[Python-Dev] cmd.Cmd.prompt should be an instance attribute
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).
While it's easy to fix this in my subclass by overriding the onecmd method
and setting self.prompt there, this seems kludgy ("Beautiful is better than
ugly"). More importantly, this whole arrangement obscures the fact that
prompt is a de facto attribute of the instance and not the class. It's not
the class that's displaying the prompt but the instance.
It seems more intuitive for cmd.Cmd to be implemented thus:
class Cmd:
PROMPT = '> '
def __init__(self):
...
self.prompt = self.PROMPT
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[Python-Dev] Re: cmd.Cmd.prompt should be an instance attribute
Sorry, that implementation should have been class Cmd: PROMPT = '> ' @property def prompt(self) -> str: return self.PROMPT ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/Q7TSFJD4EPUXXJFT5TMBUODIVL4CLR4O/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: cmd.Cmd.prompt should be an instance attribute
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-change-the-world/>
>
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[Python-Dev] Re: cmd.Cmd.prompt should be an instance attribute
Found the mypy bug:
https://github.com/python/mypy/issues/4125
On Sat, Jun 26, 2021 at 7:35 PM Daniel Walker wrote:
> 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-change-the-world/>
>>
>
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[Python-Dev] Re: cmd.Cmd.prompt should be an instance attribute
Thank you
On Sat, Jun 26, 2021 at 8:10 PM Guido van Rossum wrote:
> Okay, then Chris Barker’s explanation applies.
>
> On Sat, Jun 26, 2021 at 16:35 Daniel Walker wrote:
>
>> 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-change-the-world/>
>>>
>> --
> --Guido (mobile)
>
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