I don't think there's enough documentation on the Expressions class, as far as how to use each method.

Most are straight forward enough, but not all.
For instance:

• How do you use the named parameters in the funcion body?
• What's the difference between select and where?
• How could I use absolute value (or any other language specific, built in operator) in an expression?
• Do I have to define the absolute value operator myself, using expressions?

Also, I think there could be an index or table of contents for these. And what kind of order are they in?
There's enough of them that I have a hard time finding the one I'm looking for, because they're not alphabetical and there's no list of just their names.
This is true for many other classes.

(fields * many) - alphabetical - (index or table of contents) = hard to find.

This is not specific to Python, I've just been using it the most lately.