[FSSDK-11571] Python: Add holdout support and refactor decision logic in DefaultDecisionService

[esrakartalOpt]

Summary

• Added holdout support for decision service

Test plan

PR checks

Issues

• FSSDK-11571

[Mat001]

@esrakartalOpt
I get a new issue in this PR, this time in optimizely.py. Check this:

Problem: For holdout decisions, variation is a dict, not an entities.Variation object.
The code accesses .key attribute which will raise AttributeError.

LINE 1234-1238 - CURRENT CODE (WRONG for holdouts)

variation_key = (
flag_decision.variation.key # ❌ Assumes variation is an object
if flag_decision is not None and flag_decision.variation is not None
else None
)

Why This Fails:

• For regular experiments/rollouts: variation is entities.Variation with .key
  attribute ✅
• For holdouts: variation is a dict with ['key'] accessor ❌
• Accessing .key on a dict raises AttributeError

Proof:

• Bucketer returns dict for holdouts (bucketer.py:147): variation = next((v for v in
  variations if v.get('id') == variation_id), None)
• This dict variation is stored in Decision (decision_service.py:830):
  variation=variation

Similar Code that properly handles this (lines 1250-1253):
try:
if flag_decision.variation is not None:
variation_id = flag_decision.variation.id # Will fail for dict
except AttributeError:
self.logger.warning("flag_decision.variation has no attribute 'id'")

Required Fix:

Option 1: Handle both dict and object

try:
if flag_decision.variation is not None:
variation_key = (
flag_decision.variation['key']
if isinstance(flag_decision.variation, dict)
else flag_decision.variation.key
)
else:
variation_key = None
except (AttributeError, KeyError, TypeError):
self.logger.warning("Unable to extract variation key")
variation_key = None

Option 2: Add check for holdout source

if flag_decision.source == enums.DecisionSources.HOLDOUT:
variation_key = flag_decision.variation['key'] if flag_decision.variation else
None
else:
variation_key = flag_decision.variation.key if flag_decision.variation else None

[Mat001]

Looks good now :-)