fix(errors): emit $exception_list in canonical order (caught-first, root cause last)

[cat-ph]

Problem

posthog-python is the only SDK that emits $exception_list with the root cause first. Every other SDK with exception-chain support puts the caught/outermost exception at index 0. exceptions_from_error_tuple() walked the chain in caught-first order and then called .reverse(), flipping it to root-cause-first.

This is the Python side of the cross-SDK exception-ordering standardization: PostHog/sdk-specs#11.

Canonical order: $exception_list[0] = the caught/outermost exception; each cause is appended after its wrapper in unwrap order; the root cause is last.

Change

• Removed the .reverse() in exceptions_from_error_tuple() (posthog/exception_utils.py). Both build paths already produce the list in canonical order:
  • plain chained exceptions: walk_exception_chain yields caught-first, following __cause__ / __context__;
  • ExceptionGroup: exceptions_from_error keeps the group (parent) before its contained exceptions.
• Added an explanatory comment documenting the canonical order and why we do not reverse.
• Frame order within each stacktrace is untouched (already bottom-up / canonical).

No code in the SDK or the Django integration derives values from $exception_list positions (e.g. no issue-name/type/message picking off [0]), so there is no downstream code change needed in this repo.

Tests

Added regression tests in posthog/test/test_exception_capture.py:

• explicit cause (raise B from A) -> asserts [0] is the caught B and the last entry is root cause A;
• implicit context chain (exception raised while handling another) -> same ordering;
• ExceptionGroup (3.11+) -> asserts the group is first, members follow.

uv run pytest posthog/test/test_exception_capture.py posthog/test/test_client.py -> 145 passed (re-run after rebase onto current main). Ruff lint + format clean.

Coordination

BREAKING wire-order change. The ordering of $exception_list on the wire flips. Merge/release only after the pipeline normalization gate (cymbal) is live, so the pipeline can normalize incoming order based on $lib_version. Shipping this as a MINOR release (changeset included) so the pipeline can gate on the version. This flip should be paired with fingerprint aliasing on the pipeline side, since exception/frame fingerprints are order-sensitive.

User-visible effects

Anything downstream that reads $exception_list[0] (issue fingerprinting / naming, primary exception type + message) will now see the caught/outermost exception instead of the root cause. That is the intended, canonical behavior and matches the other SDKs — but it is a visible change for existing Python-sourced issues.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "chore: use pypi/posthog changeset key" | Re-trigger Greptile}

[github-actions]

posthog-python Compliance Report

Date: 2026-07-24 16:06:57 UTC
Duration: 338862ms

✅ 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	✅	1009ms
Required Headers.Has Authorization Bearer Header	✅	1009ms
Required Headers.Has Content Type Json	✅	1008ms
Required Headers.Has Posthog Sdk Info Format	✅	1009ms
Required Headers.Has Posthog Attempt Header	✅	1009ms
Required Headers.Has Posthog Request Id	✅	1009ms
Required Headers.Has Posthog Request Timestamp	✅	1009ms
Required Headers.Has User Agent	✅	1009ms
Body Format.Body Has Created At And Batch	✅	1009ms
Body Format.No Api Key In Body	✅	1008ms
Body Format.No Sent At In Body	✅	1009ms
Event Format.Event Has Required Root Fields	✅	1009ms
Event Format.Event Uuid Is Valid	✅	1009ms
Event Format.Event Timestamp Is Rfc3339	✅	1009ms
Event Format.Distinct Id Is String	✅	1008ms
Event Format.Distinct Id At Root Not Properties	✅	1009ms
Event Format.Custom Properties Preserved	✅	1009ms
Event Format.Set Properties Preserved	✅	1009ms
Event Format.Set Once Properties Preserved	✅	1009ms
Event Format.Groups Properties Preserved	✅	1009ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1008ms
Event Format.Event Has Required Root Fields Batch	✅	1013ms
Event Format.Event Uuid Is Valid Batch	✅	1013ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	1012ms
Event Format.Distinct Id Is String Batch	✅	1013ms
Event Format.Distinct Id At Root Not Properties Batch	✅	1012ms
Event Format.Custom Properties Preserved Batch	✅	1012ms
Event Format.Set Properties Preserved Batch	✅	1012ms
Event Format.Set Once Properties Preserved Batch	✅	1012ms
Event Format.Groups Properties Preserved Batch	✅	1012ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1012ms
Batch Behavior.Multiple Events In Single Batch	✅	1507ms
Batch Behavior.Batch Envelope Smoke	✅	1014ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1006ms
Batch Behavior.Flush At Triggers Batch	✅	1508ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1011ms
Deduplication.Generates Unique Uuids	✅	1507ms
Deduplication.Different Events Same Content Different Uuids	✅	1507ms
Deduplication.Preserves Uuid On Retry	✅	7513ms
Deduplication.Preserves Timestamp On Retry	✅	7511ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7517ms
Deduplication.No Duplicate Events In Batch	✅	1504ms
Header Behavior On Retry.Attempt Header Starts At One	✅	1008ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	14517ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7512ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3520ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7510ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1011ms
Response Format Validation.Success Response Has Ok For Each Event	✅	1507ms
Response Format Validation.Success No Retry After When All Ok	✅	1507ms
Response Format Validation.Success Retry After Present When Retry Events	✅	2511ms
Response Format Validation.Success No Retry After When Drop Only	✅	1507ms
Response Format Validation.Response Echoes Request Id	✅	1010ms
Retry Behavior.Retries On 408	✅	7511ms
Retry Behavior.Retries On 500	✅	7518ms
Retry Behavior.Retries On 503	✅	9515ms
Retry Behavior.Retries On 504	✅	7509ms
Retry Behavior.Retryable Errors Have Retry After	✅	4514ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12517ms
Retry Behavior.Does Not Retry On 400	✅	3504ms
Retry Behavior.Does Not Retry On 401	✅	3508ms
Retry Behavior.Does Not Retry On 402	✅	3509ms
Retry Behavior.Does Not Retry On 413	✅	3508ms
Retry Behavior.Does Not Retry On 415	✅	3508ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3508ms
Retry Behavior.Implements Backoff	✅	23523ms
Retry Behavior.Max Retries Respected	✅	23531ms
Partial Batch Handling.Handles 200 Full Success	✅	3002ms
Partial Batch Handling.Handles 200 With All Ok	✅	4509ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	4509ms
Partial Batch Handling.Does Not Retry Limited Events	✅	4511ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	7514ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	7513ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7511ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7510ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7514ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7516ms
Partial Batch Handling.Respects Retry After On Partial	✅	9511ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4507ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4507ms
Compression.Sends Gzip Content Encoding	✅	1008ms
Compression.No Content Encoding When Disabled	✅	1010ms
Compression.Compressed Body Is Decompressible	✅	1008ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3509ms
Event Options.Cookieless Mode Override	✅	1008ms
Event Options.Disable Skew Correction Override	✅	1009ms
Event Options.Process Person Profile Override	✅	1008ms
Event Options.Product Tour Id Override	✅	1009ms
Event Options.Unset Options Omitted	✅	1009ms
Event Options.Options Override In Batch	✅	1012ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	1009ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	1008ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	1008ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	1006ms
Request Payload.Flags Request Uses V2 Query Param	✅	1008ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	1007ms
Request Payload.Flags Request Omits Authorization Header	✅	1007ms
Request Payload.Token In Flags Body Matches Init	✅	1007ms
Request Payload.Groups Round Trip	✅	1007ms
Request Payload.Groups Default To Empty Object	✅	1007ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	1007ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	1006ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	1006ms
Request Lifecycle.No Flags Request On Init Alone	✅	503ms
Request Lifecycle.No Flags Request On Normal Capture	✅	1507ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1011ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1002ms
Retry Behavior.Retries Flags On 502	✅	1007ms
Retry Behavior.Retries Flags On 504	✅	1007ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1509ms

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