cel-python

Pure Python implementation of Google Common Expression Language, https://opensource.google/projects/cel.

The Common Expression Language (CEL) implements common semantics for expression evaluation, enabling different applications to more easily interoperate.

Key Applications

Security policy: organization have complex infrastructure and need common tooling to reason about the system as a whole

Protocols: expressions are a useful data type and require interoperability across programming languages and platforms.

This implementation has minimal dependencies, runs quickly, and can be embedded into Python-based applications. Specifically, the intent is to be part of Cloud Custodian, C7N, as part of the security policy filter.

Installation

pip install cel-python

You now have the CEL run-time available to Python-based applications.

Command Line

We can read JSON directly from stdin, making this a bit like jq.

% python -m celpy '.this.from.json * 3 + 3' <<EOF
heredoc> {"this": {"from": {"json": 13}}}
heredoc> EOF
42

It's also a desk calculator, like expr, but with float values:

% python -m celpy -n '355.0 / 113.0'
3.1415929203539825

It's not as sophistcated as bc. But, yes, this has a tiny advantage over python -c '355/113'. Most notably, the ability to embed Google CEL into other contexts where you don't really want Python's power.

It's also capable of decision-making, like test:

% echo '{"status": 3}' | python -m celpy -sb '.status == 0'
false
% echo $?
1

We can provide a -a option to define objects with specific data types. This is particularly helpful for providing protobuf message definitions.

python -m celpy -n --arg x:int=6 --arg y:int=7 'x*y'
42

If you want to see details of evaluation, use -v.

python -m celpy -v -n '[2, 4, 6].map(n, n/2)'
... a lot of output
[1, 2, 3]

Library

To follow the pattern defined in the Go implementation, there's a multi-step process for compiling a CEL expression to create a runnable "program". This program can then be applied to argument values.

>>> import celpy
>>> cel_source = """
... account.balance >= transaction.withdrawal
... || (account.overdraftProtection
... && account.overdraftLimit >= transaction.withdrawal - account.balance)
... """

>>> env = celpy.Environment()
>>> ast = env.compile(cel_source)
>>> prgm = env.program(ast)

>>> activation = {
...     "account": celpy.json_to_cel({"balance": 500, "overdraftProtection": False}),
...     "transaction": celpy.json_to_cel({"withdrawal": 600})
... }
>>> result = prgm.evaluate(activation)
>>> result
BoolType(False)

The Python classes are generally based on the object model in https://github.com/google/cel-go These types semantics are slightly different from Python's native semantics. Type coercion is not generally done. Python // truncates toward negative infinity. Go (and CEL) / truncates toward zero.

Development

The parser is based on the grammars used by Go and C++, but processed through Python Lark.

See https://github.com/google/cel-spec/blob/master/doc/langdef.md

https://github.com/google/cel-cpp/blob/master/parser/Cel.g4

https://github.com/google/cel-go/blob/master/parser/gen/CEL.g4

Notes

CEL provides a number of runtime errors that are mapped to Python exceptions.

no_matching_overload: this function has no overload for the types of the arguments.
no_such_field: a map or message does not contain the desired field.
return error for overflow: integer arithmetic overflows

There are mapped to Python celpy.evaluation.EvalError exception. The args will have a message similar to the CEL error message, as well as an underlying Python exception.

In principle CEL can pre-check types. However, see https://github.com/google/cel-spec/blob/master/doc/langdef.md#gradual-type-checking. Rather than try to pre-check types, we'll rely on Python's implementation.

Contributing

See https://cloudcustodian.io/docs/contribute.html

Code of Conduct

This project adheres to the Open Code of Conduct. By participating, you are expected to honor this code.

ddn0/cel-python