Pure Python Common Expression Language (CEL)

0.5.0 · active · verified Fri Apr 10

cel-python is a pure Python implementation of Google's Common Expression Language (CEL), currently at version 0.5.0. It provides common semantics for expression evaluation, enabling applications to interoperate through a fast, embeddable expression language. The library aims for minimal dependencies and is primarily used for security policies, such as within Cloud Custodian. Its release cadence is tied to its primary consumers.

Warnings

gotcha CEL's type semantics differ from native Python, and type coercion is generally not performed implicitly. This can lead to unexpected `TypeError`s if not handled explicitly in expressions.
Fix: Ensure explicit type conversion within CEL expressions (e.g., `double(1) + 2.5` instead of `1 + 2.5`) if mixed types are intended, or ensure consistent typing of inputs.
gotcha Division behavior (`/`) in CEL truncates towards zero, which is different from Python's integer division (`//`) that truncates towards negative infinity. This can lead to subtle differences in arithmetic results.
Fix: Be aware of the truncation behavior for division operations and test expressions thoroughly, especially with negative numbers.
gotcha CEL runtime errors are mapped to Python exceptions, specifically `celpy.evaluation.EvalError`. These errors provide a message similar to the CEL error, along with an underlying Python exception.
Fix: Implement proper try-except blocks to catch `celpy.evaluation.EvalError` when evaluating expressions to handle potential runtime issues gracefully.

Install

pip install cel-python Install stable version

Imports

Environment
```
from celpy import Environment
```
While Environment is in evaluation, it's exposed directly via celpy top-level for convenience.
celtypes
```
from celpy import celtypes
```
celtypes is a module within the celpy package, not a top-level module.

Quickstart

This example demonstrates compiling and evaluating a simple CEL expression with a string variable, using the Environment and celtypes classes.

from celpy import Environment, celtypes

decls = {"name": celtypes.StringType}
env = Environment(annotations=decls)
ast = env.compile("'Hello world! I\'m " + name + ".'")
out = env.program(ast).evaluate({"name": "CEL"})
print(out)
# Expected: Hello world! I'm CEL.

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