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Re: Newbie: static typing?

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Date	Tue, 06 Aug 2013 16:27:10 +0300
From	Burak Arslan <burak.arslan@arskom.com.tr>
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Subject	Re: Newbie: static typing?
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On 08/06/13 13:12, Rui Maciel wrote:
> Joshua Landau wrote:
>
>> What's the actual problem you're facing? Where do you feel that you
>> need to verify types?
> A standard case would be when there's a function which is designed expecting 
> that all operands support a specific interface or contain specific 
> attributes.
>
> In other words, when passing an unsupported type causes problems.
>

Hi,

First, let's get over the fact that, with dynamic typing, code fails at
runtime. Irrespective of language, you just shouldn't ship untested
code, so I say that's not an argument against dynamic typing.

This behaviour is only a problem when code fails *too late* into the
runtime -- i.e. when you don't see the offending value in the stack trace.

For example, consider you append values to a list and the values in that
list get processed somewhere else. If your code fails because of an
invalid value, your stack trace is useless, because that value should
not be there in the first place. The code should fail when appending to
that list and not when processing it.

The "too late" case is a bit tough to illustrate. This could be a rough
example: https://gist.github.com/plq/6163839 Imagine that the list there
is progressively constructed somewhere else in the code and later
processed by the sq_all function. As you can see, the stack trace is
pretty useless as we don't see how that value got there.

In such cases, you do need manual type checking.

Yet, as someone else noted, naively using isinstance() for type checking
breaks duck typing. So you should read up on abstract base classes:
http://docs.python.org/2/glossary.html#term-abstract-base-class

These said, I've been writing Python for several years now, and I only
needed to resort to this technique only once. (i was working on a
compiler) Most of the time, you'll be just fine without any manual type
checking.

Best regards,
Burak

Thread

Newbie: static typing? Rui Maciel <rui.maciel@gmail.com> - 2013-08-05 21:46 +0100
  Re: Newbie: static typing? Gary Herron <gary.herron@islandtraining.com> - 2013-08-05 14:07 -0700
    Re: Newbie: static typing? Rui Maciel <rui.maciel@gmail.com> - 2013-08-06 10:05 +0100
      Re: Newbie: static typing? Steven D'Aprano <steve@pearwood.info> - 2013-08-06 09:26 +0000
      Re: Newbie: static typing? Joshua Landau <joshua@landau.ws> - 2013-08-06 10:29 +0100
        Re: Newbie: static typing? Rui Maciel <rui.maciel@gmail.com> - 2013-08-06 11:12 +0100
          Re: Newbie: static typing? Burak Arslan <burak.arslan@arskom.com.tr> - 2013-08-06 16:27 +0300
          Re: Newbie: static typing? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-06 15:57 +0200
          Re: Newbie: static typing? Chris Angelico <rosuav@gmail.com> - 2013-08-06 15:06 +0100
          Re: Newbie: static typing? "Eric S. Johansson" <esj@harvee.org> - 2013-08-06 09:58 -0400
          Re: Newbie: static typing? Chris Angelico <rosuav@gmail.com> - 2013-08-06 15:38 +0100
      Easier to Ask Forgiveness than Permission (was: Newbie: static typing?) Ben Finney <ben+python@benfinney.id.au> - 2013-08-07 08:23 +1000
  Re: Newbie: static typing? Ian Kelly <ian.g.kelly@gmail.com> - 2013-08-05 17:38 -0600
  Re: Newbie: static typing? Ben Finney <ben+python@benfinney.id.au> - 2013-08-06 10:35 +1000
    Re: Newbie: static typing? Rui Maciel <rui.maciel@gmail.com> - 2013-08-06 10:01 +0100
      Re: Newbie: static typing? Joshua Landau <joshua@landau.ws> - 2013-08-06 10:19 +0100
        Re: Newbie: static typing? Rui Maciel <rui.maciel@gmail.com> - 2013-08-06 11:07 +0100
          Re: Newbie: static typing? Rotwang <sg552@hotmail.co.uk> - 2013-08-06 15:25 +0100
          Re: Newbie: static typing? Ben Finney <ben+python@benfinney.id.au> - 2013-08-07 08:34 +1000
      Re: Newbie: static typing? Chris Angelico <rosuav@gmail.com> - 2013-08-06 10:29 +0100
        Re: Newbie: static typing? Rui Maciel <rui.maciel@gmail.com> - 2013-08-06 11:28 +0100
          Re: Newbie: static typing? Chris Angelico <rosuav@gmail.com> - 2013-08-06 11:50 +0100
          Re: Newbie: static typing? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-08-06 18:54 -0400
          Re: Newbie: static typing? Terry Reedy <tjreedy@udel.edu> - 2013-08-06 19:02 -0400
          Re: Newbie: static typing? Chris Angelico <rosuav@gmail.com> - 2013-08-07 01:16 +0100
          RE: Newbie: static typing? "Prasad, Ramit" <ramit.prasad@jpmorgan.com.dmarc.invalid> - 2013-08-08 16:46 +0000
  Re: Newbie: static typing? Steven D'Aprano <steve@pearwood.info> - 2013-08-06 05:21 +0000
    Re: Newbie: static typing? Rui Maciel <rui.maciel@gmail.com> - 2013-08-06 10:04 +0100
  Re: Newbie: static typing? Grant Edwards <invalid@invalid.invalid> - 2013-08-06 15:05 +0000

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