feat(flags): add default_value to feature_enabled()

[posthog]

💡 Motivation and Context

Brings feature_enabled() into compliance with the cross-SDK is-feature-enabled contract in PostHog/sdk-specs: a hard SHALL, @both-tagged requirement that the SDK accept a caller-supplied boolean default and return it whenever the flag has no value (not loaded, failed request, or unknown key) — while any present value, including False and variant strings, still always wins over the default.

This was flagged in the sdk-specs compliance matrix (posthog-python.md#n10, "Is Feature Enabled — ❌ Fail"): feature_enabled() had no default_value parameter at all, unconditionally returning None on a miss with no way for a caller to force a True/False fallback. Its successor, FeatureFlagEvaluations.is_enabled(), already got a default_value parameter in 7.38.0 — this PR closes the identical gap on the legacy feature_enabled() API for parity.

Backwards compatibility

Purely additive — default_value is a new keyword-only parameter defaulting to None. Existing callers who don't pass it see byte-for-byte identical behavior (a miss still resolves to None, matching the spec's allowed variation for three-state boolean APIs). Callers can now opt in to a True/False fallback on a miss.

💚 How did you test it?

• Added two new tests to posthog/test/test_feature_flags.py: one confirming a caller-supplied default_value is returned on a missing flag or a failed /flags request, another confirming a flag with a real (even falsy) value still wins over the default.
• Ran the full test_feature_flags.py + test_evaluate_flags.py + test_client.py suites (414 tests) — all passing.
• mypy posthog/client.py — clean.
• ruff format — clean on touched files.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed. (docstring updated; no README signature to update)
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous — this PR was generated by a scheduled compliance-audit loop that reads the SDK compliance matrices in PostHog/sdk-specs and implements one well-scoped, backward-compatible gap per run. No human directed this specific change; left unassigned for the owning team to triage.

• Tooling: Claude Code (Sonnet 5), running an unattended daily loop over compliance/*.md in PostHog/sdk-specs.
• Selection: Of the open gaps across 12 SDKs, this one was chosen for being a hard spec SHALL (not just a nice-to-have), high-impact/widely-used (core flag-evaluation API), narrowly scoped (a single optional parameter), and purely additive — no existing open PR/branch addressed it.
• Scope decision: The audit note also mentioned a related-but-separate gap (no per-call tracking-suppression on is_enabled()/get_flag()); that was left out of scope here to keep this PR focused on the single contract (default_value).

Closes the posthog-python × Is Feature Enabled cell in the sdk-specs compliance matrix.

[github-actions]

posthog-python Compliance Report

Date: 2026-08-07 16:41:14 UTC
Duration: 256582ms

✅ All Tests Passed!

111/111 tests passed

---

Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	518ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	511ms
Required Headers.Has Authorization Bearer Header	✅	511ms
Required Headers.Has Content Type Json	✅	511ms
Required Headers.Has Posthog Sdk Info Format	✅	511ms
Required Headers.Has Posthog Attempt Header	✅	511ms
Required Headers.Has Posthog Request Id	✅	512ms
Required Headers.Has Posthog Request Timestamp	✅	511ms
Required Headers.Has User Agent	✅	512ms
Body Format.Body Has Created At And Batch	✅	511ms
Body Format.No Api Key In Body	✅	511ms
Body Format.No Sent At In Body	✅	512ms
Event Format.Event Has Required Root Fields	✅	512ms
Event Format.Event Uuid Is Valid	✅	511ms
Event Format.Event Timestamp Is Rfc3339	✅	512ms
Event Format.Distinct Id Is String	✅	511ms
Event Format.Distinct Id At Root Not Properties	✅	512ms
Event Format.Custom Properties Preserved	✅	511ms
Event Format.Set Properties Preserved	✅	511ms
Event Format.Set Once Properties Preserved	✅	511ms
Event Format.Groups Properties Preserved	✅	512ms
Event Format.Sdk Generates Uuid If Not Provided	✅	512ms
Event Format.Event Has Required Root Fields Batch	✅	515ms
Event Format.Event Uuid Is Valid Batch	✅	515ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	515ms
Event Format.Distinct Id Is String Batch	✅	515ms
Event Format.Distinct Id At Root Not Properties Batch	✅	514ms
Event Format.Custom Properties Preserved Batch	✅	514ms
Event Format.Set Properties Preserved Batch	✅	515ms
Event Format.Set Once Properties Preserved Batch	✅	514ms
Event Format.Groups Properties Preserved Batch	✅	514ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	515ms
Batch Behavior.Multiple Events In Single Batch	✅	520ms
Batch Behavior.Batch Envelope Smoke	✅	517ms
Batch Behavior.Flush With No Events Sends Nothing	✅	507ms
Batch Behavior.Flush At Triggers Batch	✅	1024ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	512ms
Deduplication.Generates Unique Uuids	✅	519ms
Deduplication.Different Events Same Content Different Uuids	✅	512ms
Deduplication.Preserves Uuid On Retry	✅	6517ms
Deduplication.Preserves Timestamp On Retry	✅	6522ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6524ms
Deduplication.No Duplicate Events In Batch	✅	524ms
Header Behavior On Retry.Attempt Header Starts At One	✅	511ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13530ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6522ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3021ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6517ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	512ms
Response Format Validation.Success Response Has Ok For Each Event	✅	515ms
Response Format Validation.Success No Retry After When All Ok	✅	513ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1518ms
Response Format Validation.Success No Retry After When Drop Only	✅	514ms
Response Format Validation.Response Echoes Request Id	✅	510ms
Retry Behavior.Retries On 408	✅	6517ms
Retry Behavior.Retries On 500	✅	6520ms
Retry Behavior.Retries On 503	✅	8522ms
Retry Behavior.Retries On 504	✅	6519ms
Retry Behavior.Retryable Errors Have Retry After	✅	3518ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11525ms
Retry Behavior.Does Not Retry On 400	✅	2515ms
Retry Behavior.Does Not Retry On 401	✅	2513ms
Retry Behavior.Does Not Retry On 402	✅	2515ms
Retry Behavior.Does Not Retry On 413	✅	2513ms
Retry Behavior.Does Not Retry On 415	✅	2514ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2516ms
Retry Behavior.Implements Backoff	✅	22538ms
Retry Behavior.Max Retries Respected	✅	22536ms
Partial Batch Handling.Handles 200 Full Success	✅	2513ms
Partial Batch Handling.Handles 200 With All Ok	✅	3518ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3517ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3516ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6519ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6523ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6523ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6519ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6522ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6519ms
Partial Batch Handling.Respects Retry After On Partial	✅	8521ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3517ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3521ms
Compression.Sends Gzip Content Encoding	✅	512ms
Compression.No Content Encoding When Disabled	✅	512ms
Compression.Compressed Body Is Decompressible	✅	512ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2514ms
Event Options.Cookieless Mode Override	✅	512ms
Event Options.Disable Skew Correction Override	✅	511ms
Event Options.Process Person Profile Override	✅	511ms
Event Options.Product Tour Id Override	✅	512ms
Event Options.Unset Options Omitted	✅	511ms
Event Options.Options Override In Batch	✅	516ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	510ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	512ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	511ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	12ms
Request Payload.Flags Request Uses V2 Query Param	✅	11ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	11ms
Request Payload.Flags Request Omits Authorization Header	✅	11ms
Request Payload.Token In Flags Body Matches Init	✅	12ms
Request Payload.Groups Round Trip	✅	12ms
Request Payload.Groups Default To Empty Object	✅	11ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	12ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	12ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	12ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	512ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	17ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	11ms
Retry Behavior.Retries Flags On 502	✅	315ms
Retry Behavior.Retries Flags On 504	✅	316ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	513ms