pxssh.login require either username or ssh_config

If pxssh.login is called with an ssh_config then do not require username since it can be supplied via the config and some programs do not want to manage the username at all since it would require parsing the ssh config.

This commit removes the independent username parameter when formatting cmd and instead appends the flag and the username to ssh_options if a username is provided.

If neither username nor ssh_config is provided raise a TypeError in line with the error raised natively if username was not provided as a required positional argument.

pexpect/pxssh.py

@@ -253,7 +253,7 @@ def sync_original_prompt (self, sync_multiplier=1.0):
     ### TODO: This is getting messy and I'm pretty sure this isn't perfect.
     ### TODO: I need to draw a flow chart for this.
     ### TODO: Unit tests for SSH tunnels, remote SSH command exec, disabling original prompt sync
-    def login (self, server, username, password='', terminal_type='ansi',
+    def login (self, server, username=None, password='', terminal_type='ansi',
                 original_prompt=r"[#$]", login_timeout=10, port=None,
                 auto_prompt_reset=True, ssh_key=None, quiet=True,
                 sync_multiplier=1, check_local_ip=True,
@@ -354,7 +354,13 @@ def login (self, server, username, password='', terminal_type='ansi',
                         if spawn_local_ssh==False:
                             tunnel = quote(str(tunnel))
                         ssh_options = ssh_options + ' -' + cmd_type + ' ' + str(tunnel)
-        cmd = "ssh %s -l %s %s" % (ssh_options, username, server)
+
+        if username is not None:
+            ssh_options = ssh_options + ' -l ' + username
+        elif ssh_config is None:
+            raise TypeError('login() needs either a username or an ssh_config')
+
+        cmd = "ssh %s %s" % (ssh_options, server)
         if self.debug_command_string:
             return(cmd)