index.lxp@lxpwrap=x1807_252ehtm.htm

GUI Programming with Python

              xmllib to parse documents that contain references to
              Unicode characters.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH5"><IMG
SRC="images/callouts/6.gif"
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></DT
><DD
>XML has the same kind of comments as HTML. Most
          parsers simply pass the comments, but if you want to show
          them (for instance, in a structured view of an XML document)
          or if you want to preserve the contents of the file exactly,
          you can connect a slot to the signal emitted by this function.
          </DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH6"><IMG
SRC="images/callouts/7.gif"
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></DT
><DD
>CDATA is literal data enclosed
              between <TT
CLASS="LITERAL"
>&#60;![CDATA[</TT
> and
              <TT
CLASS="LITERAL"
>]]&#62;</TT
>. A file containing </DD
><PRE
CLASS="SCREEN"
>&#60;![CDATA[surely you will be allowed to
starve to death in one of the royal parks.]]&#62;
            </PRE
><DD
><P
>will present the quote
              &#8216;surely you will be allowed to starve to death in
              one of the royal parks.' to any slot that is connected
              to <TT
CLASS="FUNCTION"
>sigCData</TT
>.</P
></DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH7"><IMG
SRC="images/callouts/8.gif"
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></DT
><DD
>This is called when the XML document
              contains processing instructions. A processing
              instruction begins with &#60;?. All special cases, such
              as the XML declaration itself, are handled by other
              methods.
          </DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH8"><IMG
SRC="images/callouts/9.gif"
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></DT
><DD
>You can declare entities in XML
              &#8212; references to something externally defined.
              Those start with &#60;!. The contents of the declaration
              will be passed on in the <TT
CLASS="VARNAME"
>data</TT
>
              argument.

          </DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH9"><IMG
SRC="images/callouts/10.gif"
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></DT
><DD
>XML is far less forgiving than HTML
              (or at least, XML has both a stricter definition and
              less easy-going parsers), and whenever an error is
              encountered, such as forgetting to close a tag, this
              method is called.
          </DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH10"><B
>(11)</B
></A
></DT
><DD
>              <TT
CLASS="FUNCTION"
>unknown_starttag</TT
> is the most
              interesting method in the
              <TT
CLASS="CLASSNAME"
>xmllib.XMLParser</TT
> class. This is
              called whenever the xmllib parser encounters a plain tag
              that is not present in its <TT
CLASS="VARNAME"
>elements</TT
>
              dictionary. That is, it will be called for all elements
              in our current implementation.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH11"><B
>(12)</B
></A
></DT
><DD
>Likewise,
              <TT
CLASS="FUNCTION"
>unknown_endtag</TT
> is called for the
              corresponding ending tags.
          </DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH12"><B
>(13)</B
></A
></DT
><DD
>Whenever the parser encounters an unresolvable numeric
          character reference, this function is called.
          </DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#DH13"><B
>(14)</B
></A
></DT
><DD
>Unknown entities are forbidden in
              XML &#8212; if you use an entity somewhere in your
              document (which you can do by placing the name of the
              entity between an ampersand and a semicolon), then it
              must be declared. However, you might want to catch
              occurrences of unknown entities and do something
              special. That's why the function
              <TT
CLASS="FUNCTION"
>unknown_entityref</TT
> is implemented
              here. By default <TT
CLASS="FUNCTION"
>unknown_entityref</TT
>
              calls the <TT
CLASS="FUNCTION"
>syntax_error()</TT
> function
              of <TT
CLASS="CLASSNAME"
>xmllib.XMLParser</TT
>.
          </DD
></DL
></DIV
><P
>The <TT
CLASS="CLASSNAME"
>TreeView</TT
> class will show the
        contents of the XML file.</P
><PRE
CLASS="PROGRAMLISTING"
>class TreeView(QListView):                                 <IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
>

    def __init__(self, *args):
        apply(QListView.__init__,(self, ) + args)
        self.stack=[]                                      <IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
>
        self.setRootIsDecorated(TRUE)                      <IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
>
        self.addColumn("Element")                          <IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
>

    def startDocument(self, tag, pubid, syslit, data):     <IMG
SRC="images/callouts/5.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(5)"></A
>
        i=QListViewItem(self)
        if tag == None: tag = "None"
        i.setText(0, tag)
        self.stack.append(i)

    def startElement(self, tag, attributes):               <IMG
SRC="images/callouts/6.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(6)"></A
>
        if tag == None: tag = "None"
        i=QListViewItem(self.stack[-1])
        i.setText(0, tag)
        self.stack.append(i)

    def endElement(self, tag):                             <IMG
SRC="images/callouts/7.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(7)"></A
>
        del(self.stack[-1])
      </PRE
><DIV
CLASS="CALLOUTLIST"
><DL
COMPACT="COMPACT"
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#TVCLASS"><IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
></DT
><DD
>The <TT
CLASS="CLASSNAME"
>TreeView</TT
>
              class is a simple subclass of PyQt's versatile
              <TT
CLASS="CLASSNAME"
>QListView</TT
> class.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#TVSTACK"><IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
></DT
><DD
>Because XML is a hierarchical file
              format, elements are neatly nested in each other. In
              order to be able to create the right treeview, we should
              keep a stack of the current element depth. The last
              element of the stack will be the parent element of all
              new elements.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#TVDECORATION"><IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
></DT
><DD
>This option sets the beginning of
              the tree at the first element, making it clear to the
              user that it's an expandable tree instead of a simple
              list.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#TVCOLS"><IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
></DT
><DD
>We present only one column in the
              listview &#8212; if you want to show the attributes of
              elements, too, you might add a few more columns.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#TVF1"><IMG
SRC="images/callouts/5.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(5)"></A
></DT
><DD
>The
              <TT
CLASS="FUNCTION"
>startDocument</TT
> function is called
              when the XML document is opened. It also starts the call
              stack by creating the first element. The first
              <TT
CLASS="CLASSNAME"
>QListViewItem</TT
> object has the
              listview as a parent; all others with have a
              <TT
CLASS="CLASSNAME"
>QListViewItem</TT
> object as parent.
              The constructor of <TT
CLASS="CLASSNAME"
>QListViewItem</TT
>
              is so overloaded that sip tends to get confused, so I
              create the item and set its text separately.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#TVF2"><IMG
SRC="images/callouts/6.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(6)"></A
></DT
><DD
>Whenever an element is opened, a
          <TT
CLASS="CLASSNAME"
>QListViewItem</TT
> item is created and
          pushed on the stack, where it becomes the parent for newly
          opened elements.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#TVF3"><IMG
SRC="images/callouts/7.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(7)"></A
></DT
><DD
>Conversely, when the element is
              closed, it is popped <SPAN
><I
CLASS="EMPHASIS"
>from</I
></SPAN
> the
              stack.</DD
></DL
></DIV
><PRE
CLASS="PROGRAMLISTING"
>def main(args):

    if (len(args) == 2):
        app = QApplication(sys.argv)

        QObject.connect(app, SIGNAL('lastWindowClosed()'),
                        app, SLOT('quit()'))
        w = TreeView()
        app.setMainWidget(w)

        o=QObject()                                        <IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
>
        p=Parser(o)                                        <IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
>
        QObject.connect(o, PYSIGNAL("sigDocType"),         <IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
>
                           w.startDocument)
        QObject.connect(o, PYSIGNAL("sigStartTag"),
                           w.startElement)
        QObject.connect(o, PYSIGNAL("sigEndTag"),
                           w.endElement)

        s=open(args[1]).read()                             <IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
>
        p.start(s)

        w.show()
        app.exec_loop()
    else:
        print "Usage: python qtparser.py FILE.xml"

if __name__=="__main__":
    main(sys.argv)
        </PRE
><DIV
CLASS="CALLOUTLIST"
><DL
COMPACT="COMPACT"
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#QOBJECT"><IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
></DT
><DD
>Here we create a
              <TT
CLASS="CLASSNAME"
>QObject</TT
> which is used to emit all
              necessary signals, since we cannot inherit from more
              than one PyQt class at the same time. Note that by using
              this technique, you don't have to subclass from
              <TT
CLASS="CLASSNAME"
>QObject</TT
> in order to be able to
              emit signals. Sometimes delegation works just as
              well.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#PARSER"><IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
></DT
><DD
>A parser object is created, with the
              <TT
CLASS="CLASSNAME"
>QObject</TT
> object as its argument.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#CONNECTIONS"><IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
></DT
><DD
>Before feeding the parser the text, all connections
              we want are made from the <TT
CLASS="CLASSNAME"
>QObject</TT
>
              object (which we passed to the parser to make sure it
              can emit signals) to the <TT
CLASS="CLASSNAME"
>TreeView</TT
>
              object that forms the main window.</DD
><DT
><A href="index.lxp@lxpwrap=x1807_252ehtm.htm#READING"><IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
></DT
><DD
>The file whose name was given on the command
              line is read and passed on to the parser. I have included
              a very small test file, <TT
CLASS="FILENAME"
>test.xml</TT
>,
              but you can use any Designer UI design file.</DD
></DL
></DIV
></DIV
><P
>This is a very simple and convenient way
      of working with XML files and PyQt gui's &#8212; but it's
      generally useful, too. The standard way of working with XML
      files and parsers allows for only one function to be called for
      each tag. Using signals and slots, you can have as many slots
      connected to each signal as you want. For instance, you can have
      not only a gui, but also an analyzer that produces statistics
      listening in on the same parsing run.</P
><DIV
CLASS="MEDIAOBJECT"
><P
><DIV
CLASS="CAPTION"
><P
>The result of parsing a Designer .ui file.</P
></DIV
></P
></DIV
><P
>On a final note, there is one bug in this
      code... See if you can find it, or consult
     <A href="index.lxp@lxpwrap=x3270_252ehtm.htm#CH2LISTVIEW">the Section called <I
>QListView and QListViewItem</I
> in Chapter 10</A
> for an explanation.</P
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