It's an old issue, but here we go:

Reversible Keys via `to_dict()` — Workaround & Benchmark

Regarding the reversible keys view — the to_dict() introduced in PR (#1288 ) can serve as a practical workaround for getting reversed unique keys, even though it's not a native __reversed__ on the views themselves.

from multidict import MultiDict

md = MultiDict([('A', 1), ('B', 2), ('C', 3), ('A', 4)])
reversed_keys = list(reversed(md.to_dict().keys()))
# ['C', 'B', 'A']

This works because to_dict() returns a standard dict (which has reversible views since Python 3.8), and it preserves the insertion order of the first occurrence of each key. For CIMultiDict, case-insensitive grouping is handled automatically:

from multidict import CIMultiDict

ci = CIMultiDict([('Content-Type', 'text/html'), ('Accept', 'json'), ('content-type', 'text/plain')])
list(reversed(ci.to_dict().keys()))
# ['Accept', 'Content-Type']

Benchmark

I ran a benchmark comparing the to_dict() approach against a pure-Python manual iteration approach (building a seen-set + reversing a list). The to_dict() approach is slower because it also materializes the full value lists, but I think the API is clean.

`to_dict().keys()` vs Manual Approach:

Iterations per test:           5000
Time unit:                     milliseconds (total for 5000 calls)

| Approach           |  Keys | V/Key | to_dict ms |  manual ms | manual/ |
|--------------------|-------|-------|------------|------------|---------|
| MultiDict (small)  |    10 |     1 |       4.78 |       4.52 |    0.95x |
| MultiDict (medium) |    50 |     5 |      60.15 |      51.21 |    0.85x |
| MultiDict (large)  |   200 |    10 |     396.18 |     326.40 |    0.82x |
| MultiDict (xlarge) |  1000 |    10 |    2113.46 |    1698.61 |    0.80x |
| CIMultiDict        |    50 |     5 |      65.25 |      52.99 |    0.81x |

=================================================
Ratio > 1 means to_dict() approach is faster than manual iteration.
=================================================

The to_dict() approach has a ~15-20% overhead. However, since to_dict() runs in C, both approaches are in the same ballpark.

Reversible Keys via to_dict() — Workaround & Benchmark

Benchmark

to_dict().keys() vs Manual Approach:

Reversible Keys via `to_dict()` — Workaround & Benchmark

`to_dict().keys()` vs Manual Approach:

Uh oh!

Make keys view reversible #448

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