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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//CN"><html><head><link rel="STYLESHEET" href="style.css" type='text/css' /><link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" /><link rel='start' href='../index.html' title='Python Documentation Index' /><link rel="first" href="tut.html" title='Python Tutorial' /><link rel='contents' href='node2.html' title="Contents" /><link rel='last' href='about.html' title='About this document...' /><link rel='help' href='about.html' title='About this document...' /><link rel="next" href="node7.html" /><link rel="prev" href="node5.html" /><link rel="parent" href="tut.html" /><link rel="next" href="node7.html" /><meta name='aesop' content='information' /><title>4. 深入流程控制 More Control Flow Tools </title></head><body><DIV CLASS="navigation"><div id='top-navigation-panel' xml:id='top-navigation-panel'><table align="center" width="100%" cellpadding="0" cellspacing="2"><tr><td class='online-navigation'><a rel="prev" title="3. Python简介 An Informal" href="node5.html"><img src='../icons/previous.png' border='0' height='32' alt='Previous Page' width='32' /></A></td><td class='online-navigation'><a rel="parent" title="Python Tutorial" href="tut.html"><img src='../icons/up.png' border='0' height='32' alt='Up One Level' width='32' /></A></td><td class='online-navigation'><a rel="next" title="5. 数据结构 Data Structures" href="node7.html"><img src='../icons/next.png' border='0' height='32' alt='Next Page' width='32' /></A></td><td align="center" width="100%" class='online-navigation'>Python Tutorial</td><td class='online-navigation'><a rel="contents" title="Table of Contents" href="node2.html"><img src='../icons/contents.png' border='0' height='32' alt='Contents' width='32' /></A></td><td class='online-navigation'><img src='../icons/blank.png' border='0' height='32' alt='' width='32' /></td><td class='online-navigation'><img src='../icons/blank.png' border='0' height='32' alt='' width='32' /></td></tr></table><div class='online-navigation'><b class="navlabel">Previous:</b><a class="sectref" rel="prev" href="node5.html">3. Python简介 An Informal</A><b class="navlabel">Up:</b><a class="sectref" rel="parent" href="tut.html">Python Tutorial</A><b class="navlabel">Next:</b><a class="sectref" rel="next" href="node7.html">5. 数据结构 Data Structures</A></div><hr /></div></DIV><!--End of Navigation Panel--><div class='online-navigation'><!--Table of Child-Links--><A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a><UL CLASS="ChildLinks"><LI><A href="node6.html#SECTION006100000000000000000">4.1 <tt class="keyword">if</tt> 语句 <tt class="keyword">if</tt> Statements</a><LI><A href="node6.html#SECTION006200000000000000000">4.2 <tt class="keyword">for</tt> 语句 <tt class="keyword">for</tt> Statements</a><LI><A href="node6.html#SECTION006300000000000000000">4.3 <tt class="function">range()</tt> 函数 The <tt class="function">range()</tt> Function</a><LI><A href="node6.html#SECTION006400000000000000000">4.4 <tt class="keyword">break</tt> 和 <tt class="keyword">continue</tt> 语句, 以及
循环中的 <tt class="keyword">else</tt> 子句
<tt class="keyword">break</tt> and <tt class="keyword">continue</tt> Statements, and
<tt class="keyword">else</tt> Clauses on Loops</a><LI><A href="node6.html#SECTION006500000000000000000">4.5 <tt class="keyword">pass</tt> 语句 <tt class="keyword">pass</tt> Statements</a><LI><A href="node6.html#SECTION006600000000000000000">4.6 Defining Functions</a><LI><A href="node6.html#SECTION006700000000000000000">4.7 深入函数定义 More on Defining Functions</a><UL><LI><A href="node6.html#SECTION006710000000000000000">4.7.1 参数默认值 Default Argument Values</a><LI><A href="node6.html#SECTION006720000000000000000">4.7.2 关键字参数 Keyword Arguments</a><LI><A href="node6.html#SECTION006730000000000000000">4.7.3 可变参数表 Arbitrary Argument Lists</a><LI><A href="node6.html#SECTION006740000000000000000">4.7.4 参数列表的分拆 Unpacking Argument Lists</a><LI><A href="node6.html#SECTION006750000000000000000">4.7.5 Lambda 形式 Lambda Forms</a><LI><A href="node6.html#SECTION006760000000000000000">4.7.6 文档字符串 Documentation Strings</a></ul></ul><!--End of Table of Child-Links--></div><HR><H1><A NAME="SECTION006000000000000000000"></A><A NAME="moreControl"></A><BR>4. 深入流程控制 More Control Flow Tools </H1><P>Besides the <tt class="keyword">while</tt> statement just introduced, Python knows
the usual control flow statements known from other languages, with
some twists.<P>除了前面介绍的 <tt class="keyword">while</tt> 语句，Python 还从别的语言中借鉴了一些流程控制功能，并有所改变。<P><H1><A NAME="SECTION006100000000000000000"></A><A NAME="if"></A><BR>4.1 <tt class="keyword">if</tt> 语句 <tt class="keyword">if</tt> Statements </H1><P>Perhaps the most well-known statement type is the
<tt class="keyword">if</tt> statement. For example:<P>也许最有名的是 if 语句。例如：<P><div class="verbatim"><pre>
>>> x = int(raw_input("Please enter an integer: "))
>>> if x < 0:
... x = 0
... print 'Negative changed to zero'
... elif x == 0:
... print 'Zero'
... elif x == 1:
... print 'Single'
... else:
... print 'More'
...</pre></div><P>There can be zero or more <tt class="keyword">elif</tt> parts, and the
<tt class="keyword">else</tt> part is optional. The keyword `<tt class="keyword">elif</tt>' is
short for `else if', and is useful to avoid excessive indentation. An
<tt class="keyword">if</tt> ... <tt class="keyword">elif</tt> ... <tt class="keyword">elif</tt> ... sequence
is a substitute for the <tt class="keyword">switch</tt> or
<tt class="keyword">case</tt> statements found in other languages.<P>可能会有零到多个 <tt class="keyword">elif</tt> 部分，<tt class="keyword">else</tt> 是可选的。关键字“<tt class="keyword">elif</tt>” 是“ else if ”的缩写，这个可以有效避免过深的缩进。if ... elif ... elif ... 序列用于替代其它语言中的 switch 或 case 语句。<P><H1><A NAME="SECTION006200000000000000000"></A><A NAME="for"></A><BR>4.2 <tt class="keyword">for</tt> 语句 <tt class="keyword">for</tt> Statements </H1><P>The <tt class="keyword">for</tt><a id='l2h-5' xml:id='l2h-5'></a> statement in Python differs a bit from
what you may be used to in C or Pascal. Rather than always
iterating over an arithmetic progression of numbers (like in Pascal),
or giving the user the ability to define both the iteration step and
halting condition (as C), Python's
<tt class="keyword">for</tt><a id='l2h-6' xml:id='l2h-6'></a> statement iterates over the items of any
sequence (a list or a string), in the order that they appear in
the sequence. For example (no pun intended):<P>Python 中的 <tt class="keyword">for</tt><a id='l2h-7' xml:id='l2h-7'></a> 语句和 C 或 Pascal 中的略有不同。通常的循环可能会依据一个等差数值步进过程（如Pascal）或由用户来定义迭代步骤和中止条件（如 C ），Python 的 <tt class="keyword">for</tt><a id='l2h-8' xml:id='l2h-8'></a> 语句依据任意序列（链表或字符串）中的子项，按它们在序列中的顺序来进行迭代。例如（没有暗指）：<P><div class="verbatim"><pre>
>>> # Measure some strings:
... a = ['cat', 'window', 'defenestrate']
>>> for x in a:
... print x, len(x)
...
cat 3
window 6
defenestrate 12</pre></div><P>It is not safe to modify the sequence being iterated over in the loop
(this can only happen for mutable sequence types, such as lists). If
you need to modify the list you are iterating over (for example, to
duplicate selected items) you must iterate over a copy. The slice
notation makes this particularly convenient:<P>在迭代过程中修改迭代序列不安全（只有在使用链表这样的可变序列时才会有这样的情况）。如果你想要修改你迭代的序列（例如，复制选择项），你可以迭代它的复本。通常使用切片标识就可以很方便的做到这一点：<P><div class="verbatim"><pre>
>>> for x in a[:]: # make a slice copy of the entire list
... if len(x) > 6: a.insert(0, x)
...
>>> a
['defenestrate', 'cat', 'window', 'defenestrate']</pre></div><P><H1><A NAME="SECTION006300000000000000000"></A><A NAME="range"></A><BR>4.3 <tt class="function">range()</tt> 函数 The <tt class="function">range()</tt> Function </H1><P>If you do need to iterate over a sequence of numbers, the built-in
function <tt class="function">range()</tt> comes in handy. It generates lists
containing arithmetic progressions:<P>如果你需要一个数值序列，内置函数range()可能会很有用，它生成一个等差级数链表。<P><div class="verbatim"><pre>
>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]</pre></div><P>The given end point is never part of the generated list;
<code>range(10)</code> generates a list of 10 values, exactly the legal
indices for items of a sequence of length 10. It is possible to let
the range start at another number, or to specify a different increment
(even negative; sometimes this is called the `step'):<P><code>range(10)</code> 生成了一个包含10个值的链表，它准确的用链表的索引值填充了这个长度为10的列表，所生成的链表中不包括范围中的结束值。也可以让range操作从另一个数值开始，或者可以指定一个不同的步进值（甚至是负数，有时这也被称为“步长”）：<P><div class="verbatim"><pre>
>>> range(5, 10)
[5, 6, 7, 8, 9]
>>> range(0, 10, 3)
[0, 3, 6, 9]
>>> range(-10, -100, -30)
[-10, -40, -70]</pre></div><P>To iterate over the indices of a sequence, combine
<tt class="function">range()</tt> and <tt class="function">len()</tt> as follows:<P>需要迭代链表索引的话，如下所示结合使 用<tt class="function">range()</tt> 和 <tt class="function">len()</tt> ：<P><div class="verbatim"><pre>
>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)):
... print i, a[i]
...
0 Mary
1 had
2 a
3 little
4 lamb</pre></div><P><H1><A NAME="SECTION006400000000000000000"></A><A NAME="break"></A><BR>4.4 <tt class="keyword">break</tt> 和 <tt class="keyword">continue</tt> 语句, 以及
循环中的 <tt class="keyword">else</tt> 子句
<tt class="keyword">break</tt> and <tt class="keyword">continue</tt> Statements, and
<tt class="keyword">else</tt> Clauses on Loops
</H1><P>The <tt class="keyword">break</tt> statement, like in C, breaks out of the smallest
enclosing <tt class="keyword">for</tt> or <tt class="keyword">while</tt> loop.<P>break 语句和 C 中的类似，用于跳出最近的一级 <tt class="keyword">for</tt> 或 <tt class="keyword">while</tt> 循环。<P>The <tt class="keyword">continue</tt> statement, also borrowed from C, continues
with the next iteration of the loop.<P><tt class="keyword">continue</tt> 语句是从 C 中借鉴来的，它表示循环继续执行下一次迭代。<P>Loop statements may have an <code>else</code> clause; it is executed when
the loop terminates through exhaustion of the list (with
<tt class="keyword">for</tt>) or when the condition becomes false (with
<tt class="keyword">while</tt>), but not when the loop is terminated by a
<tt class="keyword">break</tt> statement. This is exemplified by the following loop,
which searches for prime numbers:<P>循环可以有一个 <code>else</code> 子句;它在循环迭代完整个列表（对于 <tt class="keyword">for</tt> ）或执行条件为 false （对于 <tt class="keyword">while</tt> ）时执行，但循环被 <tt class="keyword">break</tt> 中止的情况下不会执行。以下搜索素数的示例程序演示了这个子句：<P><div class="verbatim"><pre>
>>> for n in range(2, 10):
... for x in range(2, n):
... if n % x == 0:
... print n, 'equals', x, '*', n/x
... break
... else:
... # loop fell through without finding a factor
... print n, 'is a prime number'
...
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3</pre></div><P><H1><A NAME="SECTION006500000000000000000"></A><A NAME="pass"></A><BR>4.5 <tt class="keyword">pass</tt> 语句 <tt class="keyword">pass</tt> Statements </H1><P>The <tt class="keyword">pass</tt> statement does nothing.
It can be used when a statement is required syntactically but the
program requires no action.
For example:<P><tt class="keyword">pass</tt> 语句什么也不做。它用于那些语法上必须要有什么语句，但程序什么也不做的场合，例如：<P><div class="verbatim"><pre>
>>> while True:
... pass # Busy-wait for keyboard interrupt
...</pre></div><P><H1><A NAME="SECTION006600000000000000000"></A><A NAME="functions"></A><BR>4.6 Defining Functions </H1><P>⌨️ 快捷键说明
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