Why is the argparse module so inflexible?

[Terry Reedy]

On 6/27/2013 2:18 PM, Dave Angel wrote:
> On 06/27/2013 02:05 PM, Terry Reedy wrote:
>> On 6/27/2013 8:54 AM, Andrew Berg wrote:
>>> I've begun writing a program with an interactive prompt, and it needs
>>> to parse input from the user.  I thought the argparse module would be
>>> great for this,
>>
>> It is outside argparse's intended domain of application -- parsing
>> command line arguments. The grammar for a valid string of command line
>> arguments is quite restricted.
>>
>> Argparse is not intended for interactive processing of a domain-specific
>> language (DSL). There are other parsers for that. But if the grammar for
>> your DSL is restricted to what argparse can handle, using it is an
>> interesting idea. But you need non-default usage for the non-default
>> context.
>>
>>  > but unfortunately it insists on calling sys.exit() at
>>> any sign of trouble instead of letting its ArgumentError exception
>>> propagate so that I can handle it.
>>
>> When one tell argparse that something is *required*, that means "I do
>> not want to see the user's input unless it passes this condition." After
>> seeing an error message, the user can edit the command line and re-enter.
>>
>> If you do not mean 'required' in the sense above, do not say so.
>> Catching SystemExit is another way to say 'I did not really mean
>> required, in the usual mean of that term.'.
>>
>
> That last sentence is nonsense.

Not if you understand what I said.

 > If one is parsing the line the user  enters via raw_input(),

input(), in 3.x

 > catching SystemExit so the program doesn't abort
> is perfectly reasonable.  The user should be returned to his prompt,
> which in this case is probably another loop through raw_input().

Right, because 'required' means something a little different in the 
interactive context.

I don't know if all the information in the original ArgumentError 
exception is transferred to the SystemExit exception. I expect not, and 
if so, and if multiple people are using argparse this way, it would be 
reasonable to request on the tracker that its current sys.exit behavior 
become default but optional in 3.4+. There might even be an issue 
already if one searched.

-- 
Terry Jan Reedy