lib/json_lib.py

# Copyright 2015 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""Helpful functions when parsing JSON blobs."""

from __future__ import print_function

import json
import re

from chromite.lib import osutils


def AssertIsInstance(instance, expected_type, description):
  """Raise an error if |instance| is not of |expected_type|.

  Args:
    instance: instance of a Python object.
    expected_type: expected type of |instance|.
    description: short string describing |instance| used in error reporting.
  """
  if not isinstance(instance, expected_type):
    raise ValueError(
        'Expected %s to be a %s, but found %s' %
        (description, expected_type.__name__, instance.__class__.__name__))


def GetValueOfType(a_dict, key, value_type, value_description):
  """Raise an exception if we cannot get |key| from |a_dict| with |value_type|.

  Args:
    a_dict: a dictionary.
    key: string key that should be in the dictionary.
    value_type: expected type of the value at a_dict[key].
    value_description: string describing the value used in error reporting.
  """
  try:
    value = a_dict[key]
  except KeyError:
    raise ValueError('Missing %s in JSON dictionary (key "%s")' %
                     (value_description, key))
  AssertIsInstance(value, value_type, value_description)
  return value


def PopValueOfType(a_dict, key, value_type, value_description):
  """Raise an exception if we cannnot pop |key| from |a_dict| with |value_type|.

  Args:
    a_dict: a dictionary.
    key: string key that should be in the dictionary.
    value_type: expected type of the value at a_dict[key].
    value_description: string describing the value used in error reporting.
  """
  ret = GetValueOfType(a_dict, key, value_type, value_description)
  # We were able to get that value, so the key must exist.
  a_dict.pop(key)
  return ret


def ParseJsonFileWithComments(path):
  """Parse a JSON file with bash style comments.

  Strips out comments from JSON blobs.

  Args:
    path: path to JSON file.

  Returns:
    Python representation of contents of JSON file.
  """
  prog = re.compile(r'\s*#.*')
  lines = osutils.ReadFile(path).splitlines()
  lines = ['' if prog.match(line) else line for line in lines]
  parsed_contents = json.loads('\n'.join(lines))
  return parsed_contents