fix(flags): remove lower() when evaluating feature flag payloads – these payloads are case-sensitive!

[dmarticus]

Remove lower() when evaluating feature flags + feature flag payloads – feature flags are case-sensitive. Closes #178

[greptile-apps]

PR Summary

This PR addresses case sensitivity inconsistencies in PostHog's Python client feature flag handling, ensuring uniform case-insensitive flag key matching.

• Modified get_feature_flag() in /posthog/client.py to normalize flag keys to lowercase for consistent evaluation
• Added case-insensitive payload retrieval logic in _compute_payload_locally() to match flag evaluation behavior
• Added test cases in /posthog/test/test_feature_flags.py to verify mixed-case flag keys work correctly for both evaluation and payload retrieval
• Fixed issue Feature flag evaluation is case sensitive, while feature flag payload match evaluation is evaluated as lower() #178 where flag evaluation and payload retrieval had inconsistent case sensitivity handling

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[haacked]

Is that safe? Flag keys are case sensitive:

[haacked]

I looked at the linked issue #178:

It's unclear what the desired behavior is here--either feature flags should be all lower case, or the payload evaluation with lower() should be removed.

I think the payload evaluation with lower() probably should be removed. Otherwise we could introduce a bug where two flags with the same key but different casing will have a conflict where one will never be evaluated.

As an aside, I think allowing case sensitivity in flag keys is probably not ideal. But changing that is challenging if anybody has duplicate keys. But there are ways we could handle that, such as making it an unexposed project setting. All new projects (and old projects with no duplicates) have case-insensitive keys.

[haacked]

Some more context. In posthog-python

posthog-python/posthog/client.py

Line 881 in 6764c78

payload = flag_payloads.get(str(match_value).lower(), None)

we have this code:

payload = flag_payloads.get(str(match_value).lower(), None)

Which I think may be wrong as well. However, it's tricky because match_value can be either True or False or a variant key. In the case of variant key, we want to match case sensitively. In the case of True or False, we need to do the lower() because the payload keys in those cases are going to be "true" or "false".