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<HTML
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><HEAD
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><TITLE
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>General Limitations</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Python Bindings for KDE (PyKDE-3.16.0)"
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HREF="index.html"><LINK
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REL="PREVIOUS"
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TITLE="Python Bindings for KDE (PyKDE-3.16.0)"
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HREF="index.html"><LINK
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REL="NEXT"
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TITLE="Signal and Slot Support"
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HREF="signal.html"></HEAD
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><BODY
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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CELLPADDING="0"
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COLSPAN="3"
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ALIGN="center"
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>Python Bindings for KDE (PyKDE-3.3.16.0)</TH
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></TR
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><TR
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="dcopext.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="signal.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><H1
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CLASS="SECT1"
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><A
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NAME="AEN28"
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></A
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>General Limitations</H1
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN30"
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></A
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>Python Strings, Qt Strings and Unicode</H2
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><P
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>Unicode support was added to Qt in v2.0 and to Python in v1.6. In Qt, Unicode
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support is implemented using the <TT
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CLASS="LITERAL"
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>QString</TT
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> class. It is
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important to understand that <TT
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CLASS="LITERAL"
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>QString</TT
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>s, Python string objects
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and Python Unicode objects are all different but conversions between them are
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automatic in many cases and easy to achieve manually when needed.</P
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><P
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>Whenever PyKDE expects a <TT
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CLASS="LITERAL"
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>QString</TT
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> as a function argument, a
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Python string object or a Python Unicode object can be provided instead, and
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PyKDE will do the necessary conversion automatically.</P
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><P
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>You may also manually convert Python string and Unicode objects to
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<TT
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CLASS="LITERAL"
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>QString</TT
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>s by using the <TT
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CLASS="LITERAL"
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>QString</TT
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> constructor
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as demonstrated in the following code fragment.</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>qs1 = QString('Converted Python string object')
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qs2 = QString(u'Converted Python Unicode object')</PRE
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></TD
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></TR
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></TABLE
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><P
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>In order to convert a <TT
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CLASS="LITERAL"
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>QString</TT
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> to a Python string object use
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the Python <TT
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CLASS="LITERAL"
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>str()</TT
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> function. Applying
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<TT
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CLASS="LITERAL"
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>str()</TT
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> to a null <TT
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CLASS="LITERAL"
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>QString</TT
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> and an empty
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<TT
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CLASS="LITERAL"
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>QString</TT
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> both result in an empty Python string object.</P
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><P
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>In order to convert a <TT
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CLASS="LITERAL"
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>QString</TT
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> to a Python Unicode object use
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the Python <TT
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CLASS="LITERAL"
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>unicode()</TT
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> function. Applying
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<TT
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CLASS="LITERAL"
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>unicode()</TT
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> to a null <TT
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CLASS="LITERAL"
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>QString</TT
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> and an empty
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<TT
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CLASS="LITERAL"
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>QString</TT
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> both result in an empty Python Unicode object.</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN53"
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></A
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>Access to Protected Member Functions</H2
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><P
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>When an instance of a C++ class is not created from Python it is not possible
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to access the protected member functions, or emit the signals, of that
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instance. Attempts to do so will raise a Python exception. Also, any Python
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methods corresponding to the instance's virtual member functions will never be
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called.</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN56"
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></A
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>Garbage Collection</H2
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><P
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>C++ does not garbage collect unreferenced class instances, whereas Python does.
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In the following C++ fragment both colours exist even though the first can no
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longer be referenced from within the program:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
|
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|
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>c = QColor();
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c = QColor();</PRE
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></TD
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></TR
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></TABLE
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><P
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>In the corresponding Python fragment, the first colour is destroyed when
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the second is assigned to <TT
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CLASS="LITERAL"
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>c</TT
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>:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>c = QColor()
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|
c = QColor()</PRE
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></TD
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></TR
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></TABLE
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><P
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>In Python, each colour must be assigned to different names. Typically this
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is done within class definitions, so the code fragment would be something like:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>self.c1 = QColor()
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self.c2 = QColor()</PRE
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></TD
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></TR
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></TABLE
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><P
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>Sometimes a Qt class instance will maintain a pointer to another instance and
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will eventually call the destructor of that second instance. The most common
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example is that a <TT
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CLASS="LITERAL"
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>QObject</TT
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> (and any of its sub-classes) keeps
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pointers to its children and will automatically call their destructors. In
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these cases, the corresponding Python object will also keep a reference to the
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corresponding child objects.</P
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><P
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>So, in the following Python fragment, the first <TT
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CLASS="LITERAL"
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>QLabel</TT
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> is
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not destroyed when the second is assigned to <TT
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CLASS="LITERAL"
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>l</TT
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> because the
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parent <TT
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CLASS="LITERAL"
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>QWidget</TT
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> still has a reference to it.</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>p = QWidget()
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l = QLabel('First label',p)
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l = QLabel('Second label',p)</PRE
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN72"
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></A
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>C++ Variables</H2
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><P
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>Access to C++ variables is supported. They are accessed as Python instance
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variables. For example:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>tab = QTab()
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tab.label = "First Tab"
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tab.r = QRect(10,10,75,30)</PRE
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></TD
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></TR
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></TABLE
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><P
|
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|
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>Global variables and static class variables are effectively read-only. They
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can be assigned to, but the underlying C++ variable will not be changed. This
|
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|
may change in the future.</P
|
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><P
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>Access to protected C++ class variables is not supported. This may change in
|
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|
|
the future.</P
|
|
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|
|
></DIV
|
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
|
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><A
|
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NAME="AEN78"
|
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></A
|
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|
|
|
>Multiple Inheritance</H2
|
|
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|
|
><P
|
|
|
|
|
>It is not possible to define a new Python class that sub-classes from more than
|
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|
|
one Qt class.</P
|
|
|
|
|
></DIV
|
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></DIV>
|
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|
|
<H2 CLASS="SECT2">tr() methods</H2>
|
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|
|
<P>
|
|
|
|
|
In a normal Qt installation, every descendant of QObject inherits two methods
|
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|
|
(tr (const char *) and tr (const char *, const char *) from QObject explicitly
|
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|
|
and also overloads these methods via the moc mechanism (by defining Q_OBJECT
|
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|
|
in the class declaration). KDE however is compiled with -DQT_NO_TRANSLATION,
|
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|
|
which prevents moc from creating the overloading tr() methods, and also produces
|
|
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|
|
side-effects with a normal Qt installation which was compiled without the
|
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|
|
-DQT_NO_TRANSLATION switch.
|
|
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|
|
</P>
|
|
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|
|
<P>
|
|
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|
|
PyKDE handles this situation by NOT providing tr() methods (either the inherited
|
|
|
|
|
methods from QObject or the moc generated methods) for any KDE based QObject
|
|
|
|
|
descendant. The tr() methods are static, so QObject::tr () methods are available
|
|
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|
|
via PyQt, as are tr() methods for any PyQt QObject descendant. PyKDE's handling
|
|
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|
|
of these methods has no effect on PyQt.
|
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|
|
</P>
|
|
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|
<P>Instead of the tr() methods, KDE uses corresponding i18n() methods for translating.
|
|
|
|
|
These methods are available in the tdecore module of PyKDE. For compatibility with
|
|
|
|
|
KDE, you should use the i18n methods.
|
|
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|
|
</P>
|
|
|
|
|
<H2>Socket classes</H2>
|
|
|
|
|
<P>
|
|
|
|
|
The following classes (introduced in KDE2.2.0) are NOT yet implemented:
|
|
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|
|
</P>
|
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|
|
<TABLE BORDER="0" BGCOLOR="#E0E0E0" WIDTH="100%">
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<TR>
|
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<TD>
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|
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<PRE CLASS="PROGRAMLISTING">
|
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|
|
KAddressInfo
|
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|
KExtendedSocket
|
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|
KInetSocketAddress
|
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|
|
KSocketAddress
|
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|
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KUnixSocketAddress
|
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KSocks
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</PRE>
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</TD>
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<P>
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Most of their functionality already exists in the Python socket class or in the
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KSocket class (tdecore module). These classes may be implemented at a future date
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(they require support for C socket structures and careful handling to avoid buffer
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overflow problems/exploits)
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