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GUI Programming with Python

...     def __init__(self, value):
...             self.item=value
...     def printValue(self):
...             print self.item
...
>>> firstObject=FirstClass(value="BlackAdder goes forth")
>>> firstObject.printValue
<method FirstClass.printValue of FirstClass instance at 0x80db1f4>
>>> firstObject.printValue()
BlackAdder goes forth
>>>
    </PRE
><P
>If you type neatly and without mistakes,
      the contents of the Python interpreter window might look like
      this. Let's look at what happens: we have defined a &#8216;class'
      &#8212; that's a combination of information and complex actions
      that work on the contained information. The class has a name:
      <TT
CLASS="VARNAME"
>FirstClass</TT
>. (It is customary to capitalize
      the first letter of each word in a classname).</P
><P
>A class in itself is only the
      &#8216;template', so to speak, while an object is the
      &#8216;document' &#8212; just as you can make documents out of
      templates in a wordprocessor, you can make objects from
      classes.</P
><P
>Furthermore, the class has two
      &#8216;functions' &#8212; defined with the
      <TT
CLASS="FUNCTION"
>def</TT
> statement.
    </P
><P
>The first function,
      <TT
CLASS="FUNCTION"
>__init__</TT
>, is called when you want to create
      an object. The function has two &#8216;parameters' &#8212; that is,
      two names associated with a value (which we call a &#8216;variable'
      because the value can change, though the name remains the same).
      The first parameter refers to the object &#8212; it's always
      called <TT
CLASS="VARNAME"
>self</TT
> in Python (though it is called
      <TT
CLASS="VARNAME"
>this</TT
> in Java or C++). The second parameter is
      the value we want the object to manage for us.</P
><P
>You can use a dot &#8216;.' to associate
      variables with each other. The line &#8216;self.item = value' means
      that from now on the object we refer to with
      <TT
CLASS="VARNAME"
>self</TT
> (but also, in another context, with
      <TT
CLASS="VARNAME"
>firstObject</TT
>) knows that the name
      <TT
CLASS="VARNAME"
>item</TT
> is associated with the value represented
      by the parameter <TT
CLASS="VARNAME"
>value</TT
>.</P
><P
>Cleverly, Python doesn't forget this, so
      when you create an object with the name
      <TT
CLASS="VARNAME"
>firstObject</TT
> and the
      <SPAN
><I
CLASS="EMPHASIS"
>string</I
></SPAN
> value (that is to say, some text, as
      opposed to a number) &#8216;BlackAdder goes forth', you can later call
      the <TT
CLASS="FUNCTION"
>printValue()</TT
> function, which will be
      able to do something with that value.</P
><P
>In order to call&#8212;that is, ask Python to
      execute&#8212; a function, you must add brackets after the
      function name; the parameters always go between the brackets.
      You don't have to put <TT
CLASS="VARNAME"
>self</TT
> between brackets, for
      Python does this for you. If you don't add the brackets, you are
      <SPAN
><I
CLASS="EMPHASIS"
>referring to</I
></SPAN
> the function, not asking
      Python to execute it. Python then answers you with the revealing
      sentence:</P
><PRE
CLASS="SCREEN"
>&#62;&#62;&#62; firstObject.printValue
&#60;method FirstClass.printValue of FirstClass instance at 0x80db1f4&#62;
    </PRE
><P
>This tells you what kind of an object a
      function is. <SPAN
><I
CLASS="EMPHASIS"
>Calling</I
></SPAN
> the function will
      &#8216;print' the value of <TT
CLASS="VARNAME"
>item</TT
> in your
      window:</P
><PRE
CLASS="SCREEN"
>&#62;&#62;&#62; firstObject.printValue()
BlackAdder goes forth
&#62;&#62;&#62;
    </PRE
><P
>As I said, the <TT
CLASS="VARNAME"
>self</TT
> is
      supplied by Python, because you call the function from the
      object. That is, by prefixing the variable that points to the
      object to the function name, with a dot in between. This is the
      same as typing the following code (that is, calling the function
      with the object as its first parameter). As such, the following
      two expressions are equivalent:</P
><PRE
CLASS="SCREEN"
>&#62;&#62;&#62;firstObject.printValue()
BlackAdder goes forth
&#62;&#62;&#62;FirstClass.printValue(firstObject)
BlackAdder goes forth
    </PRE
><P
>Of course, typing in all these
      instructions correctly every time you want the computer to print
      &#8216;BlackAdder goes forth' is quite a chore. To get around this,
      you can write a small text document (this is not the same as a
      Word document!) using BlackAdder's text editor, and then ask
      Python to execute it.</P
><P
>To sum up: composition of complex wholes
      from smaller parts using a debased variant of English, and
      calling things names, is what programming is all about. The rest
      is made up of rules &#8212;rules intended to make it easier for
      computer the compute to determine what it should do, and more
      difficult for you to explain yourself to the machine.</P
><DIV
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><P
></P
><TABLE
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WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Warning</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
>Please be warned that if you execute
        your programs (or scripts) from BlackAdder, all the output of
        &#8216;print' will disappear into the void. The output will
        only be shown if you start your scripts using the debugger,
        and have the Python Interpreter window open. If you merely
        type in stuff in the Interpreter window you will see all
        output.</P
></TD
></TR
></TABLE
></DIV
><P
>If this section went over your head with
      the airspeed of an unladen African swallow, don't worry. There
      is much more to programming &#8212; more than I can explain in a
      third of a chapter. Please read the Python tutorial that is
      included with Python and with BlackAdder. It is well-written and
      a little less hasty. Another good source is the free Livewires
      Python course, which you can find in PDF format at:
      http://www.livewires.org.uk/python/. I heartily recommend it as
      the best introduction to the general idea of programming I've
      ever read.</P
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