There are many places in scikit-learn where ints and floats (usually between 0 and 1) are allowed, and have different semantics.
Most of us think this is brittle. This is an issue to discuss whether we should deprecate and use a different interface, and what that interface should be.

I think the main issue arises when people pass 1.0 vs 1 and 0.0 vs 0 (though they are likely to have the same semantics).
I think we can keep the current semantics (as they are very concise) as long as we are careful about 1.0 and 1.

@GaelVaroquaux proposed to use a more explicit syntax, like "10%", which is a non-backward-compatible change and needs a full deprecation of all floats 0-1. There is a question on whether this applies to all floats between 0 and 1 or only those where also an integer is possible.

My half-baked idea would be to replace 1.0 by "all", which is quite intuitive. I'm not sure what to do about 1, though. We could require something like "one", which would be a bit inconsistent, but solve all use-cases (grid-searching from 1 to 10 will be odd, though).

However, if we do the %, then grid-searching fractions becomes pretty ugly.