why is user = form.getfirst("user", "").upper() safe?

[buran]

No, this is the solution, not the example code for error.
The example code for error is this one:

user = form.getvalue("user").upper()

now, if form.getvalue("user") returns list and you call upper() on it you will get an error.
as explained in the docs, you can test for what is returned. but there is alternative - if instead you use

user = form.getfirst("user", "").upper()

it will return just the first element (single string) or "" and upper() will not raise exception

you can also use form.getlist() in which case you will always get a list. In any case the code is more compact and clean compared to the one if you check what is returned.