If you'll hover with the mouse over the line numbers you'll see arrows - clicking on any of them will fold/collapse the code

If you want to collapse/expand all - you can do so by going to edit->code folding and choose "fold all" or "unfold all":

One thing you can do is select the Except code bloc using a regular expression, for instance using except(.|\n)*?raise.* in your case. You can then select "Find all" in the search bar, then Edit->Code Folding -> Fold .
Windows shortcut : Ctrl-Shift-[
Mac shortcut: Cmd-Alt-[

All the Except bloc will then be collapsed.

In addition to the other answers it is also possible to fold based on level as well. So for example looking at the default key bindings for fold.

Searching for fold key bindings.

So for example a foldall, or folding level 1 would be to hold Ctrl followed by pressing the sequence k and then 1:

Or folding level 2 would be to hold Ctrl followed by pressing the sequence k and then 2:

Or unfolding all would be would be to hold Ctrl followed by pressing the sequence k and then 0 or in my defaults I also seem to have it bound to the letter j:

Warning.

Pressing Ctrl+k twice will remove a line or a count of lines. But not really cause you can put them back one by one by Ctrl+u

I know this is an old question, but it still comes up high in search results and none of the answers quite do what the OP wanted.

1. select the code you don't want to be hidden
2. use "Selection" -> "Invert Selection" to select the code you do want to be hidden instead
3. use ctrl + shift + [ or Command + Option + ] to collapse the selection(s)

This will leave you with just the code you originally had selected visible.

Fold and UnFold function or class base only for MAC:

 * Fold: command + K, command + 1
* UnFold: command + K, command + J