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Check

The Check class provides a fluent interface for making assertions about policy data. The object keeps track of accessed data within the component JSON and records that for traceability purposes. The final result of a check will have not just the status (pass, fail, pending, or error), but complete information about which JSON paths were used to reach this conclusion. Designed to be used as a context manager with Python's with statement, the Check class automatically handles NoDataError (turns it into pending status), and tracks result statuses.

Constructor

Check(name, description=None, node=None)

Creates a new check instance that can be used to make assertions about the component data. If the component data is not provided, it will be loaded automatically from the environment via LUNAR_BUNDLE_PATH.

  • name (str): A unique identifier for this check

  • description (str, optional): A human-readable description of what this check validates

  • node (Node, optional): An alternate Node instance to use for this check, instead of loading it from the environment. Useful for unit testing.

Context Manager

The Check class is designed to be used as a context manager with Python's with statement. This is the recommended way to use the class as it ensures proper setup, teardown, and error handling.

with Check("check-name", "Check description") as check:
    # Make assertions using check methods
    check.assert_true(check.get_value(".path.to.data"))

When used as a context manager, the Check class:

  1. On enter: Sets up the check context and automatically loads component data if not provided

  2. On exit: Records the check result with its status and all accessed data paths

  3. Exception handling:

    • Catches and suppresses NoDataError, and sets the check status to pending, if collectors are still running.

    • Otherwise, propagates NoDataError and records as error status since NoDataError is unexpected after collectors finished.

    • Propagates other exceptions and records them as error status with the exception message

JSONPath Compatibility

We do not support the entirety of JSONPath as defined in RFC 9535. However, we do support a strict subset of the language:

  • $ is implicit at the beginning of paths.

  • You can access members of an object by key using either .dot.syntax or ['literal']['syntax'].

  • You can index into an array by number only. No wildcards or filters allowed.

Data Access Methods

get_value

get_value(path=".")

Retrieves data from the component JSON using a JSONPath expression. This method raises ValueError if the path is invalid.

Missing data behavior:

  • Raises NoDataError before collectors finished (results in pending status).

  • Raises ValueError after collectors finished (results in error status).

This means that get_value is best used when you are assuming that the data will eventually be provided by a collector.

  • path (str): JSONPath expression to query the component data (default: "." for root)

  • Returns: The value at the specified path, or raises NoDataError or ValueError.

Example:

# Get the number of lines in the README
lines = check.get_value(".readme.lines")

# Get the entire component data
all_data = check.get_value()

get_value_or_default

get_value_or_default(path=".", default=None)

Retrieves data from the component JSON using a JSONPath expression. If the value is missing for any reason, it returns the specified default instead.

  • path (str): JSON path relative to the root of the component JSON (default: ".")

  • default: The value to return if there is no value at this path

Example:

# Given this component JSON:
# {
#     "existing": {
#         "value": "hello"
#     }
# }

val = node.get_value_or_default(".existing.value", "goodbye") # val will be "hello"
val = node.get_value_or_default(".missing", "goodbye") # val will be "goodbye"

get_all_values

get_all_values(path=".")

Retrieves data from the component JSON deltas using a JSONPath expression. Use this when you collect something at the same path multiple times to help assert on all values. This method raises ValueError if the path is invalid.

Missing data behavior:

  • Raises NoDataError before collectors finished (results in pending status).

  • Raises ValueError after collectors finished (results in error status).

This means that get_all_values is best used when you are assuming that the data will eventually be provided by a collector.

  • path (str): JSONPath expression to query the component data (default: "." for root)

  • Returns: A list of all values found at the specified path, or raises NoDataError or ValueError.

Example:

# Get the number of lines in the README
lines = check.get_all_values(".readme.lines")

get_node

get_node(path)

Gets a Node at the given path. Uses lazy evaluation - no data access or path tracking until value is needed.

  • path (str): JSONPath expression to query the component data

  • Returns: A Node instance at the specified path

  • Raises: ValueError if the path syntax is invalid

The returned Node object provides methods like get_value(), get_node(), exists(), and supports iteration. See the Node class documentation for complete details.

Example:

config_node = check.get_node(".config")
if config_node.exists():
    config = config_node.get_value()

exists

exists(path=".")

Returns True if the path exists in the component data.

Missing path behavior:

  • Raises NoDataError before collectors finished (results in pending status).

  • Returns False after collectors finished.

Node-like Iteration Methods

The Check class supports iteration methods that make it behave like a Node for duck-typing compatibility. These methods allow you to iterate over the root component data.

Iterating over Check Fields

for item in check:
    # Process item

Makes Check iterable like a Node. For dictionaries, yields keys. For arrays, yields Node objects.

  • For dict-like data: Yields string keys

  • For array-like data: Yields Node objects for each array element

  • Raises: ValueError if the component data is not a dict or array, NoDataError if data is not available yet

Example:

# Iterate over top-level keys in component data
with Check("iterate-check") as check:
    for key in check:
        print(f"Top-level key: {key}")

items

items()

Get key-value pairs when the Check points to dict-like component data.

  • Returns: Iterator of (key, Node) tuples for dict-like data

  • Raises: ValueError if the component data is not a dictionary, NoDataError if data is not available yet

Example:

with Check("items-check") as check:
    for key, value_node in check.items():
        value = value_node.get_value()
        print(f"{key}: {value}")

Assertion Methods

All assertion methods have these common parameters:

  • value: The value to be asserted

  • failure_message (str, optional): Custom message to display if the assertion fails

Additionally, all assertion methods raise NoDataError if the path doesn't exist in the component data.

assert_true

assert_true(value, failure_message=None)

Asserts that a value is True.

Example:

# Assert that authentication is required
check.assert_true(check.get_value(".api.auth_required"), "API should require authentication")

assert_false

assert_false(value, failure_message=None)

Asserts that a value is False.

Example:

# Assert that the README.md file is not missing
check.assert_false(check.get_value(".readme.missing"), "README.md file should exist")

assert_equals

assert_equals(value, expected, failure_message=None)

Asserts that a value equals the expected value.

  • expected: The expected value to compare against

Example:

# Assert that the API endpoint uses GET method
check.assert_equals(check.get_value(".api.endpoints[0].method"), "GET", "Endpoint should use GET method")

assert_exists

assert_exists(path, failure_message=None)

Asserts that a path exists in the component data. If the path was not found, this method raises NoDataError before collectors finished, and fails the check after collectors finished.

Missing path behavior:

  • Raises NoDataError before collectors finished (results in pending status).

  • Fails the check after collectors finished (results in FAIL status).

Example:

check.assert_exists(".api", "API data not found")

assert_contains

assert_contains(value, expected, failure_message=None)

Asserts that a value contains the expected value (works for strings, lists, etc.).

  • expected: The value that should be contained

Example:

# Assert that the endpoint path contains a specific substring
check.assert_contains(check.get_value(".api.endpoints[0].path"), "/users")

# Assert that the tags list contains a specific tag
check.assert_contains(check.get_value(".tags"), "api")

assert_greater

assert_greater(value, expected, failure_message=None)

Asserts that a numeric value is greater than the expected value.

  • expected: The threshold value to compare against

Example:

# Assert that the code coverage is greater than 80%
check.assert_greater(check.get_value(".coverage.percentage"), 80, "Code coverage should be greater than 80%")

assert_greater_or_equal

assert_greater_or_equal(value, expected, failure_message=None)

Asserts that a numeric value is greater than or equal to the expected value.

  • expected: The threshold value to compare against

Example:

# Assert that README has at least 50 lines
check.assert_greater_or_equal(check.get_value(".readme.lines"), 50, "README should have at least 50 lines")

assert_less

assert_less(value, expected, failure_message=None)

Asserts that a numeric value is less than the expected value.

  • expected: The threshold value to compare against

Example:

# Assert that cyclomatic complexity is less than 15
check.assert_less(check.get_value(".complexity.cyclomatic"), 15, "Cyclomatic complexity should be less than 15")

assert_less_or_equal

assert_less_or_equal(value, expected, failure_message=None)

Asserts that a numeric value is less than or equal to the expected value.

  • expected: The threshold value to compare against

Example:

# Assert that build time is at most 5 minutes
check.assert_less_or_equal(check.get_value(".build.duration_minutes"), 5, "Build should take at most 5 minutes")

assert_match

assert_match(value, pattern, failure_message=None)

Asserts that a string value matches a regular expression pattern.

  • pattern (str): A regular expression pattern to match against

Example:

# Assert that version follows semantic versioning
check.assert_match(check.get_value(".version"), r"^\d+\.\d+\.\d+$", "Version should follow semantic versioning")

fail

fail(message)

Unconditionally fails the check with a given message.

  • message (str): The message to display when the check fails

Example:

check.fail("This is a policy failure")

Instance Properties

After a check has been executed (typically after exiting the with context), the following properties are available:

status

status: CheckStatus

The final status of the check after execution.

  • Type: CheckStatus

  • Values: PASS, FAIL, PENDING, or ERROR

failure_reasons

failure_reasons: List[str]

The reasons for failure when the check status is FAIL. This property contains an array of detailed error messages from any failed assertions within the check.

  • Type: List[str]

  • Available when: status is CheckStatus.FAIL

name

name: str

The name of the check as specified in the constructor.

  • Type: str

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