[Tutor] dict() versus {}

[Mark Tolonen]

This:

>>> dict(one=1,two=2,three=3)
{'three': 3, 'two': 2, 'one': 1}

Is a shortcut for the longer:

>>> dict((('one',1),('two',2),('three',3)))
{'three': 3, 'two': 2, 'one': 1}

and given how this works:

>>> def function(**kwargs):
...  print kwargs
...  
>>> function(one=1,two=2,three=3)
{'three': 3, 'two': 2, 'one': 1}

One can guess how the shortcut is implemented.

-Mark

"wormwood_3" <wormwood_3 at yahoo.com> wrote in message news:454349.94041.qm at web110811.mail.gq1.yahoo.com...
        dict( [arg]) 
  Return a new dictionary initialized from an optional positional argument or from a set of keyword arguments. If no arguments are given, return a new empty dictionary. If the positional argument arg is a mapping object, return a dictionary mapping the same keys to the same values as does the mapping object.


  But why doesn't the optional positional argument arg in this case, not being a mapping type, get evaluated?: dict(thing=1)

  And even if it makes sense for it not to be evaluated, wouldn't it be better for dict() to complain that it didn't get a string or an int as it expects for a keyword argument? Maybe I am missing the use case, so far it just seems strange to force the keyword to a string.

  -Sam






------------------------------------------------------------------------------
  From: bob gailer <bgailer at g mail.com>
  To: wormwood_3 <wormwood_3 at yahoo.com>
  Cc: Python Tutorlist <tutor at python.org>
  Sent: Wednesday, January 21, 2009 11:25:12 PM
  Subject: Re: [Tutor] dict() versus {}

  wormwood_3 wrote: 
    Hmm, looked through the latest docs, in the sections on dictionary types, don't see examples that point to this case well. Can you link to what you had in mind?


  2.1 Built-in Functions 
  ...
  dict( [mapping-or-sequence]) 
  ...
  these all return a dictionary equal to {"one": 2, "two": 3}: 
  ...
  dict(one=2, two=3) 





----------------------------------------------------------------------------
    From: bob gailer <bgailer at gmail.com>
    To: wormwood_3 <wormwood_3 at yahoo.com>
    Cc: Python Tutorlist <tutor at python.org>
    Sent: Wednesday, January 21, 2009 11:02:35 PM
    Subject: Re: [Tutor] dict() versus {}

    wormwood_3 wrote: 
      When creating a list of dictionaries through a loop, I ran into a strange issue. I'll let the code talk:

      >>> l = 'i am a special new list'.split()
      >>> t = []
      >>> for thing in l:
      ...     t.append({thing: 1})
      ... 
      >>> t
      [{'i': 1}, {'am': 1}, {'a': 1}, {'special': 1}, {'new': 1}, {'list': 1}]

      This is what I expected. {} says to make a dictionary. Thing, not being quoted, is clearing a variable, which needs to be evaluated and used as the key.

      >>> t = []
      >>> for thing in l:
      ...     t.append(dict(thing=1))
      ... 
      >>> t
      [{'thing': 1}, {'thing': 1}, {'thing': 1}, {'thing': 1}, {'thing': 1}, {'thing': 1}]

      This was what threw me. Why would the dict() function not evaluate thing? How can it take it as a literal string without quotes?
    I suggest you look dict up in the Python documentation. There it shows the result you got as an example. When in doubt read the manual.


    -- 
    Bob Gailer
    Chapel Hill NC
    919-636-4239
----------------------------------------------------------------------------
_______________________________________________
Tutor maillist  -  Tutor at python.org
http://mail.python.org/mailman/listinfo/tutor
  


  -- 
  Bob Gailer
  Chapel Hill NC
  919-636-4239


------------------------------------------------------------------------------


  _______________________________________________
  Tutor maillist  -  Tutor at python.org
  http://mail.python.org/mailman/listinfo/tutor
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/tutor/attachments/20090121/c87ea2f2/attachment-0001.htm>