How to prevent tests from running against production?

[Roy Smith]

Our deploy/configuration system includes credentials for connecting to a 
database.  We have one production database, and a variety of clones of 
that in our test and development environments.

We've got a large body of tests, written with a combination of unittest 
and nose.  Many of our tests do things which shouldn't be done against 
the production database.  I'd like to set things up so anytime any test 
code gets run, a check is made to see which database you're connected to 
and if you're connect to production, the test refuses to run.

It's easy to write a check like that in setup(), but that only gets 
called if you remember to write a setup() method (or inherit from 
something that does).  I'm looking for something that runs completely 
automatically.  I don't want somebody to be able to write something 
which causes damage that nose can discover and run when you're connected 
to the wrong database.