In:

http://psf.upfronthosting.co.za/roundup/meta/issue555

the user reports the following steps:

  user registers
  gets an email sent to them to confirm the registration
  click on link
  gets back message 'node with key "username" exists'

Need a couple of things:

1) verify username is available before sending registration
   (possibly as an option since this could be used to fish for
   usernames)
2) Change the error message to 'User "username" already exists.'

A somewhat similar problem happens when other fields, such as email address 
are invalid, see: http://psf.upfronthosting.co.za/roundup/meta/issue591
On bugs.python.org we have a detector to check them 
(https://hg.python.org/tracker/python-
dev/file/default/detectors/userauditor.py -- I don't know if Roundup does 
any attempt at validating the other fields), but this is only triggered 
when the user clicks on the confirmation link.
It would be good to have a way to trigger the detector as soon as the user 
submits, without having to wait for the confirmation email.

Fixed in in rev5976:71c68961d9f4.

The code to allow the admin to enable/disable username collisions at 
registration time (rather than confirmation) has a set of tests.

The code to pretty up the error message doesn't.
Also it's a little fragile as it overrides a specific ValueError 
exception message. If that message changes the friendlier message will 
not be displayed.

Roundup AFAICT has no way to trigger a detector since that requires a 
'commit to database' flow that only happens on confirmation. At 
registration time a small set of data is stored in the OTK database 
that doesn't trigger detectors.

This change at least fixes the primary complaint.