fhirpathrs 0.1.1

FHIRPath expression parser and interpreter

fhirpathrs

A high-performance FHIRPath engine written in Rust with Python bindings.

FHIRPath is a path-based navigation and extraction language for FHIR (Fast Healthcare Interoperability Resources) data, widely used in healthcare systems for querying clinical data, validating resources, and implementing decision support logic.

Key Features

• Fast — Rust-powered parser and interpreter, 10-600x faster than pure-Python alternatives
• Safe — Fully iterative evaluation (no call-stack recursion)
• Complete — 100+ FHIRPath functions including collections, math, strings, dates, types, and higher-order functions
• Python-native — Install with pip, pass dicts and lists, get dicts and lists back

Installation

pip install fhirpathrs

Build from source

# Rust toolchain required
pip install maturin
maturin develop --release --manifest-path crates/python/Cargo.toml

Quick Start

from fhirpathrs import evaluate

patient = {
    "resourceType": "Patient",
    "id": "pat-42",
    "active": True,
    "name": [
        {"family": "Jetson", "given": ["George", "Astro"]},
        {"family": "Jetson-Spacely", "given": ["Elroy"]},
    ],
    "address": [
        {"city": "Orbit City", "state": "Space"}
    ],
}

# Navigate fields
evaluate("name.family", patient)
# → ["Jetson", "Jetson-Spacely"]

# Filter collections
evaluate("name.where(family = 'Jetson').given", patient)
# → ["George", "Astro"]

# Boolean logic
evaluate("name.exists() and active", patient)
# → True

# Count results
evaluate("name.count()", patient)
# → 2

Python API

evaluate(expression: str, data: Any = None, constants: dict | None = None) -> Any

Parameter	Description
expression	FHIRPath expression string
data	Input data — any combination of dicts, lists, strings, numbers, bools, and None
constants	Optional external constants, accessible as %name in expressions

Returns the evaluation result as a Python object. Raises ValueError on parse errors and RuntimeError on evaluation errors.

Examples

Path Navigation

data = {"a": {"b": {"c": 42}}}
evaluate("a.b.c", data)  # → 42

Filtering with where()

bundle = {
    "entry": [
        {"resource": {"resourceType": "Observation", "status": "final", "value": 120}},
        {"resource": {"resourceType": "Condition", "status": "active"}},
        {"resource": {"resourceType": "Observation", "status": "final", "value": 80}},
    ]
}

evaluate(
    "entry.resource.where(resourceType = 'Observation').value",
    bundle,
)
# → [120, 80]

QuestionnaireResponse Navigation

data = {
    "QuestionnaireResponse": {
        "resourceType": "QuestionnaireResponse",
        "status": "completed",
        "item": [
            {
                "linkId": "blood-pressure",
                "item": [
                    {"linkId": "systolic", "answer": [{"valueInteger": 120}]},
                    {"linkId": "diastolic", "answer": [{"valueInteger": 80}]},
                ],
            },
            {
                "linkId": "heart-rate",
                "answer": [{"valueInteger": 72}],
            },
        ],
    }
}

# %QuestionnaireResponse resolves from the data context
evaluate(
    "%QuestionnaireResponse.repeat(item).where(linkId = 'systolic').answer.valueInteger",
    data,
)
# → [120]

evaluate(
    "%QuestionnaireResponse.repeat(item).where(linkId = 'heart-rate').answer.valueInteger",
    data,
)
# → [72]

# Collect all answers across nesting levels
evaluate("%QuestionnaireResponse.repeat(item).answer.valueInteger", data)
# → [72, 120, 80]

External Constants

data = {"items": [{"value": 10}, {"value": 50}, {"value": 90}]}

evaluate(
    "items.where(value > %threshold).count()",
    data,
    constants={"threshold": 25},
)
# → 2

Arithmetic and String Functions

evaluate("1 + 2 * 3")               # → 7
evaluate("name.length()", {"name": "hello"})  # → 5
evaluate("name.upper()", {"name": "hello"})   # → "HELLO"

Aggregation

data = {"scores": [10, 20, 30, 40, 50]}

evaluate("scores.sum()", data)   # → 150
evaluate("scores.avg()", data)   # → 30.0
evaluate("scores.min()", data)   # → 10
evaluate("scores.max()", data)   # → 50

# Custom aggregation
evaluate("scores.aggregate($total + $this, 0)", data)  # → 150

FHIR Date and Quantity Literals

evaluate("@2024-01-15")           # → "2024-01-15"
evaluate("@T14:30:00")            # → "14:30:00"
evaluate("10 'mg'")               # → {"value": 10.0, "unit": "mg"}

Pydantic Models

from pydantic import BaseModel

class Patient(BaseModel):
    resourceType: str = "Patient"
    id: str
    active: bool

patient = Patient(id="pat-42", active=True)
evaluate("active", patient.model_dump())  # → True

Type Testing and Conversion

evaluate("(1).is(Integer)")              # → True
evaluate("(1.5).is(Decimal)")            # → True
evaluate("42.toString()")               # → "42"
evaluate("'123'.toInteger()")           # → 123

Supported Functions

Collection

empty() exists() count() first() last() single() tail() take() skip() combine() distinct() isDistinct() intersect() exclude() subsetOf() supersetOf() not() hasValue() union() children() descendants() ofType()

Higher-Order

where() select() all() repeat() repeatAll() sort() iif() coalesce() aggregate() defineVariable()

Math

abs() ceiling() floor() round() truncate() sqrt() exp() ln() log() power()

String

indexOf() substring() startsWith() endsWith() contains() upper() lower() replace() matches() matchesFull() replaceMatches() length() toChars() split() join() trim() lastIndexOf() encode() decode() escape() unescape()

Type Conversion

toString() toInteger() toDecimal() toBoolean() toDate() toDateTime() toTime() toQuantity() toLong() convertsToInteger() convertsToDecimal() convertsToBoolean() convertsToString() convertsToDate() convertsToDateTime() convertsToTime() convertsToQuantity() convertsToLong()

Aggregate

allTrue() anyTrue() allFalse() anyFalse() sum() avg() min() max()

Utility

now() today() timeOfDay() type() precision() lowBoundary() highBoundary() trace()

Date/Time Components

year() month() day() hour() minute() second() millisecond() timezone() duration() difference() comparable()

Operators

+ - * / div mod = != ~ !~ < > <= >= and or xor implies is as in contains | &

Performance

Benchmarked against fhirpathpy (pure Python, ANTLR-based):

Benchmark	fhirpathrs	fhirpathpy	Speedup
Field access (small object)	1.6 us	73 us	47x
Nested path (a.b.c.d.e)	2.4 us	268 us	103x
Arithmetic (1 + 2 * 3 - 4)	2.3 us	1,411 us	627x
where() filter (10 items)	14 us	745 us	54x
where() filter (1000 items)	1,578 us	15,504 us	10x
Patient field navigation	3.8 us	130 us	34x
Patient where().given	6.3 us	389 us	61x
Bundle filter+count (100 entries)	191 us	2,482 us	13x
100-chained .where(true)	102 us	50,099 us	489x
100-nested iif()	152 us	crashes	--

Architecture

crates/
├── parser/        # FHIRPath string → AST (nom + iterative Pratt parser)
├── interpreter/   # AST + data → result (iterative dispatch loop)
└── python/        # PyO3 bindings exposing evaluate()

All three layers are fully iterative — no call-stack recursion anywhere.

License

MIT