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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="node10.html" /><link rel="prev" href="node8.html" /><link rel="parent" href="tut.html" /><link rel="next" href="node10.html" /><meta name='aesop' content='information' /><title>7. 输入和输出 Input and Output </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="6. 模块 Modules" href="node8.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="8. 错误和异常 Errors and" href="node10.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="node8.html">6. 模块 Modules</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="node10.html">8. 错误和异常 Errors and</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="node9.html#SECTION009100000000000000000">7.1 设计输出格式 Fancier Output Formatting</a><LI><A href="node9.html#SECTION009200000000000000000">7.2 读写文件 Reading and Writing Files</a><UL><LI><A href="node9.html#SECTION009210000000000000000">7.2.1 文件对象（file object）的方法 Methods of File Objects</a><LI><A href="node9.html#SECTION009220000000000000000">7.2.2 <tt class="module">pickle</tt> 模块 <tt class="module">pickle</tt> Module</a></ul></ul><!--End of Table of Child-Links--></div><HR><H1><A NAME="SECTION009000000000000000000"></A><A NAME="io"></A><BR>7. 输入和输出 Input and Output </H1><P>There are several ways to present the output of a program; data can be
printed in a human-readable form, or written to a file for future use.
This chapter will discuss some of the possibilities.<P>有几种方法可以表现程序的输出结果；数据可以用可读的结构打印，也可以写入文件供以后使用。本章将会讨论几种可行的做法。<P><H1><A NAME="SECTION009100000000000000000"></A><A NAME="formatting"></A><BR>7.1 设计输出格式 Fancier Output Formatting </H1><P>So far we've encountered two ways of writing values: <em>expression
statements</em> and the <tt class="keyword">print</tt> statement. (A third way is using
the <tt class="method">write()</tt> method of file objects; the standard output file
can be referenced as <code>sys.stdout</code>. See the Library Reference for
more information on this.)<P>我们有两种大相径庭的输出值方法：<em>表达式语句</em>和 <tt class="keyword">print</tt> 语句。（第三种访求是使用文件对象的 <tt class="method">write()</tt> 方法，标准文件输出可以参考 <code>sys.stdout</code>。详细内容参见库参考手册。）<P>Often you'll want more control over the formatting of your output than
simply printing space-separated values. There are two ways to format
your output; the first way is to do all the string handling yourself;
using string slicing and concatenation operations you can create any
lay-out you can imagine. The standard module
<tt class="module">string</tt><a id='l2h-43' xml:id='l2h-43'></a> contains some useful operations
for padding strings to a given column width; these will be discussed
shortly. The second way is to use the <code>%</code> operator with a
string as the left argument. The <code>%</code> operator interprets the
left argument much like a <tt class="cfunction">sprintf()</tt>-style format
string to be applied to the right argument, and returns the string
resulting from this formatting operation.<P>可能你经常想要对输出格式做一些比简单的打印空格分隔符更为复杂的控制。有两种方法可以格式化输出。第一种是由你来控制整个字符串，使用字符切片和联接操作就可以创建出任何你想要的输出形式。标准模块 <tt class="module">string</tt><a id='l2h-44' xml:id='l2h-44'></a> 包括了一些操作，将字符串填充入给定列时，这些操作很有用。随后我们会讨论这部分内容。第二种方法是使用 <code>%</code> 操作符，以某个字符串做为其左参数。 <code>%</code> 操作符将左参数解释为类似于 <tt class="cfunction">sprintf()</tt> 风格的格式字符串，并作用于右参数，从该操作中返回格式化的字符串。<P>One question remains, of course: how do you convert values to strings?
Luckily, Python has ways to convert any value to a string: pass it to
the <tt class="function">repr()</tt> or <tt class="function">str()</tt> functions. Reverse quotes
(<code>``</code>) are equivalent to <tt class="function">repr()</tt>, but their use is
discouraged.<P>当然，还有一个问题，如何将（不同的）值转化为字符串？很幸运，Python总是把任意值传入 <tt class="function">repr()</tt> 或 <tt class="function">str()</tt> 函数，转为字符串。相对而言引号 (<code>``</code>)等价于<tt class="function">repr()</tt>，不过不提倡这样用。<P>The <tt class="function">str()</tt> function is meant to return representations of
values which are fairly human-readable, while <tt class="function">repr()</tt> is
meant to generate representations which can be read by the interpreter
(or will force a <tt class="exception">SyntaxError</tt> if there is not equivalent
syntax). For objects which don't have a particular representation for
human consumption, <tt class="function">str()</tt> will return the same value as
<tt class="function">repr()</tt>. Many values, such as numbers or structures like
lists and dictionaries, have the same representation using either
function. Strings and floating point numbers, in particular, have two
distinct representations.<P>函数 <tt class="function">str()</tt> 用于将值转化为适于人阅读的形式，而 <tt class="function">repr()</tt> 转化为供解释器读取的形式（如果没有等价的语法，则会发生 <tt class="exception">SyntaxError</tt> 异常） 某对象没有适于人阅读的解释形式的话， <tt class="function">str()</tt> 会返回与 <tt class="function">repr()</tt> 等同的值。很多类型，诸如数值或链表、字典这样的结构，针对各函数都有着统一的解读方式。字符串和浮点数，有着独特的解读方式。<P>Some examples:<P><div class="verbatim"><pre>
>>> s = 'Hello, world.'
>>> str(s)
'Hello, world.'
>>> repr(s)
"'Hello, world.'"
>>> str(0.1)
'0.1'
>>> repr(0.1)
'0.10000000000000001'
>>> x = 10 * 3.25
>>> y = 200 * 200
>>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
>>> print s
The value of x is 32.5, and y is 40000...
>>> # The repr() of a string adds string quotes and backslashes:
... hello = 'hello, world\n'
>>> hellos = repr(hello)
>>> print hellos
'hello, world\n'
>>> # The argument to repr() may be any Python object:
... repr((x, y, ('spam', 'eggs')))
"(32.5, 40000, ('spam', 'eggs'))"
>>> # reverse quotes are convenient in interactive sessions:
... `x, y, ('spam', 'eggs')`
"(32.5, 40000, ('spam', 'eggs'))"</pre></div><P>Here are two ways to write a table of squares and cubes:<P>以下两种方法可以输出平方和立方表：<P><div class="verbatim"><pre>
>>> for x in range(1, 11):
... print repr(x).rjust(2), repr(x*x).rjust(3),
... # Note trailing comma on previous line
... print repr(x*x*x).rjust(4)
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
>>> for x in range(1,11):
... print '%2d %3d %4d' % (x, x*x, x*x*x)
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000</pre></div><P>(Note that one space between each column was added by the way
<tt class="keyword">print</tt> works: it always adds spaces between its arguments.)<P>（需要注意的是使用 <tt class="keyword">print</tt> 方法时每两列之间有一个空格：它总是在参数之间加一个空格。）<P>This example demonstrates the <tt class="method">rjust()</tt> method of string objects,
which right-justifies a string in a field of a given width by padding
it with spaces on the left. There are similar methods
<tt class="method">ljust()</tt> and <tt class="method">center()</tt>. These
methods do not write anything, they just return a new string. If
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