default variable in python $_

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  • rh0dium
    #1

    default variable in python $_

    Hi all,

    So I have this simple little routine.. say like this..


    def foo()
    return {"a":"b", "b":"c"}

    if foo():
    print "Have foo"


    Now I want the dictionary item a (ie. b)

    How can I do it the above way or do I still have to go like this..

    def foo()
    return {"a":"b", "b":"c"}

    z = foo()
    if z:
    print "Have foo"
    print z['a']

    This is where $_ in perl is awesome - There must be a default variable
    in python right?

    Tags: ", python "_
    • Georg Brandl
      #2
      Re: default variable in python $_

      rh0dium wrote:
      Hi all,
      >
      So I have this simple little routine.. say like this..
      >
      >
      def foo()
      return {"a":"b", "b":"c"}
      >
      if foo():
      print "Have foo"
      >
      >
      Now I want the dictionary item a (ie. b)
      >
      How can I do it the above way or do I still have to go like this..
      >
      def foo()
      return {"a":"b", "b":"c"}
      >
      z = foo()
      if z:
      print "Have foo"
      print z['a']
      >
      This is where $_ in perl is awesome - There must be a default variable
      in python right?
      Why should there? When you're learning a new language, the first thing you
      should do is to empty your mind and stop expecting concepts known from other
      languages. Instead, try to grasp what the essence of the new language is.

      In the case of Python, one credo is "explicit is better than implicit".
      IMO, this precludes, among other things, the notion of a "default variable".

      Where is the problem with your second snippet?

      Georg

        Comment

        • bearophileHUGS@lycos.com
          #3
          Re: default variable in python $_

          rh0dium:
          This is where $_ in perl is awesome - There must be a default variable
          in python right?
          A default variable may add bugs to your code, and newbies of the
          language may see it coming from air, so Python avoids such things. The
          only Python "default variable" I know of is the _ that when used in the
          Python shell, it (seem to) stores the last not-None result (inside
          scripts it is a normal name sometimes used to 'ignore' some values):
          >>a = 1
          >>_
          Traceback (most recent call last):
          ...
          NameError: name '_' is not defined
          >>print 2
          2
          >>_
          Traceback (most recent call last):
          ...
          NameError: name '_' is not defined
          >>5 / 3
          1
          >>_
          1
          >>None
          >>_
          1
          >>2
          2
          >>_
          2

          Bye,
          bearophile

            Comment

            • MaR
              #4
              Re: default variable in python $_


              rh0dium wrote:
              Hi all,
              >
              So I have this simple little routine.. say like this..
              >
              >
              def foo()
              return {"a":"b", "b":"c"}
              >
              if foo():
              print "Have foo"
              >
              >
              Now I want the dictionary item a (ie. b)
              >
              How can I do it the above way or do I still have to go like this..
              >
              def foo()
              return {"a":"b", "b":"c"}
              >
              z = foo()
              if z:
              print "Have foo"
              print z['a']
              >
              This is where $_ in perl is awesome - There must be a default variable
              in python right?
              As said in earlier response, such a default variable is *dangerous* at
              best!
              Not knowing much about Perl and guessing that the $_ is a global
              storage space, my immediate thought is; What happens if you have
              multiple threads?

              The example is too simplified to give any clues as to what you are
              needing the feature for.
              Guessing that you want to generate some dict() and use the result once
              and then discard, write:

              foo_default = 1
              def foo(): return(generate _some_dict())

              you can further write

              foo().get('a', foo_default)

              giving compact code and ensuring that you get a well defined result
              regardless what dictionary keys there are.

                Comment

                • rh0dium
                  #5
                  Re: default variable in python $_

                  Hi Maria,

                  This is exactly what I was looking for. I (as others have asked me to)
                  cleared my head of the other languages, but was mearly giving perl as
                  an example indicating the compactness I was after.

                  Thanks Maria!!

                  MaR wrote:
                  rh0dium wrote:
                  Hi all,

                  So I have this simple little routine.. say like this..


                  def foo()
                  return {"a":"b", "b":"c"}

                  if foo():
                  print "Have foo"


                  Now I want the dictionary item a (ie. b)

                  How can I do it the above way or do I still have to go like this..

                  def foo()
                  return {"a":"b", "b":"c"}

                  z = foo()
                  if z:
                  print "Have foo"
                  print z['a']

                  This is where $_ in perl is awesome - There must be a default variable
                  in python right?
                  >
                  As said in earlier response, such a default variable is *dangerous* at
                  best!
                  Not knowing much about Perl and guessing that the $_ is a global
                  storage space, my immediate thought is; What happens if you have
                  multiple threads?
                  >
                  The example is too simplified to give any clues as to what you are
                  needing the feature for.
                  Guessing that you want to generate some dict() and use the result once
                  and then discard, write:
                  >
                  foo_default = 1
                  def foo(): return(generate _some_dict())
                  >
                  you can further write
                  >
                  foo().get('a', foo_default)
                  >
                  giving compact code and ensuring that you get a well defined result
                  regardless what dictionary keys there are.

                    Comment

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