Python cmd shell: handling undefined commands - python-3.x

Question

I'm writing a shell that must be able to take an unlimited number of commands. I can't figure out from the docs (https://docs.python.org/3.6/library/cmd.html) which say:
Cmd.default(line) Method called on an input line when the command
prefix is not recognized. If this method is not overridden, it prints
an error message and returns.
I must be writing the default() method incorrectly?
I've tried this:
import cmd
class MyShell(cmd.Cmd):
def default():
print('you entered this unknown command: ')
if __name__ == '__main__':
MyShell().cmdloop()
but get this (when I enter 'hi' in the shell):
(Cmd) hi
Traceback (most recent call last):
File "/Users/david/anaconda/lib/python3.6/cmd.py", line 214, in onecmd
func = getattr(self, 'do_' + cmd)
AttributeError: 'MyShell' object has no attribute 'do_hi'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "bitmessage_shell.py", line 9, in <module>
MyShell().cmdloop()
File "/Users/david/anaconda/lib/python3.6/cmd.py", line 138, in cmdloop
stop = self.onecmd(line)
File "/Users/david/anaconda/lib/python3.6/cmd.py", line 216, in onecmd
return self.default(line)
TypeError: default() takes 0 positional arguments but 2 were given

Answer 1

def default():
print('you entered this unknown command: ')
that doesn't work in a class. You need at least the object argument (self), or make the method static/class using #staticmethod or #classmethod decorators (but not very convenient, you may need the object state at some point)
Moreover, the parent class seems to pass the line, so now you need 2 arguments:
def default(self,line):
print('you entered this unknown command: ',line)