gflags_validators.py

#!/usr/bin/env python

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"""Module to enforce different constraints on flags.

A validator represents an invariant, enforced over a one or more flags.
See 'FLAGS VALIDATORS' in gflags.py's docstring for a usage manual.
"""

__author__ = 'olexiy@google.com (Olexiy Oryeshko)'


class Error(Exception):
  """Thrown If validator constraint is not satisfied."""


class Validator(object):
  """Base class for flags validators.

  Users should NOT overload these classes, and use gflags.Register...
  methods instead.
  """

  # Used to assign each validator an unique insertion_index
  validators_count = 0

  def __init__(self, checker, message):
    """Constructor to create all validators.

    Args:
      checker: function to verify the constraint.
        Input of this method varies, see SimpleValidator and
          DictionaryValidator for a detailed description.
      message: string, error message to be shown to the user
    """
    self.checker = checker
    self.message = message
    Validator.validators_count += 1
    # Used to assert validators in the order they were registered (CL/18694236)
    self.insertion_index = Validator.validators_count

  def Verify(self, flag_values):
    """Verify that constraint is satisfied.

    flags library calls this method to verify Validator's constraint.
    Args:
      flag_values: gflags.FlagValues, containing all flags
    Raises:
      Error: if constraint is not satisfied.
    """
    param = self._GetInputToCheckerFunction(flag_values)
    if not self.checker(param):
      raise Error(self.message)

  def GetFlagsNames(self):
    """Return the names of the flags checked by this validator.

    Returns:
      [string], names of the flags
    """
    raise NotImplementedError('This method should be overloaded')

  def PrintFlagsWithValues(self, flag_values):
    raise NotImplementedError('This method should be overloaded')

  def _GetInputToCheckerFunction(self, flag_values):
    """Given flag values, construct the input to be given to checker.

    Args:
      flag_values: gflags.FlagValues, containing all flags.
    Returns:
      Return type depends on the specific validator.
    """
    raise NotImplementedError('This method should be overloaded')


class SimpleValidator(Validator):
  """Validator behind RegisterValidator() method.

  Validates that a single flag passes its checker function. The checker function
  takes the flag value and returns True (if value looks fine) or, if flag value
  is not valid, either returns False or raises an Exception."""
  def __init__(self, flag_name, checker, message):
    """Constructor.

    Args:
      flag_name: string, name of the flag.
      checker: function to verify the validator.
        input  - value of the corresponding flag (string, boolean, etc).
        output - Boolean. Must return True if validator constraint is satisfied.
          If constraint is not satisfied, it should either return False or
          raise Error.
      message: string, error message to be shown to the user if validator's
        condition is not satisfied
    """
    super(SimpleValidator, self).__init__(checker, message)
    self.flag_name = flag_name

  def GetFlagsNames(self):
    return [self.flag_name]

  def PrintFlagsWithValues(self, flag_values):
    return 'flag --%s=%s' % (self.flag_name, flag_values[self.flag_name].value)

  def _GetInputToCheckerFunction(self, flag_values):
    """Given flag values, construct the input to be given to checker.

    Args:
      flag_values: gflags.FlagValues
    Returns:
      value of the corresponding flag.
    """
    return flag_values[self.flag_name].value


class DictionaryValidator(Validator):
  """Validator behind RegisterDictionaryValidator method.

  Validates that flag values pass their common checker function. The checker
  function takes flag values and returns True (if values look fine) or,
  if values are not valid, either returns False or raises an Exception.
  """
  def __init__(self, flag_names, checker, message):
    """Constructor.

    Args:
      flag_names: [string], containing names of the flags used by checker.
      checker: function to verify the validator.
        input  - dictionary, with keys() being flag_names, and value for each
          key being the value of the corresponding flag (string, boolean, etc).
        output - Boolean. Must return True if validator constraint is satisfied.
          If constraint is not satisfied, it should either return False or
          raise Error.
      message: string, error message to be shown to the user if validator's
        condition is not satisfied
    """
    super(DictionaryValidator, self).__init__(checker, message)
    self.flag_names = flag_names

  def _GetInputToCheckerFunction(self, flag_values):
    """Given flag values, construct the input to be given to checker.

    Args:
      flag_values: gflags.FlagValues
    Returns:
      dictionary, with keys() being self.lag_names, and value for each key
        being the value of the corresponding flag (string, boolean, etc).
    """
    return dict([key, flag_values[key].value] for key in self.flag_names)

  def PrintFlagsWithValues(self, flag_values):
    prefix = 'flags '
    flags_with_values = []
    for key in self.flag_names:
      flags_with_values.append('%s=%s' % (key, flag_values[key].value))
    return prefix + ', '.join(flags_with_values)

  def GetFlagsNames(self):
    return self.flag_names