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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="node6.html" /><link rel="prev" href="node4.html" /><link rel="parent" href="tut.html" /><link rel="next" href="node6.html" /><meta name='aesop' content='information' /><title>3. Python简介 An Informal Introduction to Python </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="2. 使用Python解释器 Using the" href="node4.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="4. 深入流程控制 More Control" href="node6.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="node4.html">2. 使用Python解释器 Using the</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="node6.html">4. 深入流程控制 More Control</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="node5.html#SECTION005100000000000000000">3.1 将Python当作计算器使用 Using Python as a Calculator</a><UL><LI><A href="node5.html#SECTION005110000000000000000">3.1.1 数值 Numbers</a><LI><A href="node5.html#SECTION005120000000000000000">3.1.2 字符串 Strings</a><LI><A href="node5.html#SECTION005130000000000000000">3.1.3 Unicode 字符串 Unicode Strings</a><LI><A href="node5.html#SECTION005140000000000000000">3.1.4 链表 Lists</a></ul><LI><A href="node5.html#SECTION005200000000000000000">3.2 开始编程 First Steps Towards Programming</a></ul><!--End of Table of Child-Links--></div><HR><H1><A NAME="SECTION005000000000000000000"></A><A NAME="informal"></A><BR>3. Python简介 An Informal Introduction to Python </H1><P>In the following examples, input and output are distinguished by the
presence or absence of prompts ("<tt class="samp">><code>></code>> </tt>" and "<tt class="samp">... </tt>"): to repeat
the example, you must type everything after the prompt, when the
prompt appears; lines that do not begin with a prompt are output from
the interpreter. Note that a secondary prompt on a line by itself in an example means
you must type a blank line; this is used to end a multi-line command.<P>在后面的例子中，区分输入和输出的方法是看是否有提示符（"<tt class="samp">»> </tt>"
和"<tt class="samp">... </tt>"）：想要重现这些例子的话，你就要在提示符显示后输入所有的一切；没有以提示符开始的行，是解释器输出的信息。需要注意的是示例中的从属提示符用于多行命令的结束，它表示你需要输入一个空行。<P>Many of the examples in this manual, even those entered at the
interactive prompt, include comments. Comments in Python start with
the hash character, "<tt class="character">#</tt>", and extend to the end of the
physical line. A comment may appear at the start of a line or
following whitespace or code, but not within a string literal. A hash
character within a string literal is just a hash character.<P>本手册中的很多示例都包括注释，甚至有一些在交互提示符中折行。Python中的注释以符号
"<tt class="character">#</tt>"
起始，一直到当前行的结尾。注释可能出现在一行的开始，也可能跟在空格或程序代码之后，但不会出现在字符串中，字符串中的
"<tt class="character">#</tt>" 号只代表 "<tt class="character">#</tt>" 号。<P>Some examples:<P>示例：<P><div class="verbatim"><pre>
# this is the first comment
SPAM = 1 # and this is the second comment
# ... and now a third!
STRING = "# This is not a comment."</pre></div><P><H1><A NAME="SECTION005100000000000000000"></A><A NAME="calculator"></A><BR>3.1 将Python当作计算器使用 Using Python as a Calculator </H1><P>Let's try some simple Python commands. Start the interpreter and wait
for the primary prompt, "<tt class="samp">><code>></code>> </tt>". (It shouldn't take long.)<P>让我们试验一些简单的 Python
命令。启动解释器然后等待主提示符"<tt class="samp">><code>></code>> </tt>"出现（这用不了太久）。<P><H2><A NAME="SECTION005110000000000000000"></A><A NAME="numbers"></A><BR>3.1.1 数值 Numbers </H2><P>The interpreter acts as a simple calculator: you can type an
expression at it and it will write the value. Expression syntax is
straightforward: the operators <code>+</code>, <code>-</code>, <code>*</code> and
<code>/</code> work just like in most other languages (for example, Pascal
or C); parentheses can be used for grouping. For example:<P>解释器的行为就像是一个计算器。你可以向它输入一个表达式，它会返回结果。表达式的语法简明易懂：<code>+</code>，<code>-</code>，<code>*</code>，<code>/</code>和大多数语言中的用法一样（比如C或Pascal），括号用于分组。例如：<P><div class="verbatim"><pre>
>>> 2+2
4
>>> # This is a comment
... 2+2
4
>>> 2+2 # and a comment on the same line as code
4
>>> (50-5*6)/4
5
>>> # Integer division returns the floor:
... 7/3
2
>>> 7/-3
-3</pre></div><P>Like in C, the equal sign ("<tt class="character">=</tt>") is used to assign a value to a
variable. The value of an assignment is not written:<P>像c一样，等号（"<tt class="character">=</tt>"）用于给变量赋值。被分配的值是只读的。<P><div class="verbatim"><pre>
>>> width = 20
>>> height = 5*9
>>> width * height
900</pre></div><P>A value can be assigned to several variables simultaneously:<P>同一个值可以同时赋给几个变量：<P><div class="verbatim"><pre>
>>> x = y = z = 0 # Zero x, y and z
>>> x
0
>>> y
0
>>> z
0</pre></div><P>There is full support for floating point; operators with mixed type
operands convert the integer operand to floating point:<P>Python完全支持浮点数，不同类型的操作数混在一起时，操作符会把整型转化为浮点数。<P><div class="verbatim"><pre>
>>> 3 * 3.75 / 1.5
7.5
>>> 7.0 / 2
3.5</pre></div><P>Complex numbers are also supported; imaginary numbers are written with
a suffix of "<tt class="samp">j</tt>" or "<tt class="samp">J</tt>". Complex numbers with a nonzero
real component are written as "<tt class="samp">(<var>real</var>+<var>imag</var>j)</tt>", or can
be created with the "<tt class="samp">complex(<var>real</var>, <var>imag</var>)</tt>" function.<P>Python 也同样支持复数，虚部由一个后缀"<tt class="samp">j</tt>"或者"<tt class="samp">J</tt>"来表示。带有非零实部的复数记为"<tt class="samp"><var>real</var>+<var>imag</var>j)</tt>"，或者也可以通过"<tt class="samp">complex(<var>real</var>, <var>img</var>)</tt>"函数创建。<P><div class="verbatim"><pre>
>>> 1j * 1J
(-1+0j)
>>> 1j * complex(0,1)
(-1+0j)
>>> 3+1j*3
(3+3j)
>>> (3+1j)*3
(9+3j)
>>> (1+2j)/(1+1j)
(1.5+0.5j)</pre></div><P>Complex numbers are always represented as two floating point numbers,
the real and imaginary part. To extract these parts from a complex
number <var>z</var>, use <code><var>z</var>.real</code> and <code><var>z</var>.imag</code>.<P>复数总是由实部和虚部两部分浮点数来表示。可以从 <code><var>z</var>.real</code>
和 <code><var>z</var>.imag</code> 得到复数<var>z</var>的实部和虚部。<P><div class="verbatim"><pre>
>>> a=1.5+0.5j
>>> a.real
1.5
>>> a.imag
0.5</pre></div><P>The conversion functions to floating point and integer
(<tt class="function">float()</tt>, <tt class="function">int()</tt> and <tt class="function">long()</tt>) don't
work for complex numbers -- there is no one correct way to convert a
complex number to a real number. Use <code>abs(<var>z</var>)</code> to get its
magnitude (as a float) or <code>z.real</code> to get its real part.<P>用于向浮点数和整型转化的函数（float(), int() 和 long()）不能对复数起作用－－没有什么方法可以将复数转化为实数。可以使用abs(z)取得它的模，也可以通过z.real得到它的实部。<P><div class="verbatim"><pre>
>>> a=3.0+4.0j
>>> float(a)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: can't convert complex to float; use abs(z)
>>> a.real
3.0
>>> a.imag
4.0
>>> abs(a) # sqrt(a.real**2 + a.imag**2)
5.0
>>></pre></div><P>In interactive mode, the last printed expression is assigned to the
variable <code>_</code>. This means that when you are using Python as a
desk calculator, it is somewhat easier to continue calculations, for
example:<P>交互模式下，最近一次表达式输出保存在 <code>_</code> 变量中。这意味着把 Python
当做桌面计算器使用时，可以方便的进行连续计算，例如：<P><div class="verbatim"><pre>
>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06
>>></pre></div><P>This variable should be treated as read-only by the user. Don't
explicitly assign a value to it -- you would create an independent
local variable with the same name masking the built-in variable with
its magic behavior.<P>这个变量对于用户来说是只读的。不要试图去给它赋值－－限于 Python
的语法规则，你只会创建一个同名的局部变量覆盖它。<P><H2><A NAME="SECTION005120000000000000000"></A><A NAME="strings"></A><BR>3.1.2 字符串 Strings </H2><P>Besides numbers, Python can also manipulate strings, which can be
expressed in several ways. They can be enclosed in single quotes or
double quotes:<P>除了数值， Python
还可以通过几种不同的方法操作字符串。字符串用单引号或双引号标识：<P><div class="verbatim"><pre>
>>> 'spam eggs'
'spam eggs'
>>> 'doesn\'t'
"doesn't"
>>> "doesn't"
"doesn't"
>>> '"Yes," he said.'
'"Yes," he said.'
>>> "\"Yes,\" he said."
'"Yes," he said.'
>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'</pre></div><P>String literals can span multiple lines in several ways. Continuation
lines can be used, with a backslash as the last character on the line
indicating that the next line is a logical continuation of the line:<P>字符串可以通过几种方式分行。可以在行加反斜杠做为继续符，这表示下一行是当前行的逻辑沿续。<P><div class="verbatim"><pre>
hello = "This is a rather long string containing\n\
several lines of text just as you would do in C.\n\
Note that whitespace at the beginning of the line is\
significant."
print hello</pre></div><P>Note that newlines would still need to be embedded in the string using
<code>\n</code>; the newline following the trailing backslash is
discarded. This example would print the following:<P>注意换行用 <code>\n</code> 来表示；反斜杠后面的新行标识（newline，缩写“n”）会转换为换行符，示例会按如下格式打印：<P><div class="verbatim"><pre>
This is a rather long string containing
several lines of text just as you would do in C.
Note that whitespace at the beginning of the line is significant.</pre></div><P>If we make the string literal a ``raw'' string, however, the
<code>\n</code> sequences are not converted to newlines, but the backslash
at the end of the line, and the newline character in the source, are
both included in the string as data. Thus, the example:<P>然而，如果我们创建一个“行”("raw")字符串，<code>\
n</code>序列就不会转为换行，源码中的反斜杠和换行符n都会做为字符串中的数据处理。如下所示：<P><div class="verbatim"><pre>
hello = r"This is a rather long string containing\n\
several lines of text much as you would do in C."
print hello</pre></div><P>would print:<P>会打印为：<P><div class="verbatim"><pre>
This is a rather long string containing\n\
several lines of text much as you would do in C.</pre></div><P>Or, strings can be surrounded in a pair of matching triple-quotes:
<code>"""</code> or <code>'<code>'</code>'</code>. End of lines do not need to be escaped
when using triple-quotes, but they will be included in the string.<P>另外，字符串可以用一对三重引号”””或'''来标识。三重引号中的字符串在行尾不需要换行标记，所有的格式都会包括在字符串中。<P><div class="verbatim"><pre>
print """
Usage: thingy [OPTIONS]
-h Display this usage message
-H hostname Hostname to connect to
"""</pre></div><P>produces the following output:<P>⌨️ 快捷键说明
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