The batch entry points evaluate a whole set in one native call and report failures per item, so one bad input never poisons its neighbours:

from datalogic_py import BatchItemError

# One rule, many payloads:
results = sess.evaluate_batch(rule, [d0, d1, d2])
# Many rules, one payload (the rule-set / feature-flag shape):
results = sess.evaluate_many([r0, r1], data)

for i, r in enumerate(results):
    if isinstance(r, BatchItemError):   # not raised — a result object
        print(f"item {i} failed: {r.message} ({r.tag}, operator={r.operator})")
    else:
        print(f"item {i}: {r}")         # the item's JSON string

Exceptions are reserved for argument problems (a rule compiled by a different engine, a non-handle list element, …). Typed and batch evaluations take data handles only; rules must belong to the session's engine, and sessions stay single-threaded.

Custom operators

Pass custom_operators={"name": callable} to Engine(...). Each callable receives the operator's pre-evaluated arguments as a JSON-array string and returns a JSON string of the result:

import json
from datalogic_py import Engine

engine = Engine(custom_operators={
    "double": lambda args_json: json.dumps(json.loads(args_json)[0] * 2),
})
engine.eval_str('{"double": [21]}', '{}')  # "42"

Built-ins win: a custom registration of a built-in name (+, if, var, ...) never dispatches. Callbacks run with the GIL held.

Engine configuration

Pass config= to Engine(...) to change evaluation semantics. The value is a dict (or a JSON string) with an optional preset plus per-field overrides. Unknown keys raise EvaluateError, so typos fail loudly:

from datalogic_py import Engine, EvaluateError

strict = Engine(config={"preset": "strict"})
try:
    strict.eval({"+": ["", 1]}, {})   # strict rejects non-numeric coercion
except EvaluateError as e:
    print(e.error_type)

lenient = Engine(config={"division_by_zero": "return_null"})
lenient.eval({"/": [1.5, 0]}, {})     # None

The preset applies first; the remaining keys override individual fields on top of it. Every binding shares this JSON schema and parses it with the same core code, so a config that works here works in the WASM and Node bindings too. The full semantics of each knob are documented on the Rust crate's EvaluationConfig.

Error handling

All exceptions descend from DataLogicError:

Two error_type tags come from the binding itself rather than the engine, mirroring the C ABI: "TypeMismatch" (a typed evaluation whose result has the wrong type) and "InvalidArgument" (e.g. a rule compiled by a different engine passed to a session's handle-based entry points). Per-item batch failures don't raise at all — they surface as BatchItemError values (.tag, .message, .operator) in the result list.

from datalogic_py import Engine, EvaluateError

engine = Engine()
try:
    engine.eval({"+": ["x", 1]}, {})  # arithmetic on a non-numeric string raises
except EvaluateError as e:
    print(e.error_type)  # a runtime error tag
    print(e.operator)    # "+"
    print(e.path)        # JSON-pointer-style path through the compiled tree

Threading

Type conversion

The dict-input path walks Python objects directly into the engine's arena representation (with a pythonize fallback for the long tail — behaviour is identical either way, only speed differs):

Fast direct walk: dict, list, tuple, str, int, float, bool, None.

Handled via the fallback: set/frozenset (become JSON arrays, iteration order), container/scalar subclasses (IntEnum, OrderedDict, …), mappings and dataclasses.

Conversion details worth knowing:

• float('nan') / float('inf') become JSON null (they have no JSON encoding)
• ints above 2^63 - 1 up to 2^64 - 1 degrade to float; beyond that they raise ParseError
• dict keys must be str (anything else raises ParseError) and objects are presented to the engine in sorted-key order, so object-iteration results are deterministic
• result dicts also come back key-sorted

Not supported — these raise ParseError with a clear message:

• datetime.datetime, datetime.date — convert to ISO string at the Python edge
• decimal.Decimal — convert to float or str
• bytes, bytearray

For payloads with exotic types, use rule.evaluate_str(json_text) and bring your own JSON encoder (e.g. with default=str).

Templating mode

engine = Engine(templating=True)
rule = engine.compile({
    "name": {"var": "user.name"},
    "ok": {">": [{"var": "score"}, 50]},
})
rule.evaluate({"user": {"name": "Ada"}, "score": 99})
# -> {"name": "Ada", "ok": True}

Execution tracing

Engine.evaluate_with_trace(logic, data) evaluates with step-by-step tracing and returns a JSON string envelope. The shape is identical to the WASM binding's evaluateWithTrace, so the React debugger component can consume it directly:

import json
from datalogic_py import Engine

engine = Engine()
trace = json.loads(engine.evaluate_with_trace(
    '{">": [{"var": "score"}, 50]}',
    '{"score": 75}',
))
trace["result"]           # True
trace["expression_tree"]  # {"id", "expression", "children"} tree
trace["steps"]            # per-node execution log, in evaluation order

Both arguments are JSON strings. Runtime failures do not raise: the envelope carries an error message and a structured_error object instead, alongside the steps recorded up to the failure. Tracing skips the optimizer so every operator in the rule appears in the trace; use it for debugging, not hot paths.

Performance

Geomean across 50 operator benchmark suites (Apple M2 Pro, median of 3 runs; pairwise shared-suite ratios per the methodology): the native Rust core evaluates at 8.9 ns/op, 7.9× faster than json-logic-engine (compiled, the fastest JS engine), 30.6× faster than jsonlogic-rs (the closest Rust alternative), and 104.2× faster than the json-logic-js reference implementation. The WASM build under Node measures 901.1 ns geomean (101× native); on Node servers, prefer @goplasmatic/datalogic-node.

The pyo3 boundary adds a small per-call marshalling cost on top of the core numbers; the dict paths use direct Python ↔ arena walks, so that cost scales with payload node count, not with a JSON round-trip. Use rule.evaluate_str(json_text) when you already have a JSON string, and a DataHandle when the same payload is evaluated repeatedly — on the boundary harness's 8 KB workload, session.evaluate_data_str measures ~1.3 µs/op against ~12 µs for session.evaluate_str (the per-call JSON parse) and ~24 µs for the dict path (the per-call conversion walk). Every evaluate call releases the GIL, so a multi-threaded server gains real parallelism on top of the engine's native speed.

Learn more

• datalogic-rs repository
• Rust crate deep-dive
• Documentation — Python
• Online playground
• JSONLogic specification

License

Apache-2.0. See the main repository for source and contribution guidelines.