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<table border="0" cellspacing="0" cellpadding="3" width="100%"><tr><td> <div align="center" id="bldcontent"> <a href="../default.htm"><img src="../images/opendocs.png" width="63" height="76" border="0"></a> <br> <div class="symbol">Your OpenSource Publisher™</div> </div> </td></tr></table> <div align="center" class="author"> <a href="../products.lxp">Products</a> | <a href="../wheretobuy.lxp">Where to buy</a> | <a href="../bookstore.lxp">Retailers</a> | <a href="../faq.lxp">FAQ</a> | <a href="../writeforus.lxp">Write for Us.</a> | <a href="#contact">Contact Us.</a> </div> <table border="0" cellspacing="3" cellpadding="0" width="100%"><tr><td width="100%"> <div class="content"> <table border="0" cellspacing="2" cellpadding="0" width="100%"><tr><td width="100%"> <div align="center"><H4 CLASS="AUTHOR"><A NAME="AEN5">Boudewijn Rempt</A><br><a href="../../https@secure.linuxports.com/opendocs/default.htm"><img src=odpyqt125.png></a><br>ISBN: 0-97003300-4-4<br><a href="../../https@secure.linuxports.com/opendocs/default.htm">Available from bookstores everywhere or you can order it here.</a><p>You can download the source files for the book <a href="pyqtsrc.tgz">(code / eps) here.</a><hr></div> <HTML><HEAD><TITLE>String conversions</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.72"><LINKREL="HOME"TITLE="GUI Programming with Python: QT Edition"HREF="book1.htm"><LINKREL="UP"TITLE="String Objects in Python and Qt"HREF="c2029.htm"><LINKREL="PREVIOUS"TITLE="String Objects in Python and Qt"HREF="c2029.htm"><LINKREL="NEXT"TITLE="QCString — simple strings in PyQt"HREF="x2104.htm"></HEAD><BODYCLASS="SECT1"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#0000FF"VLINK="#840084"ALINK="#0000FF"><DIVCLASS="NAVHEADER"><TABLESUMMARY="Header navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">GUI Programming with Python: QT Edition</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><A accesskey="P" href="index.lxp@lxpwrap=c2029_252ehtm.htm">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom">Chapter 8. String Objects in Python and Qt</TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><A accesskey="N" href="index.lxp@lxpwrap=x2104_252ehtm.htm">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="SECT1"><H1CLASS="SECT1">String conversions</A></H1><P>I will return to <TTCLASS="CLASSNAME">QCString</TT> later, as <TTCLASS="CLASSNAME">QString</TT> is used everywhere in Qt. All user-visible text is set using <TTCLASS="CLASSNAME">QString</TT>s, and if a widget returns some text, it will return a <TTCLASS="CLASSNAME">QString</TT> as well. It should become clear that the only way to work comfortably with strings in Python and Qt is to have some automatic conversion between Python string objects and <TTCLASS="CLASSNAME">QString</TT>s. </P><P>The conversion from a Python string to a <TTCLASS="CLASSNAME">QString</TT> is completely transparent. Any Qt class method that asks for a <TTCLASS="CLASSNAME">QString</TT> as an argument accepts any Python string (or Python Unicode string). The other way around isn't all that transparent, unfortunately. When you ask, for example, a <TTCLASS="CLASSNAME">QMultiLineEdit</TT> widget for its contents, you get a <TTCLASS="CLASSNAME">QString</TT>. If you try to use Python's regular expression engine on this object, of if you try to write it to a file, you will be surprised at the results: </P><DIVCLASS="EXAMPLE"></A><P><B>Example 8-1. qstring1.py — conversion from <TTCLASS="CLASSNAME">QString</TT> to a Python string.</B></P><PRECLASS="PROGRAMLISTING">## qstring1.py - saving a QString to a file#from qt import *# Construct a Python stringpyString = """Now is the summer of our sweet content,Made o'er-cast winter by these Tudor clouds.And I that am not shaped for black-faced war,"""# Construct a Qt QStringqtString=QString("""I that am rudely cast and want true majesty,Am forced to fight,To set sweet England free.I pray to Heaven we fare well,And all who fight us go to Hell.""")f=open("richard", "w+")f.write(pyString)f.flush()f.write(qtString)f.close() </PRE></DIV><P>If you run this script, you'll get the following output:</P><PRECLASS="SCREEN">boud@calcifer:~/doc/opendoc/ch4 > python qstring1.pyTraceback (most recent call last): File "qstring1.py", line 26, in ? f.write(qtString)TypeError: read-only character buffer, instanceboud@calcifer:~/doc/opendoc/ch4 > </PRE><P>There are good reasons for this behavior. Returning <TTCLASS="CLASSNAME">QString</TT>s from widgets gives the developer access to all the neat Qt string handling functionality. A Qt string is mutable, in contrast to a Python string, and having the Qt <TTCLASS="CLASSNAME">QString</TT> makes it easier to change the contents in place. Lastly, returning a Qt string instead of a Python string avoids a somewhat costly conversion which might not be needed if all you want to do is to stuff the text in another Qt widget.</P><P>Of course, the downside is that if you want to treat a QString object as a Python string, you'll have to convert it yourself, using one of the Python built-in functions <TTCLASS="FUNCTION">str()</TT> or <TTCLASS="FUNCTION">unicode()</TT>. Adapting the previous script makes it work as expected:</P><DIVCLASS="EXAMPLE"></A><P><B>Example 8-2. qstring2.py - second try of saving a <TTCLASS="CLASSNAME">QString</TT> to a file</B></P><PRECLASS="PROGRAMLISTING">## qstring2.py - saving a QString to a file#from qt import *# Construct a Python stringpyString = """Now is the summer of our sweet content,Made o'er-cast winter by these Tudor clouds.And I that am not shaped for black-faced war,"""# Construct a Qt QStringqtString=QString("""I that am rudely cast and want true majesty,Am forced to fight,To set sweet England free.I pray to Heaven we fare well,And all who fight us go to Hell.""")f=open("richard", "w+")f.write(pyString)f.flush()f.write(str(qtString))f.close() </PRE></DIV><P>I don't need to show you screen output here — it just works. You will have to pay attention to what happens with the strings you receive from Qt widgets. If you want to write the contents to a file, database, or to mangle the string with Python modules, you will need to explicitly convert the QString object to Python strings. If you want to feed the string to another widget, you don't have to do anything.</P></DIV><DIVCLASS="NAVFOOTER"><HRALIGN="LEFT"WIDTH="100%"><TABLESUMMARY="Footer navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top"><A accesskey="P" href="index.lxp@lxpwrap=c2029_252ehtm.htm">Prev</A></TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><A accesskey="H" href="index.lxp@lxpwrap=book1_252ehtm">Home</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top"><A accesskey="N" href="index.lxp@lxpwrap=x2104_252ehtm.htm">Next</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">String Objects in Python and Qt</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><A accesskey="U" href="index.lxp@lxpwrap=c2029_252ehtm.htm">Up</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">QCString — simple strings in PyQt</TD></TR></TABLE></DIV></BODY></HTML> </td> </tr> </table> </td> </tr> </table> ⌨️ 快捷键说明
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