Support None value for telemetry values.

tools/telemetry/telemetry/value/none_values.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.
+
+MERGE_FAILURE_REASON = "Unable to merge values with None value."

[chrishenry, 2014/08/15 04:43:20]

The merge is successful tho, so maybe "Merging values containing None value
results in a None value" (or a better phrasing of that)

[ariblue, 2014/08/15 22:10:26]

Done.

+
+class NoneValueMissingReason(Exception):
+  pass
+
+class ValueMustHaveNoneValue(Exception):
+  pass
+
+def ValidateValue(value, none_value_reason):

[nednguyen, 2014/08/15 15:50:04]

Maybe ValidateNoneAbleValue. 

[ariblue, 2014/08/15 22:10:26]

How about ValidateNullableValue?

+  if value is None and none_value_reason is None:
+    raise NoneValueMissingReason()
+  if value is not None and none_value_reason is not None:
+    raise ValueMustHaveNoneValue()

[chrishenry, 2014/08/15 04:43:20]

Should this just be part of __init__.py? Thoughts?

[nednguyen, 2014/08/15 15:50:04]

I find the situation of partly "adding the new field at leaves" + partly "adding
the validation at root" quite awkward.

[chrishenry, 2014/08/15 16:32:40]

Some raw thoughts from my end on this. I don't think we should pursue it in this
patch, but worth mulling over.

Maybe we can formalize the concept of value trait for this: NoneableValueTrait.
We can either make it a base class or a composable trait. Not sure if it's going
to look ok or not.

Something like this:

class NoneableValueTrait(ValueTrait):
  def __init__(self, value=None, none_value_reason=None):
    self._ValidateNoneTrait(value, none_value_reason)
    self._none_value_reason = none_value_reason

  @property
  def none_value_reason(): ...

  def FillDict(dict):
    base_obj['none_value_reason'] = self.none_value_reason

  @classmethod
  def ReviveFromDict(dict, value_class):
    if dict['none_value_reason'] and
value_class.IsTraitSupported(NoneValueTrait)
    return new NoneValueTrait(dict)

  def IsMergable(value): ...

  def MergeLikeValues(...): ?????? <- don't know how this looks like or how this
is called, maybe we can pair and discuss briefly later.

class Value(object):
  def __init__(..., traits=None):
    self._traits = traits if traits is not None else []

  def AsDict():
    base_dict = {...}
    for trait in self._traits:
      trait.FillDict(base_dict)

  etc...

[ariblue, 2014/08/15 22:10:26]

Acknowledged.