common-expression-language

Common Expression Language (CEL)

The Common Expression Language (CEL) is a non-Turing complete language designed for simplicity, speed, and safety. CEL is primarily used for evaluating expressions in a variety of applications, such as policy evaluation, state machine transitions, and graph traversals.

This Python package wraps the Rust implementation cel-interpreter.

Install from PyPI:

pip install common-expression-language

Basic usage:

from cel import evaluate

expression = "age > 21"
result = evaluate(expression, {"age": 18})
print(result)  # False

Simply pass the CEL expression and a dictionary of context to the evaluate function. The function returns the result of the expression evaluation converted to Python primitive types.

CEL supports a variety of operators, functions, and types

evaluate(
    'resource.name.startsWith("/groups/" + claim.group)', 
    {
        "resource": {"name": "/groups/hardbyte"},
        "claim": {"group": "hardbyte"}
    }
)
True

Custom Python Functions

This Python library supports user defined Python functions in the context:

from cel import evaluate

def is_adult(age):
    return age > 21

evaluate("is_adult(age)", {'is_adult': is_adult, 'age': 18})
# False

You can also explicitly create a Context object:

from cel import evaluate, Context

def is_adult(age):
    return age > 21

context = Context()
context.add_function("is_adult", is_adult)
context.update({"age": 18})

evaluate("is_adult(age)", context)
# False

Testing

uv run pytest --log-cli-level=debug

Future work

Command line interface

The package (plans to) provides a command line interface for evaluating CEL expressions:

$ python -m cel '1 + 2'
3

Separate compilation and Execution steps