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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>Constructions</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="Introduction to Python"HREF="c653.htm"><LINKREL="PREVIOUS"TITLE="The Rules"HREF="x719.htm"><LINKREL="NEXT"TITLE="Conclusion"HREF="x879.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=x719_252ehtm.htm">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom">Chapter 4. Introduction to Python</TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><A accesskey="N" href="index.lxp@lxpwrap=x879_252ehtm.htm">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="SECT1"><H1CLASS="SECT1">Constructions</A></H1><P>Python, like all languages, gives you constructions for looping, branching and jumping. In addition, since Python 2.2, you can also use iterators and generators.</P><DIVCLASS="SECT2"><H2CLASS="SECT2">Looping</A></H2><P>You do not use counters to loop in Python. Rather, you use sequences of objects to loop over. Those objects can of course also be be numbers, generated by either <TTCLASS="FUNCTION">range</TT> or <TTCLASS="FUNCTION">xrange</TT>:</P><PRECLASS="SCREEN">Python 2.1.1 (#1, Aug 11 2001, 20:14:53)[GCC 2.95.2 19991024 (release)] on linux2Type "copyright", "credits" or "license" for more information.>>> aList=["a","b","c"]>>> for item in aList:... print item...abc>>> for counter in range(3):... print counter...012>>> </PRE><P>Another loop repeats a block of statements while a certain expression evaluates to true:</P><PRECLASS="SCREEN">>>> a=0>>> while a < 3:... print a... a+=1...012 </PRE><P>The <TTCLASS="FUNCTION">break</TT> statement breaks execution out of a loop; the <TTCLASS="FUNCTION">continue</TT> statement continues immediately with the next iteration.</P><P>Iterators define a <TTCLASS="FUNCTION">__iter__</TT> and a <TTCLASS="FUNCTION">next()</TT> function to allow easy looping:</P><PRECLASS="PROGRAMLISTING"># iter.py - an iteratorclass MyIterator: def __init__(self, start): self.start = start def __iter__(self): return self def next(self): if self.start < 10: self.start += 1 return self.start else: raise StopIterationfor i in MyIterator(1): print i </PRE><P>Generators are functions that return a function that can yield a result part-way compution, and resume later:</P><PRECLASS="PROGRAMLISTING"># generator.pyfrom __future__ import generatorsdef MyGenerator(): count = 0 while count < 10: yield count count += 1gen = MyGenerator()try: while 1: print gen.next()except StopIteration: print "finished" </PRE><P>Note how <TTCLASS="FUNCTION">yield</TT> returns the number, but <TTCLASS="VARNAME">count</TT> is still increased.</P></DIV><DIVCLASS="SECT2"><H2CLASS="SECT2">Branching</A></H2><P>The number zero, empty lists, dictionaries, tuples and the object <TTCLASS="VARNAME">None</TT> all evaluate to false; (almost) everything else is true. You create branches using the <TTCLASS="FUNCTION">if</TT> statement.</P><PRECLASS="SCREEN">>>> a=1>>> if a:... print "true"... else:... print "false"...true>>> if a==0:... print "a was zero"... elif a == None:... print "a was none"... else:... print "a was zero nor none"...a was zero nor none </PRE><P>The operator <TTCLASS="FUNCTION">==</TT> tests for equality, while <TTCLASS="FUNCTION">!=</TT> (or the deprecated <TTCLASS="FUNCTION"><></TT>) tests for inequality. The operator <TTCLASS="FUNCTION">is</TT> tests for identity: that is, whether two references point to (unless you <TTCLASS="LITERAL">import division from future</TT> in Python 2.2) the same object:</P><PRECLASS="SCREEN">>>> from qt import *>>> a=QString("bla")>>> b=QString("bla")>>> a is b0>>> c=a>>> a is c1>>> a="bla">>> b="bla">>> a is b1>>> id(a)135455928>>> id(b)135455928 </PRE><P>As you can see, Python does some optimizations that reuse the same string object if the string contents are the same.</P></DIV><DIVCLASS="SECT2"><H2CLASS="SECT2">Exceptions</A></H2><P>As every modern programming language must have, Python contains an error catching construction. This is the try: ... except... construction.</P><PRECLASS="SCREEN">>>> try:... 1/0... except ZeroDivisionError:... print "Zerodivisionerror"...Zerodivisionerror </PRE><P>You can also create your own exceptions that can carry significant data about the causes of the error:</P><PRECLASS="SCREEN">>>> class BlaError:... def __init__(self, value):... self.value = value... def __str__(self):... return repr(self.value)>>> try:... raise BlaError("Bla happened - that's bad!")... except BlaError, error:... print error...Bla happened - that's bad! </PRE><P>If you want to catch several different exceptions, you have to create a tuple of all the exceptions you want to catch:</P><PRECLASS="SCREEN">>>> try:... print "bla"... except (ValueError, ZeroDivisionError):... print "that's bad"... bla </PRE><P>Finally, you can define something that should happen when all errors have been handled in the <TTCLASS="FUNCTION">finally</TT> block:</P><PRECLASS="SCREEN">>>> try:... 1/0... finally:... print "finally"...finallyTraceback (most recent call last): File "<stdin>", line 2, in ?ZeroDivisionError: integer division or modulo by zero </PRE></DIV><DIVCLASS="SECT2"><H2CLASS="SECT2">Classes</A></H2><P>Classes are defined with the <TTCLASS="FUNCTION">class</TT> keyword. Python classes can inherit from zero, one, or more other classes, but from only one PyQt class.</P><P>Classes are initialized using the code in the <TTCLASS="FUNCTION">__init__</TT> method. There are other special methods, like <TTCLASS="FUNCTION">__str__</TT>, which should return a string representation of the class. Consult the Python language reference for a complete list of these.</P><PRECLASS="SCREEN">>>>class A:pass...>>> class B(A):... def __init__(self, val):... self.val = val... def __str__(self):... return str(self.val)...>>> b=B(10)>>> print b10>>> </PRE></DIV></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=x719_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=x879_252ehtm.htm">Next</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">The Rules</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><A accesskey="U" href="index.lxp@lxpwrap=c653_252ehtm.htm">Up</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">Conclusion</TD></TR></TABLE></DIV></BODY></HTML> </td> </tr> </table> </td> </tr> </table> ⌨️ 快捷键说明
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