Simple Class/Variable passing question

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  • monkeyboy
    #1

    Simple Class/Variable passing question

    Hello,

    I'm new to python, and PythonCard. In the code below, I'm trying to
    create a member variable (self.currValue ) of the class, then just pass
    it to a simple function (MainOutputRout ine) to increment it. I thought
    Python "passed by reference" all variables, but the member variable
    doesn't seem to be incremented. If I make the function just increment
    the member variable directly, without passing it in, it works fine?

    In the code below, "self.currValue " stays at 5, and value evaluates to
    1? Any insight would be appreciated...


    class TestModel(model .Background):

    def on_initialize(s elf,event):
    self.currValue = 5

    def on_startBtn_mou seClick(self, event):
    self.MainOutput Routine(self.cu rrValue)
    self.OutputRout ine(self.currVa lue)

    def OutputRoutine(s elf,value):
    self.components .txtBox1.text = str(value)

    def MainOutputRouti ne(self,value):
    value = value + 1
    Tags: None
    • Matt Nordhoff
      #2
      Re: Simple Class/Variable passing question

      monkeyboy wrote:
      Hello,
      >
      I'm new to python, and PythonCard. In the code below, I'm trying to
      create a member variable (self.currValue ) of the class, then just pass
      it to a simple function (MainOutputRout ine) to increment it. I thought
      Python "passed by reference" all variables, but the member variable
      doesn't seem to be incremented. If I make the function just increment
      the member variable directly, without passing it in, it works fine?
      >
      In the code below, "self.currValue " stays at 5, and value evaluates to
      1? Any insight would be appreciated...
      >
      >
      class TestModel(model .Background):
      >
      def on_initialize(s elf,event):
      self.currValue = 5
      >
      def on_startBtn_mou seClick(self, event):
      self.MainOutput Routine(self.cu rrValue)
      self.OutputRout ine(self.currVa lue)
      >
      def OutputRoutine(s elf,value):
      self.components .txtBox1.text = str(value)
      >
      def MainOutputRouti ne(self,value):
      value = value + 1
      That's not how Python works. When you call
      "self.MainOutpu tRoutine(self.c urrValue)", in that method's scope, the
      local name "value" points to the same object as self.currValue does.
      When you do "value = value + 1", the local name "value" now points to a
      different object. That has no bearing on self.currValue.

      Err, I can't find a guide here. Um, read the language spec? I dunno.

      However:
      >>my_list = [1]
      >>def function(l):
      .... l.append(2)
      >>function(my_l ist)
      >>my_list
      [1, 2]

      That's because function() is *mutating* the list; it's not changing what
      the "l" name points to. It's calling the "append" method of the list
      object, which changes said list object. If it were doing, say, "l = 42",
      that would only rebind the function's local name "l":
      >>my_list = [1]
      >>def function(l):
      .... l = 42
      >>function(my_l ist)
      >>my_list
      [1]

      Note that strings and integers are immutable, so whenever you think
      you're mutating them (e.g. "s.replace( 'a', 'b')" or "i += 1"), you're
      actually getting a whole new, different object, with all that that implies.
      --

        Comment

        • monkeyboy
          #3
          Re: Simple Class/Variable passing question

          On Jun 19, 6:37 pm, Matt Nordhoff <mnordh...@matt nordhoff.comwro te:
          monkeyboy wrote:
          Hello,
          >
          I'm new to python, and PythonCard. In the code below, I'm trying to
          create a member variable (self.currValue ) of the class, then just pass
          it to a simple function (MainOutputRout ine) to increment it. I thought
          Python "passed by reference" all variables, but the member variable
          doesn't seem to be incremented. If I make the function just increment
          the member variable directly, without passing it in, it works fine?
          >
          In the code below, "self.currValue " stays at 5, and value evaluates to
          1? Any insight would be appreciated...
          >
          class TestModel(model .Background):
          >
          def on_initialize(s elf,event):
          self.currValue = 5
          >
          def on_startBtn_mou seClick(self, event):
          self.MainOutput Routine(self.cu rrValue)
          self.OutputRout ine(self.currVa lue)
          >
          def OutputRoutine(s elf,value):
          self.components .txtBox1.text = str(value)
          >
          def MainOutputRouti ne(self,value):
          value = value + 1
          >
          That's not how Python works. When you call
          "self.MainOutpu tRoutine(self.c urrValue)", in that method's scope, the
          local name "value" points to the same object as self.currValue does.
          When you do "value = value + 1", the local name "value" now points to a
          different object. That has no bearing on self.currValue.
          >
          Err, I can't find a guide here. Um, read the language spec? I dunno.
          >
          However:
          >
          >my_list = [1]
          >def function(l):
          ... l.append(2)
          >function(my_li st)
          >my_list
          >
          [1, 2]
          >
          That's because function() is *mutating* the list; it's not changing what
          the "l" name points to. It's calling the "append" method of the list
          object, which changes said list object. If it were doing, say, "l = 42",
          that would only rebind the function's local name "l":
          >
          >my_list = [1]
          >def function(l):
          ... l = 42
          >function(my_li st)
          >my_list
          >
          [1]
          >
          Note that strings and integers are immutable, so whenever you think
          you're mutating them (e.g. "s.replace( 'a', 'b')" or "i += 1"), you're
          actually getting a whole new, different object, with all that that implies.
          --
          Thank you, I haven't used python for a couple of years, and I didn't
          recall that aspect of the language. I'll have to dig out my O'Reilly
          book,

            Comment

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