feat: add device_id to flags request payload

[andehen]

Add device_id parameter to all feature flag methods, similar to how
distinct_id is handled. The device_id is included in the flags request
payload sent to the server.

• Add device_id parameter to get_flags_decision and related methods
• Add context support via set_context_device_id() for automatic fallback
• Add tests for explicit device_id and context-based device_id

This is required for supporting device_id as bucketing identifier.

[greptile-apps]

Greptile Summary

Overview

This PR adds support for device_id as a bucketing identifier in feature flag evaluations. The implementation adds device_id parameter to feature flag methods and context support via set_context_device_id().

What Works Well

✅ Client methods: All Client class feature flag methods properly accept device_id parameter and pass it through to the flags API
✅ Context support: set_context_device_id() and get_context_device_id() are correctly implemented in the context system
✅ Fallback logic: Device ID properly falls back to context when not explicitly provided, matching the pattern used for distinct_id
✅ Request payload: The device_id is correctly included in the flags request payload sent to the server
✅ Test coverage: Tests verify explicit device_id passing and context-based device_id for Client methods

Critical Issue Found

❌ Missing device_id in module-level functions: All module-level feature flag functions in posthog/__init__.py are missing the device_id parameter:

• posthog.feature_enabled()
• posthog.get_feature_flag()
• posthog.get_all_flags()
• posthog.get_feature_flag_result()
• posthog.get_feature_flag_payload()
• posthog.get_all_flags_and_payloads()

While users can work around this by using set_context_device_id(), this creates an inconsistency: these functions require distinct_id as an explicit parameter (even though it can also come from context). For completeness and consistency with the stated goal of "adding device_id parameter to all feature flag methods", these functions should also accept an optional device_id parameter.

Recommendation

Add the optional device_id parameter to all module-level feature flag functions in posthog/__init__.py and pass it through to the underlying Client methods. This ensures API consistency and fully delivers on the PR's objective.

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_{1 file reviewed, 1 comment}

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[greptile-apps]

Additional Comments (1)

posthog/__init__.py
The feature_enabled() function is missing the device_id parameter. According to the PR description, "device_id parameter should be added to all feature flag methods."

While users can use set_context_device_id() to set device_id via context, this creates an inconsistency: the function requires distinct_id as an explicit parameter even though distinct_id can also come from context in Client methods. For consistency with the Client implementation and to fully support the stated goal, this function should accept an optional device_id parameter.

The same issue applies to:

• get_feature_flag() (line 551)
• get_all_flags() (line 601)
• get_feature_flag_result() (line 643)
• get_feature_flag_payload() (line 686)
• get_all_flags_and_payloads() (line 731)

All these module-level functions should be updated to accept an optional device_id parameter and pass it through to the underlying Client methods via _proxy().

Prompt To Fix With AI

This is a comment left during a code review.
Path: posthog/__init__.py
Line: 500:548

Comment:
The `feature_enabled()` function is missing the `device_id` parameter. According to the PR description, "device_id parameter should be added to all feature flag methods."

While users can use `set_context_device_id()` to set device_id via context, this creates an inconsistency: the function requires `distinct_id` as an explicit parameter even though `distinct_id` can also come from context in Client methods. For consistency with the Client implementation and to fully support the stated goal, this function should accept an optional `device_id` parameter.

The same issue applies to:
- `get_feature_flag()` (line 551)
- `get_all_flags()` (line 601)
- `get_feature_flag_result()` (line 643)
- `get_feature_flag_payload()` (line 686)
- `get_all_flags_and_payloads()` (line 731)

All these module-level functions should be updated to accept an optional `device_id` parameter and pass it through to the underlying Client methods via `_proxy()`.

How can I resolve this? If you propose a fix, please make it concise.

[haacked]

This looks good!

[greptile-apps]

_{6 files reviewed, 1 comment}

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