---
id: cel-expr-python
version: "0.1.3"
license: Apache-2.0
license_treatment: permissive
maintenance: active
---

# cel-expr-python — The CEL Python runtime

License: permissive · Maintenance: active · Downloads: 234.1K/mo

## What it is and what it does

cel-expr-python is a Python binding to the CEL C++ runtime, enabling you to parse, compile, and evaluate CEL expressions—a Google-designed language for policy evaluation and access control. It lets you define typed variables, compile expressions once for reuse, and evaluate them efficiently against different data sets. The package supports protobuf messages as first-class values, custom Python functions, optional type checking at compile time, and memory-efficient evaluation via shared arenas and activations.

You typically use it to build policy engines, authorization systems, or rule evaluators where expressions need to be safe, sandboxed, and fast. The wrapper handles the C++ complexity, exposing a straightforward Python API for environment setup, expression compilation, and evaluation with optional configuration for descriptor pools, containers, and extensions.

Use it for:

- Authorization and access control: evaluate policies like 'user.role in ["admin"] && request.resource == "sensitive_data"'.
- Rule engines in microservices: compile and cache expressions once, then evaluate them across many requests with different variable bindings.
- Protobuf-based validation: check constraints on protobuf message fields within CEL expressions.
- Custom business logic: define domain-specific functions and reuse them across multiple compiled expressions.
- Memory-constrained evaluation: use Arena for batch evaluation to release temporary objects as a group.

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

A Python wrapper for the CEL C++ implementation that compiles and evaluates Common Expression Language (CEL) expressions with support for custom functions, protobuf messages, and type checking.

Yes, if you need a safe, efficient expression evaluator for policy or rule engines. The package is actively maintained, has no known vulnerabilities, supports modern Python versions with precompiled wheels, and offers a clean API for both simple and advanced use cases (custom functions, protobuf integration, memory optimization). Install friction is moderate but manageable; the Apache-2.0 license is unrestrictive.

## Install

pip install cel-expr-python
uv add cel-expr-python
poetry add cel-expr-python

## Installing cel-expr-python

Before you install: Medium install friction due to compiled wheels for multiple Python versions (3.11–3.14) and platforms (macOS, Linux, Windows). Active maintenance with a recent release (50 days ago) and an active repository.

License in practice: Apache-2.0 permissive license allows commercial and private use with minimal restrictions; you must include a copy of the license and state significant changes.

Quickstart:

pip install cel-expr-python

from cel_expr_python import cel

cel_env = cel.NewEnv(variables={"x": cel.Type.INT, "y": cel.Type.INT})
expr = cel_env.compile("x + y > 10")
result = expr.eval(data={"x": 7, "y": 4})
print(result.value())  # True

Requires Python 3.11 or later; precompiled wheels available for macOS (Intel and ARM), Linux (x86_64 and aarch64), and Windows.

Verify before relying:

- Performance characteristics compared to direct CEL C++ usage or alternative expression evaluators.
- Whether custom extensions beyond math and string libraries are documented or supported.
- Serialization format stability and forward/backward compatibility guarantees.

## Package facts

- License: Apache-2.0 (permissive)
- Python support: supports_current
- Install friction: medium
- Maintenance: active
- Downloads: 234.1K/month (top 15,000 on PyPI)
- Known vulnerabilities: none known

## Tags

CEL expression evaluation, common expression language python, policy evaluation engine, expression compiler runtime, CEL protobuf support, custom function expressions, type-checked expression language, policy-evaluation, expression-language, protobuf-integration

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