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Date: Wed, 14 Mar 2012 17:26:19 +0000
From: MRAB <python@mrabarnett.plus.com>
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Subject: Re: Enchancement suggestion for argparse: intuit type from default
References: <jjocvo$5i2$1@panix2.panix.com> <87zkbkgp67.fsf@benfinney.id.au> <4f6094c5$0$29972$c3e8da3$5496439d@news.astraweb.com> <87399bgw18.fsf@benfinney.id.au> <roy-AEEEC5.09304414032012@news.panix.com>
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On 14/03/2012 13:30, Roy Smith wrote:
> In article<87399bgw18.fsf@benfinney.id.au>,
>   Ben Finney<ben+python@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.