deb-mistral/mistral/expressions.py

# -*- coding: utf-8 -*-
#
# Copyright 2013 - Mirantis, Inc.
#
#    Licensed under the Apache License, Version 2.0 (the "License");
#    you may not use this file except in compliance with the License.
#    You may obtain a copy of the License at
#
#        http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS,
#    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
#    See the License for the specific language governing permissions and
#    limitations under the License.

import abc
import copy
import re
import six
import yaql

from mistral.openstack.common import log as logging
from mistral import yaql_utils

LOG = logging.getLogger(__name__)


class Evaluator(object):
    """Expression evaluator interface.

    Having this interface gives the flexibility to change the actual expression
    language used in Mistral DSL for conditions, output calculation etc.
    """

    @classmethod
    @abc.abstractmethod
    def evaluate(cls, expression, context):
        """Evaluates the expression against the given data context.

        :param expression: Expression string
        :param context: Data context
        :return: Expression result
        """
        pass

    @classmethod
    @abc.abstractmethod
    def is_expression(cls, expression):
        """Check expression string and decide whether it is expression or not.

        :param expression: Expression string
        :return: True if string is expression
        """
        pass


class YAQLEvaluator(Evaluator):
    @classmethod
    def evaluate(cls, expression, data_context):
        LOG.debug("Evaluating YAQL expression [expression='%s', context=%s]"
                  % (expression, data_context))

        result = yaql.parse(expression).evaluate(
            data=data_context,
            context=yaql_utils.create_yaql_context()
        )

        LOG.debug("YAQL expression result: %s" % result)

        return result

    @classmethod
    def is_expression(cls, s):
        # TODO(rakhmerov): It should be generalized since it may not be YAQL.
        return s and s.startswith('$')


class InlineYAQLEvaluator(YAQLEvaluator):
    # Put YAQL-specific regexp pattern here.
    # Use form {$.any_symbols_except'}'} to find an expression.
    find_expression_pattern = re.compile("\{\$\.*[^\}]*\}")

    @classmethod
    def evaluate(cls, expression, data_context):
        LOG.debug(
            "Evaluating inline YAQL expression [expression='%s', context=%s]"
            % (expression, data_context)
        )

        if super(InlineYAQLEvaluator, cls).is_expression(expression):
            return super(InlineYAQLEvaluator,
                         cls).evaluate(expression, data_context)

        result = expression
        found_expressions = cls.find_inline_expressions(expression)

        if found_expressions:
            for expr in found_expressions:
                trim_expr = expr.strip("{}")

                evaluated = super(InlineYAQLEvaluator, cls).evaluate(
                    trim_expr,
                    data_context
                )

                replacement = str(evaluated)
                result = result.replace(expr, replacement)

        LOG.debug("Inline YAQL expression result: %s" % result)

        return result

    @classmethod
    def is_expression(cls, s):
        return s

    @classmethod
    def find_inline_expressions(cls, s):
        return cls.find_expression_pattern.findall(s)


# TODO(rakhmerov): Make it configurable.
_EVALUATOR = InlineYAQLEvaluator


def evaluate(expression, context):
    # Check if the passed value is expression so we don't need to do this
    # every time on a caller side.
    if not isinstance(expression, six.string_types) or \
            not _EVALUATOR.is_expression(expression):
        return expression

    return _EVALUATOR.evaluate(expression, context)


def _evaluate_item(item, context):
    if isinstance(item, six.string_types):
        try:
            return evaluate(item, context)
        except AttributeError as e:
            LOG.debug("Expression %s is not evaluated, [context=%s]: %s"
                      % (item, context, e))
            return item
    else:
        return evaluate_recursively(item, context)


def evaluate_recursively(data, context):
    data = copy.copy(data)

    if not context:
        return data

    if isinstance(data, dict):
        for key in data:
            data[key] = _evaluate_item(data[key], context)
    elif isinstance(data, list):
        for index, item in enumerate(data):
            data[index] = _evaluate_item(item, context)
    elif isinstance(data, six.string_types):
        return _evaluate_item(data, context)

    return data