[Python-Dev] Re: cmd.Cmd.prompt should be an instance attribute

26 Jun 2021


      I wasn't looking at the type stub but cmd.py itself.  It has

PROMPT = '(Cmd) '
...

class Cmd:
    prompt = PROMPT
    ...

On Sat, Jun 26, 2021 at 6:04 PM Guido van Rossum  wrote:
...
On Sat, Jun 26, 2021 at 9:25 AM Daniel Walker  wrote:
...
I was recently using the cmd module for a project where my CLI
could connect to and interact with another host.  I implemented prompt in
such a way that it would show the IP address when connected.  I.e.,
class MyCmd(cmd.Cmd):
    ...
@property
    def prompt(self) -> str:
        if self.remote_host.connected():
            return f'> ({self.remote_host.ip}) '
        else:
            return '> '
This worked perfectly fine... until I ran mypy.  mypy complained because,
in cmd.Cmd, prompt is a class attribute.
Looking at cmd.py, this seems like an odd design choice as all of the
references to cmd are through the instance (i.e., self.prompt).
You misread the typeshed stub. Where you see these lines in cmd.pyi
class Cmd:
    prompt: str
    identchars: str
    ruler: str
    ...
those are all instance attribute declarations.
I think that you're running into a different mypy bug, which is that you
can't override a plain attribute with a property in a subclass.
I think there's already a bug for that in the mypy tracker, but I can't
find it right now.
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-c...