cedarpy 4.8.4

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Cedar Python

 

cedarpy helps you use the (Rust) Cedar Policy library from Python. You can use cedarpy to:

• check whether a request is authorized by the Cedar Policy engine
• validate policies against a schema
• format policies

cedarpy releases correspond to the following Cedar Policy engine versions:

Cedar Policy (engine) release	cedarpy release	cedarpy branch
v4.8.2	v4.8.4	main
v4.7.2	v4.7.1	release/4.7.x
v4.1.0	v4.1.0	release/4.1.x
v2.2.0	v0.4.1	release/2.2.x

Beginning with v4.1.0, cedarpy's version number indicates the Cedar Policy engine major and minor version that it is based on. cedarpy increases the patch number when releasing backwards-compatible changes and bug fixes. So the cedarpy and Cedar Engine patch versions can and will diverge. Select the cedarpy version that provides the Cedar Policy language and engine features you need.

cedarpy packages are available for the following platforms:

Operating System	Processor Architectures	Python
Linux	x86_64, aarch64	3.9 - 3.14
Mac	x86_64, aarch64	3.11 - 3.14
Windows	x86_64	3.9 - 3.14

Note: This project is not officially supported by AWS or the Cedar Policy team.

Using the library

Releases of cedarpy are available on PyPi. You can install the latest release with:

pip install cedarpy

(See the Developing section for how to use artifacts you've built locally.)

Authorizing access with Cedar policies in Python

Now you can use the library to authorize access with Cedar from your Python project using the is_authorized function. Here's an example of basic use:

from cedarpy import is_authorized, AuthzResult, Decision

policies: str = "//a string containing cedar policies"
entities: list = [  # a list of Cedar entities; can also be a json-formatted string of Cedar entities
    {"uid": {"__entity": { "type" : "User", "id" : "alice" }}, "attrs": {}, "parents": []}
    # ...
]
request = {
    "principal": 'User::"bob"',
    "action": 'Action::"view"',
    "resource": 'Photo::"1234-abcd"',
    "context": {}
}

authz_result: AuthzResult = is_authorized(request, policies, entities)

# so you can assert on the decision like:
assert Decision.Allow == authz_result.decision

# or use the 'allowed' convenience method 
assert authz_result.allowed

# or even via AuthzResult's attribute subscripting support 
assert authz_result['allowed']

The AuthzResult class also provides diagnostics and metrics for the access evaluation request.

See the unit tests for more examples of use and expected behavior.

Authorize a batch of requests

You can also authorize a batch of requests with the is_authorized_batch function. is_authorized_batch accepts a list of requests to evaluate against shared policies, entities, and schema.

Batch authorization is often much more efficient (+10x) than processing authorization requests one by one with is_authorized. This is because the most expensive part of the authorization process is transforming the policies, entities, and schema into objects that Cedar can evaluate. See RFC: support batch authorization requests for details.

Here's an example of how to use is_authorized_batch and the optional request-result correlation_id:

batch_id:str = randomstr()
requests: List[dict] = []
for action_name in action_names:
    requests.append({
        "principal": f'User::"{user_id}"',
        "action": f'Action::"{action_name}"',
        "resource": f'Resource::"{resource_id}"',
        "context": context_keys,
        "correlation_id": f"authz_req::{batch_id}-{action_name}"
    })

# ... resolve get policies, entities, schema ...

# process authorizations in batch
authz_results: List[AuthzResult] = is_authorized_batch(requests=requests, policies=policies, entities=entities, schema=schema)

# ... verify results came back in correct order via correlation_id ...
for request, result, in zip(requests, authz_results):
    assert request.get('correlation_id') == result.correlation_id

cedar-py returns the list of AuthzResult objects in the same order as the list of requests provided in the batch.

The above example also supplies an optional correlation_id in the request so that you can verify results are returned in the correct order or otherwise map a request to a result.

Partially authorizing a request with unknowns

Sometimes you can't fully evaluate a request up front. A resource entity may not be loaded from the database yet, the caller may not have picked a resource, or context may be filled in by a downstream service. is_authorized_partial evaluates whatever is known and returns residual policies for the parts that aren't.

is_authorized_partial returns either:

• Decision.Allow or Decision.Deny when the unknowns can't change the outcome, or
• Decision.NoDecision plus residual policies for the caller to re-evaluate once the unknowns are bound.

from cedarpy import is_authorized_partial, PartialAuthzResult, Decision

# Allow access to all principals when the resource is public
policies: str = 'permit(principal, action, resource) when { resource.public == true };'

# Resource entity hasn't been loaded from the database yet
entities: list = []

request = {
    "principal": 'User::"alice"',
    "action": 'Action::"view"',
    "resource": 'Photo::"photo1"',
    "context": {},
}

result: PartialAuthzResult = is_authorized_partial(request, policies, entities)

# Cedar can't decide yet: the policy depends on resource.public,
# but Photo::"photo1" hasn't been loaded.
assert result.decision == Decision.NoDecision

# Which entities Cedar needs you to load before it can decide:
assert result.diagnostics.unknown_entities == ['Photo::"photo1"']

# Which policies are still unresolved, identified by the auto-generated policy id:
assert result.diagnostics.nontrivial_residuals == ['policy0']

Now you can load the unknown entities and re-evaluate access with is_authorized to get a final decision:

from cedarpy import is_authorized

# Load Photo::"photo1" from the database and transform to its Cedar entity representation
entities = [
    {"uid": {"__entity": {"type": "Photo", "id": "photo1"}},
     "attrs": {"public": True}, "parents": []}
]

final_result = is_authorized(request, policies, entities) # new entities; same request and policies
assert final_result.decision == Decision.Allow

Unlike is_authorized, an absent or None context is treated as an unknown rather than empty. Pass context={} for an explicitly empty context.

Note: A partial-eval result is not a final authorization decision. Schema type-checking, particularly the per-action context type, is skipped while fields remain unknown. See the is_authorized_partial docstring for the full caveats.

See tests/unit/test_authorize_partial.py for usage details, including binding unknowns and re-running is_authorized once the request is complete.

Validating policies against a schema

You can use validate_policies to validate Cedar policies against a schema before deploying them. Validation catches common mistakes like typos in entity types, invalid actions, type mismatches, and unsafe access to optional attributes—errors that would otherwise cause policies to silently fail at runtime.

This is particularly useful in CI/CD pipelines to catch policy errors before they reach production. See the Cedar validation documentation for details on what the validator checks.

Here's an example of basic use:

from cedarpy import validate_policies, ValidationResult

policies: str = "// a string containing Cedar policies"
schema: str = "// a Cedar schema as JSON string, Cedar schema string, or Python dict"

result: ValidationResult = validate_policies(policies, schema)

# so you can check validation passed like:
assert result.validation_passed

# or use ValidationResult in a boolean context
assert result  # True if validation passed

# and if validation fails, iterate over errors:
for error in result.errors:
    print(f"error: {error}")

The ValidationResult class provides the validation outcome and a list of ValidationError objects when validation fails.

See the unit tests for more examples of use and expected behavior.

Formatting Cedar policies

You can use format_policies to pretty-print Cedar policies according to convention.

from cedarpy import format_policies

policies: str = """
    permit(
        principal,
        action == Action::"edit",
        resource
    )
    when {
        resource.owner == principal
    };
"""

print(format_policies(policies))
# permit (
#   principal,
#   action == Action::"edit",
#   resource
# )
# when { resource.owner == principal };

Developing

You'll need a few things to get started:

• Python +3.9
• Rust and cargo

This project is built on the PyO3 and maturin projects. These projects are designed to enable Python to use Rust code and vice versa.

The most common development commands are in the Makefile

Create virtual env

First create a Python virtual environment for this project with: make venv-dev

In addition to creating a dedicated virtual environment, this will install cedar-py's dependencies.

If this works you should be able to run the following command:

maturin --help

Build and run cedar-py tests

Ensure the cedar-py virtual environment is active by sourcing it in your shell:

source venv-dev/bin/activate

Now run:

make quick

The make quick command will build the Rust source code with maturin and run the project's tests with pytest.

If all goes well, you should see output like:

(venv-dev) swedish-chef:cedar-py skuenzli$ make quick
Performing quick build
set -e ;\
	maturin develop ;\
	pytest
📦 Including license file "/path/to/cedar-py/LICENSE"
🔗 Found pyo3 bindings
🐍 Found CPython 3.9 at /path/to/cedar-py/venv-dev/bin/python
📡 Using build options features from pyproject.toml
Ignoring maturin: markers 'extra == "dev"' don't match your environment
Ignoring pip-tools: markers 'extra == "dev"' don't match your environment
Ignoring pytest: markers 'extra == "dev"' don't match your environment
💻 Using `MACOSX_DEPLOYMENT_TARGET=11.0` for aarch64-apple-darwin by default
   Compiling cedarpy v0.1.0 (/path/to/cedar-py)
    Finished dev [unoptimized + debuginfo] target(s) in 3.06s
📦 Built wheel for CPython 3.9 to /var/folders/k2/tnw8n1c54tv8nt4557pfx3440000gp/T/.tmpO6aj6c/cedarpy-0.1.0-cp39-cp39-macosx_11_0_arm64.whl
🛠 Installed cedarpy-0.1.0
================================================================================================ test session starts ================================================================================================
platform darwin -- Python 3.9.12, pytest-7.4.0, pluggy-1.2.0
rootdir: /path/to/cedar-py
configfile: pyproject.toml
testpaths: tests/unit
collected 10 items

tests/unit/test_authorize.py::AuthorizeTestCase::test_authorize_basic_ALLOW PASSED                                                                                                                            [ 10%]
tests/unit/test_authorize.py::AuthorizeTestCase::test_authorize_basic_DENY PASSED                                                                                                                             [ 20%]

... snip ... # a bunch of tests passing - please write more!
tests/unit/test_import_module.py::InvokeModuleTestFunctionTestCase::test_invoke_parse_test_policy PASSED                                                                                                      [100%]

================================================================================================ 10 passed in 0.51s =================================================================================================

Integration tests

This project supports validating correctness with official Cedar integration tests. To run those tests you'll need to retrieve the cedar-integration-tests data with:

make submodules

Then you can run:

make integration-tests

cedar-py currently passes 69 of the 74 tests defined in the example_use_cases, multi, ip, and decimal suites. The integration tests also validate policies against schemas when shouldValidate is set in the test definition. See test_cedar_integration_tests.py for details.

Using locally-built artifacts

If you used make quick above, then a development build of the cedarpy module will already be installed in the virtual environment.

If you want to use your local cedarpy changes in another Python environment, you'll need to build a release with:

make release

The release process will build a wheel and output it into target/wheels/

Now you can install that file with pip, e.g.:

pip install --force-reinstall /path/to/cedar-py/target/wheels/ccedarpy-*.whl

Contributing

This project is in its early stages and contributions are welcome. Please check the project's GitHub issues for work we've already identified.

Some ways to contribute are:

• Use the project and report experience and issues
• Document usage and limitations
• Enhance the library with additional functionality you need
• Add test cases, particularly those from cedar-integration-tests

You can reach people interested in this project in the #cedar-py channel of the Cedar Policy Slack workspace.