How to assign default values to psql variables?

Hi,

I have a psql script relying on variables passed in on the command line
with -v.

Is there any way to assign a default value in case the -v option is
forgotten?

Here is an example with pgtap:

select diag('should be printed only in verbose mode') where :testverbose;

This can be called with -v testverbose=true or -v testverbose=false.
However, if the assignment is forgotten, it breaks. I'd like to behave it
as if testverbose=false was passed.

Thanks.