If one activates a virtualenv while PS1 is empty (eg a non-login shell), then deactivates it, the environment name will be left in the prompt. This can then cause lots of confusion as scripts assume "PS1 is non-empty" == "interactive shell"
$ export PS1=
(myEnv) # env name is still here after deactivate
$ export PS1=moo
moo . myEnv/bin/activate
moo # env name is gone
A fix for this looks reasonably easy. I'll try to sort out a patch for it.
activate.sh: handle empty (but set) vars
Account for defined but empty shell variables
Fixes #407, Thanks to #723