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Re: Bitten by my C/Java experience

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Date Mon, 4 May 2015 16:26:30 -0500
From Tim Chase <python.list@tim.thechases.com>
To Andrew Cooper <amc96@cam.ac.uk>
Cc python-list@python.org
Subject Re: Bitten by my C/Java experience
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On 2015-05-04 21:57, Andrew Cooper wrote:
> On 04/05/2015 18:43, Ian Kelly wrote:
> > 
> > Some other gotchas that aren't necessarily related to C/Java but
> > can be surprising nonetheless:
> > 
> > *    () is a zero-element tuple, and (a, b) is a two-element
> > tuple, but (a) is not a one-element tuple. Tuples are created by
> > commas, not parentheses, so use (a,) instead.
> 
> * {} is an empty set(), not dict().
> 
> Particularly subtle when combined with **kwargs
> 
> $ python3
> Python 3.4.0 (default, Apr 11 2014, 13:05:11)
> [GCC 4.8.2] on linux
> Type "help", "copyright", "credits" or "license" for more
> information.
> >>> def foo(**kwargs):
> ...   return { (k, kwargs[k]) for k in kwargs }
> ...
> >>> foo()
> set()
> >>> foo(a=1)
> {('a', 1)}
> >>>

It's a dict:

Python 3.2.3 (default, Feb 20 2013, 14:44:27) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> type({})
<class 'dict'>


What you're seeing is that your generator creates single-element
tuples in a set constructor (note that your last item isn't
"{'a': 1}". Try instead

>>> def foo(**kwargs):
...     return {k: kwargs[k] for k in kwargs}
... 
>>> foo()
{}
>>> foo(a=42)
{'a': 42}

Note the colons, indicating that it's a dict.  You're using the
dict() syntax:

  dict((k,v) for k,v in some_iter())

-tkc

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Thread

Bitten by my C/Java experience Cecil Westerhof <Cecil@decebal.nl> - 2015-05-04 17:20 +0200
  Re: Bitten by my C/Java experience Tobiah <toby@tobiah.org> - 2015-05-04 10:18 -0700
  Re: Bitten by my C/Java experience Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2015-05-04 19:32 +0200
    Re: Bitten by my C/Java experience Chris Angelico <rosuav@gmail.com> - 2015-05-05 03:40 +1000
      Re: Bitten by my C/Java experience albert@spenarnc.xs4all.nl (Albert van der Horst) - 2015-05-08 10:47 +0000
        Re: Bitten by my C/Java experience Chris Angelico <rosuav@gmail.com> - 2015-05-08 21:32 +1000
  Re: Bitten by my C/Java experience Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-04 11:43 -0600
    Re: Bitten by my C/Java experience Andrew Cooper <amc96@cam.ac.uk> - 2015-05-04 21:57 +0100
      Re: Bitten by my C/Java experience random832@fastmail.us - 2015-05-04 17:16 -0400
      Re: Bitten by my C/Java experience Tim Chase <python.list@tim.thechases.com> - 2015-05-04 16:26 -0500
  Re: Bitten by my C/Java experience Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-05-04 18:59 +0100
  Re: Bitten by my C/Java experience Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-04 13:39 -0600
    Re: Bitten by my C/Java experience Cecil Westerhof <Cecil@decebal.nl> - 2015-05-04 22:28 +0200
      Re: Bitten by my C/Java experience Dave Angel <davea@davea.name> - 2015-05-04 19:17 -0400
  Re: Bitten by my C/Java experience Terry Reedy <tjreedy@udel.edu> - 2015-05-04 16:06 -0400
  Re: Bitten by my C/Java experience BartC <bc@freeuk.com> - 2015-05-04 23:02 +0100
    Re: Bitten by my C/Java experience Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-05 18:19 +1000
      Re: Bitten by my C/Java experience BartC <bc@freeuk.com> - 2015-05-05 14:20 +0100
        Re: Bitten by my C/Java experience Rustom Mody <rustompmody@gmail.com> - 2015-05-05 09:24 -0700
          Re: Bitten by my C/Java experience Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-05-06 14:38 +0200
            Re: Bitten by my C/Java experience Rustom Mody <rustompmody@gmail.com> - 2015-05-06 06:03 -0700
        Re: Bitten by my C/Java experience Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-05-06 23:19 +1200
          Re: Bitten by my C/Java experience BartC <bc@freeuk.com> - 2015-05-06 13:40 +0100
            Re: Bitten by my C/Java experience Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-07 02:03 +1000
              Re: Bitten by my C/Java experience Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-05-06 21:13 +0100
      Re: Bitten by my C/Java experience Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-05-06 23:15 +1200
    Re: Bitten by my C/Java experience random832@fastmail.us - 2015-05-06 09:11 -0400
    Re: Bitten by my C/Java experience Chris Angelico <rosuav@gmail.com> - 2015-05-06 23:16 +1000

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