@pitrou, I think usability is a lot more valuable than cross-feature "formal consistency" here.  I've been extracting bit fields for decades, and always think of them in terms of "least-significant bit and number of bits".  Perhaps the underlying difference is that nobody ever thinks of bit positions as being _between_ bits - instead we always think of "bit i" as being the bit with binary value 2**i.  It's more of a math concept than an indexing concept.

For a bit _array_ I'd agree slicing semantics would make more sense.  But Python ints have infinite width, and "index 0" is at the rightmost position (not the leftmost position - there is no leftmost position).

I'd also like to avoid the nuisance of having to implement all the goofy slicing possibilities, like non-unit strides and negative strides.  Not that they're "goofy" in general - they're goofy in the context of extracting bits from an integer.  Again a bit array is a different kind of beast.