Source code for runway.cfngin.lookups.handlers.default

"""Lookup to provide a default value."""

from __future__ import annotations

from typing import TYPE_CHECKING, Any, ClassVar

from ....lookups.handlers.base import LookupHandler

if TYPE_CHECKING:
    from ....context import CfnginContext




class DefaultLookup(LookupHandler["CfnginContext"]):
    """Lookup to provide a default value."""

    TYPE_NAME: ClassVar[str] = "default"
    """Name that the Lookup is registered as."""



    @classmethod
    def handle(cls, value: str, context: CfnginContext | None = None, **_: Any) -> Any:
        """Use a value from the environment or fall back to a default value.

        Allows defaults to be set at the config file level.

        Args:
            value: Parameter(s) given to this lookup. ``<env_var>::<default value>``
            context: Context instance.

        Example:
            ::

                Groups: ${default app_security_groups::sg-12345,sg-67890}

            If ``app_security_groups`` is defined in the environment, its
            defined value will be returned. Otherwise, ``sg-12345,sg-67890``
            will be the returned value.

        """
        try:
            env_var_name, default_val = value.split("::", 1)
        except ValueError:
            raise ValueError(
                f"Invalid value for default: {value}. Must be in <env_var>::<default value> format."
            ) from None

        if context and env_var_name in context.parameters:
            return context.parameters[env_var_name]
        return default_val