Python

It was just an example. What does your script do that wouldn't be configurable in ssh config

It's config language is designed for that, too. The following should work:

# Set default username across all hosts
Host *
    User default_username

or

# To match "host1..3"
Match Host host1.example.com host2.example.com host3.example.com
    User default_username

I still think the script is a good idea. I do something similar with a shell script to add entries. Your choice of Python would make maintaining and parsing a lot more straightforward than shell.

The “host” is just your friendly name for the connection, not necessarily the hostname of the remote host. You can specify the same username or hostname as many times as you want. My config is made up of mostly blocks like this:

Host server1
    HostName server1.you.com
    User your_ssh_username
    IdentityFile ~/.ssh/yourprivatekey.pem
  
Host server2
    HostName server2.you.com
    User your_ssh_username
    IdentityFile ~/.ssh/yourprivatekey.pem