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<HTML
><HEAD
><TITLE
>General Limitations</TITLE
><META
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>Python Bindings for KDE (PyKDE-3.15)</TH
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN28"
></A
>General Limitations</H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN30"
></A
>Python Strings, Qt Strings and Unicode</H2
><P
>Unicode support was added to Qt in v2.0 and to Python in v1.6. In Qt, Unicode
support is implemented using the <TT
CLASS="LITERAL"
>QString</TT
> class. It is
important to understand that <TT
CLASS="LITERAL"
>QString</TT
>s, Python string objects
and Python Unicode objects are all different but conversions between them are
automatic in many cases and easy to achieve manually when needed.</P
><P
>Whenever PyKDE expects a <TT
CLASS="LITERAL"
>QString</TT
> as a function argument, a
Python string object or a Python Unicode object can be provided instead, and
PyKDE will do the necessary conversion automatically.</P
><P
>You may also manually convert Python string and Unicode objects to
<TT
CLASS="LITERAL"
>QString</TT
>s by using the <TT
CLASS="LITERAL"
>QString</TT
> constructor
as demonstrated in the following code fragment.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>qs1 = QString('Converted Python string object')
qs2 = QString(u'Converted Python Unicode object')</PRE
></TD
></TR
></TABLE
><P
>In order to convert a <TT
CLASS="LITERAL"
>QString</TT
> to a Python string object use
the Python <TT
CLASS="LITERAL"
>str()</TT
> function. Applying
<TT
CLASS="LITERAL"
>str()</TT
> to a null <TT
CLASS="LITERAL"
>QString</TT
> and an empty
<TT
CLASS="LITERAL"
>QString</TT
> both result in an empty Python string object.</P
><P
>In order to convert a <TT
CLASS="LITERAL"
>QString</TT
> to a Python Unicode object use
the Python <TT
CLASS="LITERAL"
>unicode()</TT
> function. Applying
<TT
CLASS="LITERAL"
>unicode()</TT
> to a null <TT
CLASS="LITERAL"
>QString</TT
> and an empty
<TT
CLASS="LITERAL"
>QString</TT
> both result in an empty Python Unicode object.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN53"
></A
>Access to Protected Member Functions</H2
><P
>When an instance of a C++ class is not created from Python it is not possible
to access the protected member functions, or emit the signals, of that
instance. Attempts to do so will raise a Python exception. Also, any Python
methods corresponding to the instance's virtual member functions will never be
called.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN56"
></A
>Garbage Collection</H2
><P
>C++ does not garbage collect unreferenced class instances, whereas Python does.
In the following C++ fragment both colours exist even though the first can no
longer be referenced from within the program:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>c = QColor();
c = QColor();</PRE
></TD
></TR
></TABLE
><P
>In the corresponding Python fragment, the first colour is destroyed when
the second is assigned to <TT
CLASS="LITERAL"
>c</TT
>:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>c = QColor()
c = QColor()</PRE
></TD
></TR
></TABLE
><P
>In Python, each colour must be assigned to different names. Typically this
is done within class definitions, so the code fragment would be something like:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>self.c1 = QColor()
self.c2 = QColor()</PRE
></TD
></TR
></TABLE
><P
>Sometimes a Qt class instance will maintain a pointer to another instance and
will eventually call the destructor of that second instance. The most common
example is that a <TT
CLASS="LITERAL"
>QObject</TT
> (and any of its sub-classes) keeps
pointers to its children and will automatically call their destructors. In
these cases, the corresponding Python object will also keep a reference to the
corresponding child objects.</P
><P
>So, in the following Python fragment, the first <TT
CLASS="LITERAL"
>QLabel</TT
> is
not destroyed when the second is assigned to <TT
CLASS="LITERAL"
>l</TT
> because the
parent <TT
CLASS="LITERAL"
>QWidget</TT
> still has a reference to it.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>p = QWidget()
l = QLabel('First label',p)
l = QLabel('Second label',p)</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN72"
></A
>C++ Variables</H2
><P
>Access to C++ variables is supported. They are accessed as Python instance
variables. For example:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>tab = QTab()
tab.label = "First Tab"
tab.r = QRect(10,10,75,30)</PRE
></TD
></TR
></TABLE
><P
>Global variables and static class variables are effectively read-only. They
can be assigned to, but the underlying C++ variable will not be changed. This
may change in the future.</P
><P
>Access to protected C++ class variables is not supported. This may change in
the future.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN78"
></A
>Multiple Inheritance</H2
><P
>It is not possible to define a new Python class that sub-classes from more than
one Qt class.</P
></DIV
></DIV>
<H2 CLASS="SECT2">tr() methods</H2>
<P>
In a normal Qt installation, every descendant of QObject inherits two methods
(tr (const char *) and tr (const char *, const char *) from QObject explicitly
and also overloads these methods via the moc mechanism (by defining Q_OBJECT
in the class declaration). KDE however is compiled with -DQT_NO_TRANSLATION,
which prevents moc from creating the overloading tr() methods, and also produces
side-effects with a normal Qt installation which was compiled without the
-DQT_NO_TRANSLATION switch.
</P>
<P>
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
via PyQt, as are tr() methods for any PyQt QObject descendant. PyKDE's handling
of these methods has no effect on PyQt.
</P>
<P>Instead of the tr() methods, KDE uses corresponding i18n() methods for translating.
These methods are available in the kdecore module of PyKDE. For compatibility with
KDE, you should use the i18n methods.
</P>
<H2>Socket classes</H2>
<P>
The following classes (introduced in KDE2.2.0) are NOT yet implemented:
</P>
<TABLE BORDER="0" BGCOLOR="#E0E0E0" WIDTH="100%">
<TR>
<TD>
<PRE CLASS="PROGRAMLISTING">
KAddressInfo
KExtendedSocket
KInetSocketAddress
KSocketAddress
KUnixSocketAddress
KSocks
</PRE>
</TD>
</TR>
</TABLE>
<P>
Most of their functionality already exists in the Python socket class or in the
KSocket class (kdecore module). These classes may be implemented at a future date
(they require support for C socket structures and careful handling to avoid buffer
overflow problems/exploits)
</P>
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