Enchancement suggestion for argparse: intuit type from default

MRAB python at mrabarnett.plus.com
Wed Mar 14 18:26:19 CET 2012

On 14/03/2012 13:30, Roy Smith wrote:
> In article<87399bgw18.fsf at benfinney.id.au>,
>   Ben Finney<ben+python at benfinney.id.au>  wrote:
>>  Right. I dislike proposals for run-time type inference in Python, since
>>  they are too magical.
>>  Especially since we're talking about user input (arguments from the
>>  command line to the program); that requires more explicit declarations
>>  and checking, not less.
>>  >  What if you want an argument --foo that will accept arbitrary types? Then
>>  >  you would need some way to tell argparse not to infer the type from the
>>  >  default.
>>  So we would then need to special-case the special-case? Even more reason
>>  to dislike this proposal.
>>  >  Explicit declarations should be used only for the uncommon cases where
>>  >  type inference cannot cope.
>>  That's our point of disagreement, then: I think explicit declarations
>>  should be required regarding user input.
> I wasn't suggesting that the type be inferred from what the user
> entered.  I was suggesting it be inferred from what the programmer had
> done (i.e. what value they had given the 'default' parameter).
In other words, if there's a default but no explicit type, then the
type is the type of the default.

> It's already inferred that the type is a string if you don't give it any
> value.  What possible meaning could:
> parser.add_argument('--foo', default=100)
> have?  If I run the program with:
> $ prog
> then foo defaults to the integer 100, but if I run it with:
> $ prog --foo=100
> then I get the string "100"?  Surely there's not much of a use case for
> that.

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