Implement python mojo bindings unions.

mojo/public/python/mojo_bindings/reflection.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """The metaclasses used by the mojo python bindings."""
 
 import itertools
 
 # pylint: disable=F0401
 import mojo_bindings.serialization as serialization
@@ skipping 117 matching lines @@
 
   # Prevent adding new attributes, or mutating constants.
   def __setattr__(cls, key, value):
     raise AttributeError('can\'t set attribute')
 
   # Prevent deleting constants.
   def __delattr__(cls, key):
     raise AttributeError('can\'t delete attribute')
 
 
+class MojoUnionType(type):
+
+  def __new__(mcs, name, bases, dictionary):
+    dictionary['__slots__'] = ('_fields', '_tag', '_data')

[qsr, 2015/07/17 10:53:58]

Instead of keeping the _tag, you can keep a reference to the current field. This
will allows you to no keep tag_names in the closure.

[azani, 2015/07/17 20:38:15]

Done.

+    descriptor = dictionary.pop('DESCRIPTOR', {})
+
+    fields = descriptor.get('fields', [])
+    tag_names = {field.index: field.name for field in fields}
+    tags = {field.name: field.index for field in fields}
+    def _BuildUnionProperty(field):
+
+      # pylint: disable=W0212
+      def Get(self):
+        if self._tag != tags[field.name]:
+          raise AttributeError('%s is not currently set' % field.name,
+              field.name, tag_names[self._tag])

[qsr, 2015/07/17 10:53:58]

What do the 2nd and 3rd arguments do?

[azani, 2015/07/17 20:38:15]

All the arguments are printed when you print the AttributeError instance and
they are available as a list if you look at the instance's "args" field.

+        return self._data
+
+      # pylint: disable=W0212
+      def Set(self, value):
+        self._tag = tags[field.name]

[qsr, 2015/07/17 10:53:58]

Why is tags[field.name] different than field.index?

[azani, 2015/07/17 20:38:15]

Done.

+        self._data = field.field_type.Convert(value)
+
+      return property(Get, Set)
+
+    for field in fields:
+      dictionary[field.name] = _BuildUnionProperty(field)
+
+    def UnionInit(self, **kwargs):
+      items = kwargs.items()
+      if len(items) == 0:
+        return
+
+      if len(items) > 1:
+        raise TypeError('only 1 member may be set on a union.')
+
+      setattr(self, items[0][0], items[0][1])
+    dictionary['__init__'] = UnionInit
+
+    serializer = serialization.UnionSerializer(fields)
+    def SerializeUnionInline(self, handle_offset=0):
+      return serializer.SerializeInline(self, handle_offset)
+    dictionary['SerializeInline'] = SerializeUnionInline
+
+    def SerializeUnion(self, handle_offset=0):
+      return serializer.Serialize(self, handle_offset)
+    dictionary['Serialize'] = SerializeUnion
+
+    def DeserializeUnion(cls, context):
+      return serializer.Deserialize(context, cls)
+    dictionary['Deserialize'] = classmethod(DeserializeUnion)
+
+    class Tags(object):
+      __metaclass__ = MojoEnumType
+      VALUES = [(field.name, field.index) for field in fields]
+    dictionary['Tags'] = Tags
+
+    def GetTag(self):
+      return self._tag
+    dictionary['tag'] = property(GetTag, None)
+
+    def GetData(self):
+      return self._data
+    dictionary['data'] = property(GetData, None)
+
+    def UnionEq(self, other):
+      return (
+          (type(self) is type(other))
+          and (self.tag == other.tag)
+          and (self.data == other.data))
+    dictionary['__eq__'] = UnionEq
+
+    def UnionNe(self, other):
+      return not self.__eq__(other)
+    dictionary['__ne__'] = UnionNe
+
+    def UnionStr(self):
+      return '<%s.%s(%s): %s>' % (
+          self.__class__.__name__,
+          tag_names[self.tag],
+          self.tag,
+          self.data)
+    dictionary['__str__'] = UnionStr
+    dictionary['__repr__'] = UnionStr
+
+    return type.__new__(mcs, name, bases, dictionary)
+
+
 class InterfaceRequest(object):
   """
   An interface request allows to send a request for an interface to a remote
   object and start using it immediately.
   """
 
   def __init__(self, handle):
     self._handle = handle
 
   def IsPending(self):
@@ skipping 56 matching lines @@
     if type(self) is not type(other):
       return False
     for field in fields:
       if getattr(self, field.name) != getattr(other, field.name):
         return False
     return True
   return _Eq
 
 def _StructNe(self, other):
   return not self.__eq__(other)