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Declarative Validation of XMLRPC Responses in Python

From: "andrew cooke" <andrew@...>

Date: Thu, 23 Oct 2008 09:17:10 -0300 (CLST)

This is a fairly nice bit of code I wrote yesterday.  I wanted to validate
XMLRPC responses against a specification.  What I finally ended up with
was code like:

GET_VALUES = Array(Struct(ObjectName=Type(basestring),
                          ObjectClass=Type(basestring))
                   .either(ParameterList=Array(Struct(
                                ParameterName=Type(basestring),
                                ParameterClass=Type(basestring),
                                ParameterValue=Type(basestring),
                                ParameterTime=Optional(Type(int)))))
                   .or_(ParameterName=Type(basestring),
                        ParameterClass=Type(basestring),
                        ParameterValue=Type(basestring),
                        ParameterTime=Optional(Type(int))))
GET_VALUES.validate(client.GetValues(...))

And this would check the response from the client to make sure that it
matched that give above (it checks that: all fields received are in the
spec; all non-optional fields are present; maps and lists and values have
the correct types; the general nested structure is correct).

Note that "ObjectName" and the like are the names used in the XML
response.  They are specified in the declaration above (and are not
hard-coded below).

For Python's standard XMLRPC libraries, "structs" are returned as dicts
and "arrays" are lists.  The code is below.  Apart from the either/or,
which is a bit specific to this particular application, it should be
useful in general.

Andrew

def limit(obj):
  '''
  Reduce text size.
  '''
  text = str(obj)
  if len(text) > 100:
    text = text[0:100] + '...'
  return text

class Type(object):
  '''
  Specification for a simple value.
  '''

  def __init__(self, type):
    self.__type = type

  def validate(self, value):
    '''
    Check type of value.
    '''
    if not isinstance(value, self.__type):
      raise Exception('%s not of type %s' %
                      (limit(value), self.__type))

class Optional(object):
  '''
  Specification for verifying an optional value - does
  nothing, but is detected by Struct.
  '''

  def __init__(self, delegate):
    self.__delegate = delegate

  def validate(self, value):
    '''
    Pass on to delegate.
    '''
    self.__delegate.validate(value)

class Array(Type):
  '''
  Specification for verifying an array of values.
  '''

  def __init__(self, value):
    super(Array, self).__init__(list)
    self.__value = value

  def validate(self, value):
    '''
    Pass each value to delegate.
    '''
    super(Array, self).validate(value)
    for entry in value:
      self.__value.validate(entry)

class Struct(Type):
  '''
  Specification for verifying a struct of values.
  '''

  def __init__(self, **map):
    super(Struct, self).__init__(dict)
    self.__map = map
    self.__either = {}
    self.__or = {}
    self.__choice = None

  def either(self, **choice):
    '''
    Add first of a choice of two maps (chained)
    '''
    self.__either = choice
    return self

  def or_(self, **choice):
    '''
    Add second a choice of two maps
    '''
    self.__or = choice
    return self

  def validate(self, value):
    '''
    Check all entries.
    '''
    super(Struct, self).validate(value)
    self.__choice = None
    for name in value:
      if not self.__validate_choice(name, value[name]):
        if name not in self.__map:
          raise Exception('%s has unexpected content %s' %
                  (limit(value), name))
        else:
          self.__map[name].validate(value[name])
    for name in self.__map:
      if name not in value and \
          not isinstance(self.__map[name], Optional):
        raise Exception('%s is missing %s' %
                        (limit(value), name))
    if self.__choice is None and \
        (self.__either or self.__or):
      raise Exception('%s did not select either choice %s/%s'
              % (limit(value),
                 self.__either.keys(), self.__or.keys()))
    if self.__choice:
      for name in self.__choice:
        if name not in value and \
            not isinstance(self.__choice[name], Optional):
          raise Exception('%s is missing %s' %
                          (limit(value), name))

  def __validate_choice(self, name, value):
    '''
    Check choices
    '''
    # resolve choice once
    if self.__choice is None:
      if name in self.__either:
        self.__choice = self.__either
      elif name in self.__or:
        self.__choice = self.__or
    # check against resolved choice
    if self.__choice is not None:
      if name in self.__choice:
        self.__choice[name].validate(value)
        return True
    return False

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