The sibling comment about renaming is good - particularly with regard to the first function. However I think the other two are better suited to documentation rather than trying to make the function signature explain itself. Sooner or later you'll just remember that it takes a float, not the variable name.

Certainly calling things like "x_returnsNaNIfNot0to1" works, but I find it a bit ugly and it gets complicated if you have multiple or more complex constraints.

This is where languages with docstrings are nice. In Python all I would do is add a """ comment describing the inputs and the return values.

You then have a) a comment describing the code; and b) documentation that's standard so people can pull it up with pydoc or ? in Ipython. When I'm working in Jupyter, I often hit shift-tab to check what a function is expecting.