trouble with cmd.Cmd and prompting

Tue Jan 3 10:57:26 EST 2017

Cameron Simpson wrote:
> On 03Jan2017 00:14, Dennis Lee Bieber <wlfraed at ix.netcom.com> wrote:
>>On Tue, 3 Jan 2017 11:33:15 +1100, Cameron Simpson <cs at zip.com.au>
>>declaimed the following:
>>>I'm using cmd.Cmd to write a little FTP-like command line to interface to
>>>a storage system of mine and encountering weird behaviour. When I enter a
>>>command the next prompt appears _before_ the associated operation runs,
>>>or so it appears.
>><SNIP>
>>>Has anyone seen this kind of thing before?
>>>>Haven't used the module but there is something I find intriguing in the
>>help system
>>-=-=-=-=-
>> Cmd.precmd(line)
>> Hook method executed just before the command line is interpreted, but
>>after the input prompt is generated and issued.
>>-=-=-=-=-
>>"... AFTER the input prompt is ... issued"
>>>>I don't know, but that sure sounds to me like the cmd object tends to
>>process one line behind... Though that behavior is not shown in the turtle
>>example in the help system.
>> Hmm. Interesting. I had read that text to imply (based on what I imagined
> _should_ happen) that the flow of control went:
>> cmdloop:
> issue prompt>
> line=input() # or readline
> line=precmd(line)
> stop=onecmd(line) # which calls do_blah...
> stop=postcmd(stop,line)
> if stop:
> break
>> but your reading of it suggests that this is possible:
>> issue prompt>
> cmdloop:
> line=input() # or readline
> issue prompt>
> line=precmd(line)
> stop=onecmd(line) # which calls do_blah...
> stop=postcmd(stop,line)
> if stop:
> break
>> The text for Cmd.cmdloop starts with this:
>> Repeatedly issue a prompt, accept input, parse an initial prefix off the
> received input, and dispatch to action methods, passing them the
> remainder of the line as argument.
>> which argues for the former, and was what I naively expected.
>> I guess I'd better dig out the source; I dislike going that far, not
> merely out of laziness, but also because the source is not the spec.
>> Thanks for the suggestion,
> Cameron Simpson <cs at zip.com.au>

I don't believe Dennis' reading of the docs is correct, and the relevant 
part of the source (Python 3.4) does not show anything to support it:
 while not stop:
 if self.cmdqueue:
 line = self.cmdqueue.pop(0)
 else:
 if self.use_rawinput:
 try:
 line = input(self.prompt)
 except EOFError:
 line = 'EOF'
 else:
 self.stdout.write(self.prompt)
 self.stdout.flush()
 line = self.stdin.readline()
 if not len(line):
 line = 'EOF'
 else:
 line = line.rstrip('\r\n')
 line = self.precmd(line)
 stop = self.onecmd(line)
 stop = self.postcmd(stop, line)
 self.postloop()
Is there something asynchronous in your command? Perhaps you can post a 
minimal runnable example that shows the odd behaviour.