3. An Informal Introduction to Python

3. Python 的非正式介绍

In the following examples, input and output are distinguished by the presence or absence of prompts (>>> and ...): 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.
在下面的例子中，通过提示符 (>>> 与 ...) 的出现与否来区分输入和输出：如果你想复现这些例子，当提示符出现后，你必须在提示符后键入例子中的每一个词；不以提示符开头的那些行是解释器的输出。注意例子中某行中出现第二个提示符意味着你必须键入一个空白行；这是用来结束多行命令的。

Many of the examples in this manual, even those entered at the interactive prompt, include comments. Comments in Python start with the hash character, "#", 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. Since comments are to clarify code and are not interpreted by Python, they may be omitted when typing in examples.
这个手册中的许多例子都包含注释，甚至交互性命令中也有。Python中的注释以井号 "#" 开头，并且一直延伸到该文本行结束为止。注释可以出现在一行的开头或者是空白和代码的后边，但是不能出现在字符串中间。字符串中的井号就是井号。因为注释是用来阐明代码的，不会被 Python 解释，所以在键入这些例子时，注释是可以被忽略的。

Some examples:
几个例子:

   # this is the first comment 
   spam = 1  # and this is the second comment 
             # ... and now a third! 
   text = "# This is not a comment because it's inside quotes."

3.1. Using Python as a Calculator

3.1. Python 作为计算器使用

Let's try some simple Python commands. Start the interpreter and wait for the primary prompt, ">>>". (It shouldn't take long.)
让我们尝试一些简单的 Python 命令。启动解释器，等待界面中的提示符，">>>" （这应该花不了多少时间）。

3.1.1. Numbers

3.1.1. 数字

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 "+", "-", "*" and "/" work just like in most other languages (for example, Pascal or C); parentheses ("()") can be used for grouping. For example:
解释器就像一个简单的计算器一样：你可以在里面输入一个表达式然后它会写出答案。表达式的语法很直接：运算符 "+"、"-"、"*"、"/" 的用法和其他大部分语言一样（比如 Pascal 或者 C 语言）；括号 ("()") 用来分组。比如:

   >>> 2 + 2 
   4 
   >>> 50 - 5*6 
   20 
   >>> (50 - 5*6) / 4 
   5.0 
   >>> 8 / 5  # division always returns a floating point number 
   1.6

The integer numbers (e.g. "2", "4", "20") have type "int", the ones with a fractional part (e.g. "5.0", "1.6") have type "float". We will see more about numeric types later in the tutorial.
整数（比如 "2"、"4"、"20" ）有 "int" 类型，有小数部分的（比如 "5.0"、"1.6" ）有 "float" 类型。在这个手册的后半部分我们会看到更多的数值类型。

Division ("/") always returns a float. To do floor division and get an integer result (discarding any fractional result) you can use the "//" operator; to calculate the remainder you can use "%":
除法运算 ("/") 永远返回浮点数类型。如果要做 floor division 得到一个整数结果（忽略小数部分）你可以使用 "//" 运算符；如果要计算余数，可以使用 "%"

   >>> 17 / 3  # classic division returns a float 
   5.666666666666667 
   >>> 
   >>> 17 // 3  # floor division discards the fractional part 
   5 
   >>> 17 % 3  # the % operator returns the remainder of the division 
   2 
   >>> 5 * 3 + 2  # result * divisor + remainder 
   17

With Python, it is possible to use the "**" operator to calculate powers [1]:
在Python中，可以使用 "**" 运算符来计算乘方 [1]

   >>> 5 ** 2  # 5 squared 
   25 
   >>> 2 ** 7  # 2 to the power of 7 
   128

The equal sign ("=") is used to assign a value to a variable. Afterwards, no result is displayed before the next interactive prompt:
等号 ("=") 用于给一个变量赋值。然后在下一个交互提示符之前不会有结果显示出来:

   >>> width = 20 
   >>> height = 5 * 9 
   >>> width * height 
   900

If a variable is not "defined" (assigned a value), trying to use it will give you an error:
如果一个变量未定义(未赋值)，试图使用它时会向你提示错误:

   >>> n  # try to access an undefined variable 
   Traceback (most recent call last): 
     File "", line 1, in  
   NameError: name 'n' is not defined

There is full support for floating point; operators with mixed type operands convert the integer operand to floating point:
Python中提供浮点数的完整支持；包含多种混合类型运算数的运算会把整数转换为浮点数:

   >>> 4 * 3.75 - 1 
   14.0

In interactive mode, the last printed expression is assigned to the variable "_". This means that when you are using Python as a desk calculator, it is somewhat easier to continue calculations, for example:
在交互模式下，上一次打印出来的表达式被赋值给变量 "_"。这意味着当你把Python用作桌面计算器时，继续计算会相对简单，比如:

   >>> tax = 12.5 / 100 
   >>> price = 100.50 
   >>> price * tax 
   12.5625 
   >>> price + _ 
   113.0625 
   >>> round(_, 2) 
   113.06

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.
这个变量应该被使用者当作是只读类型。不要向它显式地赋值——你会创建一个和它名字相同独立的本地变量，它会使用魔法行为屏蔽内部变量。

In addition to "int" and "float", Python supports other types of numbers, such as "Decimal" and "Fraction". Python also has built-in support for complex numbers, and uses the "j" or "J" suffix to indicate the imaginary part (e.g. "3+5j").
除了 "int" 和 "float"，Python也支持其他类型的数字，例如 "Decimal" 或者"Fraction"。Python 也内置对复数的支持，使用后缀 "j" 或者 "J" 就可以表示虚数部分（例如 "3+5j" ）。

3.1.2. Strings

3.1.2. 字符串

Besides numbers, Python can also manipulate strings, which can be expressed in several ways. They can be enclosed in single quotes ("'...'") or double quotes (""..."") with the same result [2]. "\" can be used to escape quotes:
除了数字，Python 也可以操作字符串。字符串有多种形式，可以使用单引号（"'……'"），双引号（""……""）都可以获得同样的结果 [2]。反斜杠 "\" 可以用来转义:

   >>> 'spam eggs'  # single quotes 
   'spam eggs' 
   >>> 'doesn\'t'  # use \' to escape the single quote... 
   "doesn't" 
   >>> "doesn't"  # ...or use double quotes instead 
   "doesn't" 
   >>> '"Yes," they said.' 
   '"Yes," they said.' 
   >>> "\"Yes,\" they said." 
   '"Yes," they said.' 
   >>> '"Isn\'t," they said.' 
   '"Isn\'t," they said.'

In the interactive interpreter, the output string is enclosed in quotes and special characters are escaped with backslashes. While this might sometimes look different from the input (the enclosing quotes could change), the two strings are equivalent. The string is enclosed in double quotes if the string contains a single quote and no double quotes, otherwise it is enclosed in single quotes. The "print()" function produces a more readable output, by omitting the enclosing quotes and by printing escaped and special characters:
在交互式解释器中，输出的字符串外面会加上引号，特殊字符会使用反斜杠来转义。虽然有时这看起来会与输入不一样（外面所加的引号可能会改变），但两个字符串是相同的。如果字符串中有单引号而没有双引号，该字符串外将加双引号来表示，否则就加单引号。 "print()" 函数会生成可读性更强的输出，即略去两边的引号，并且打印出经过转义的特殊字符:

   >>> '"Isn\'t," they said.' 
   '"Isn\'t," they said.' 
   >>> print('"Isn\'t," they said.') 
   "Isn't," they said. 
   >>> s = 'First line.\nSecond line.'  # \n means newline 
   >>> s  # without print(), \n is included in the output 
   'First line.\nSecond line.' 
   >>> print(s)  # with print(), \n produces a new line 
   First line. 
   Second line.

If you don't want characters prefaced by "\" to be interpreted as special characters, you can use raw strings by adding an "r" before the first quote:
如果你不希望前置了 "\" 的字符转义成特殊字符，可以使用 原始字符串 方式，在引号前添加 "r" 即可:

   >>> print('C:\some\name')  # here \n means newline! 
   C:\some 
   ame 
   >>> print(r'C:\some\name')  # note the r before the quote 
   C:\some\name

String literals can span multiple lines. One way is using triple- quotes: """"..."""" or "'''...'''". End of lines are automatically included in the string, but it's possible to prevent this by adding a "\" at the end of the line. The following example:
字符串字面值可以跨行连续输入。一种方式是用三重引号：""""..."""" 或"'''...'''"。字符串中的回车换行会自动包含到字符串中，如果不想包含，在行尾添加一个 "\" 即可。如下例:

   print("""\ 
   Usage: thingy [OPTIONS] 
        -h                        Display this usage message 
        -H hostname               Hostname to connect to 
   """)

produces the following output (note that the initial newline is not included):
将产生如下输出（注意最开始的换行没有包括进来）:

   Usage: thingy [OPTIONS] 
        -h                        Display this usage message 
        -H hostname               Hostname to connect to

Strings can be concatenated (glued together) with the "+" operator, and repeated with "*":
字符串可以用 "+" 进行连接（粘到一起），也可以用 "*" 进行重复:

   >>> # 3 times 'un', followed by 'ium' 
   >>> 3 * 'un' + 'ium' 
   'unununium'

Two or more string literals (i.e. the ones enclosed between quotes) next to each other are automatically concatenated.
相邻的两个或多个 字符串字面值 （引号引起来的字符）将会自动连接到一起.

   >>> 'Py' 'thon' 
   'Python'

This feature is particularly useful when you want to break long strings:
把很长的字符串拆开分别输入的时候尤其有用:

   >>> text = ('Put several strings within parentheses ' 
   ...         'to have them joined together.') 
   >>> text 
   'Put several strings within parentheses to have them joined together.'

This only works with two literals though, not with variables or expressions:
只能对两个字面值这样操作，变量或表达式不行:

   >>> prefix = 'Py' 
   >>> prefix 'thon'  # can't concatenate a variable and a string literal 
     File "", line 1 
       prefix 'thon' 
                   ^ 
   SyntaxError: invalid syntax 
   >>> ('un' * 3) 'ium' 
     File "", line 1 
       ('un' * 3) 'ium' 
                      ^ 
   SyntaxError: invalid syntax

If you want to concatenate variables or a variable and a literal, use "+":
如果你想连接变量，或者连接变量和字面值，可以用 "+" 号:

   >>> prefix + 'thon' 
   'Python'

Strings can be indexed (subscripted), with the first character having index 0. There is no separate character type; a character is simply a string of size one:
字符串是可以被索引（下标访问）的，第一个字符索引是 0。单个字符并没有特殊的类型，只是一个长度为一的字符串:

   >>> word = 'Python' 
   >>> word[0]  # character in position 0 
   'P' 
   >>> word[5]  # character in position 5 
   'n'

Indices may also be negative numbers, to start counting from the right:
索引也可以用负数，这种会从右边开始数:

   >>> word[-1]  # last character 
   'n' 
   >>> word[-2]  # second-last character 
   'o' 
   >>> word[-6] 
   'P'

Note that since -0 is the same as 0, negative indices start from -1.
注意 -0 和 0 是一样的，所以负数索引从 -1 开始。

In addition to indexing, slicing is also supported. While indexing is used to obtain individual characters, slicing allows you to obtain substring:
除了索引，字符串还支持切片。索引可以得到单个字符，而切片可以获取子字符串:

   >>> word[0:2]  # characters from position 0 (included) to 2 (excluded) 
   'Py' 
   >>> word[2:5]  # characters from position 2 (included) to 5 (excluded) 
   'tho'

Note how the start is always included, and the end always excluded. This makes sure that "s[:i] + s[i:]" is always equal to "s":
注意切片的开始总是被包括在结果中，而结束不被包括。这使得 "s[:i] +s[i:]" 总是等于 "s"

   >>> word[:2] + word[2:] 
   'Python' 
   >>> word[:4] + word[4:] 
   'Python'

Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.
切片的索引有默认值；省略开始索引时默认为0，省略结束索引时默认为到字符串的结束:

   >>> word[:2]   # character from the beginning to position 2 (excluded) 
   'Py' 
   >>> word[4:]   # characters from position 4 (included) to the end 
   'on' 
   >>> word[-2:]  # characters from the second-last (included) to the end 
   'on'

One way to remember how slices work is to think of the indices as pointing between characters, with the left edge of the first character numbered 0. Then the right edge of the last character of a string of n characters has index n, for example:
您也可以这么理解切片：将索引视作指向字符之间，第一个字符的左侧标为0，最后一个字符的右侧标为 n ，其中 n 是字符串长度。例如:

    +---+---+---+---+---+---+ 
    | P | y | t | h | o | n | 
    +---+---+---+---+---+---+ 
    0   1   2   3   4   5   6 
   -6  -5  -4  -3  -2  -1

The first row of numbers gives the position of the indices 0...6 in the string; the second row gives the corresponding negative indices. The slice from i to j consists of all characters between the edges labeled i and j, respectively.
第一行数标注了字符串非负的索引的位置，第二行标注了对应的负的索引。那么从 i 到 j 的切片就包括了标有 i 和 j 的位置之间的所有字符。

For non-negative indices, the length of a slice is the difference of the indices, if both are within bounds. For example, the length of "word[1:3]" is 2.
对于使用非负索引的切片，如果索引不越界，那么得到的切片长度就是起止索引之差。例如， "word[1:3]" 的长度为2.

Attempting to use an index that is too large will result in an error:
使用过大的索引会产生一个错误:

   >>> word[42]  # the word only has 6 characters 
   Traceback (most recent call last): 
     File "", line 1, in  
   IndexError: string index out of range

However, out of range slice indexes are handled gracefully when used for slicing:
但是，切片中的越界索引会被自动处理:

   >>> word[4:42] 
   'on' 
   >>> word[42:] 
   ''

Python strings cannot be changed --- they are immutable. Therefore, assigning to an indexed position in the string results in an error:
Python 中的字符串不能被修改，它们是 immutable 的。因此，向字符串的某个索引位置赋值会产生一个错误:

   >>> word[0] = 'J' 
   Traceback (most recent call last): 
     File "", line 1, in  
   TypeError: 'str' object does not support item assignment 
   >>> word[2:] = 'py' 
   Traceback (most recent call last): 
     File "", line 1, in  
   TypeError: 'str' object does not support item assignment

If you need a different string, you should create a new one:
如果需要一个不同的字符串，应当新建一个:

   >>> 'J' + word[1:] 
   'Jython' 
   >>> word[:2] + 'py' 
   'Pypy'

The built-in function "len()" returns the length of a string:
内建函数 "len()" 返回一个字符串的长度:

   >>> s = 'supercalifragilisticexpialidocious' 
   >>> len(s) 
   34

See also:
参见:

Text Sequence Type --- str

     Strings are examples of *sequence types*, and support the common 
     operations supported by such types. 
 
文本序列类型 --- str     字符串是一种 *序列类型* ，因此也支持序列类型的各种操作。 
 
  String Methods      Strings support a large number of methods for basic 
     transformations and searching. 
 
字符串的方法     字符串支持许多变换和查找的方法。 
 
  Formatted string literals      String literals that have embedded expressions. 
 
格式化字符串字面值     内嵌表达式的字符串字面值。 
 
  Format String Syntax      Information about string formatting with "str.format()". 
 
格式字符串语法     使用 "str.format()" 进行字符串格式化。 
 
  printf-style String Formatting      The old formatting operations invoked when strings are the left 
     operand of the "%" operator are described in more detail here. 
 
printf 风格的字符串格式化     这里详述了使用 "%" 运算符进行字符串格式化。 
 
 
 3.1.3. Lists

3.1.3. 列表

Python knows a number of compound data types, used to group together other values. The most versatile is the list, which can be written as a list of comma-separated values (items) between square brackets. Lists might contain items of different types, but usually the items all have the same type.
Python 中可以通过组合一些值得到多种复合数据类型。其中最常用的列表，可以通过方括号括起、逗号分隔的一组值得到。一个列表可以包含不同类型的元素，但通常使用时各个元素类型相同:

   >>> squares = [1, 4, 9, 16, 25] 
   >>> squares 
   [1, 4, 9, 16, 25]

Like strings (and all other built-in sequence types), lists can be indexed and sliced:
和字符串（以及各种内置的 sequence 类型）一样，列表也支持索引和切片:

   >>> squares[0]  # indexing returns the item 
   1 
   >>> squares[-1] 
   25 
   >>> squares[-3:]  # slicing returns a new list 
   [9, 16, 25]

All slice operations return a new list containing the requested elements. This means that the following slice returns a shallow copy of the list:
All slice operations return a new list containing the requestedelements. This means that the following slice returns a shallow copyof the list:

   >>> squares[:] 
   [1, 4, 9, 16, 25]

Lists also support operations like concatenation:
列表同样支持拼接操作:

   >>> squares + [36, 49, 64, 81, 100] 
   [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Unlike strings, which are immutable, lists are a mutable type, i.e. it is possible to change their content:
与 immutable 的字符串不同, 列表是一个 mutable 类型，就是说，它自己的内容可以改变:

   >>> cubes = [1, 8, 27, 65, 125]  # something's wrong here 
   >>> 4 ** 3  # the cube of 4 is 64, not 65! 
   64 
   >>> cubes[3] = 64  # replace the wrong value 
   >>> cubes 
   [1, 8, 27, 64, 125]

You can also add new items at the end of the list, by using the "append()" method (we will see more about methods later):
你也可以在列表结尾，通过 "append()" 方法添加新元素 (我们会在后面解释更多关于方法的内容):

   >>> cubes.append(216)  # add the cube of 6 
   >>> cubes.append(7 ** 3)  # and the cube of 7 
   >>> cubes 
   [1, 8, 27, 64, 125, 216, 343]

Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:
给切片赋值也是可以的，这样甚至可以改变列表大小，或者把列表整个清空:

   >>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g'] 
   >>> letters 
   ['a', 'b', 'c', 'd', 'e', 'f', 'g'] 
   >>> # replace some values 
   >>> letters[2:5] = ['C', 'D', 'E'] 
   >>> letters 
   ['a', 'b', 'C', 'D', 'E', 'f', 'g'] 
   >>> # now remove them 
   >>> letters[2:5] = [] 
   >>> letters 
   ['a', 'b', 'f', 'g'] 
   >>> # clear the list by replacing all the elements with an empty list 
   >>> letters[:] = [] 
   >>> letters 
   []

The built-in function "len()" also applies to lists:
内置函数 "len()" 也可以作用到列表上:

   >>> letters = ['a', 'b', 'c', 'd'] 
   >>> len(letters) 
   4

It is possible to nest lists (create lists containing other lists), for example:
也可以嵌套列表 (创建包含其他列表的列表), 比如说:

   >>> a = ['a', 'b', 'c'] 
   >>> n = [1, 2, 3] 
   >>> x = [a, n] 
   >>> x 
   [['a', 'b', 'c'], [1, 2, 3]] 
   >>> x[0] 
   ['a', 'b', 'c'] 
   >>> x[0][1] 
   'b'

3.2. First Steps Towards Programming

3.2. 走向编程的第一步

Of course, we can use Python for more complicated tasks than adding two and two together. For instance, we can write an initial sub- sequence of the Fibonacci series as follows:
当然，我们可以将 Python 用于更复杂的任务，而不是仅仅两个和两个一起添加。例如，我们可以编写斐波那契数列的初始子序列，如下所示:

   >>> # Fibonacci series: 
   ... # the sum of two elements defines the next 
   ... a, b = 0, 1 
   >>> while a < 10: 
   ...     print(a) 
   ...     a, b = b, a+b 
   ... 
   0 
   1 
   1 
   2 
   3 
   5 
   8

This example introduces several new features.
这个例子引入了几个新的特点。

• The first line contains a multiple assignment: the variables "a" and "b" simultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right- hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.
• 第一行含有一个 多重赋值: 变量 "a" 和 "b" 同时得到了新值 0 和 1.最后一行又用了一次多重赋值, 这体现出了右手边的表达式，在任何赋值发生之前就被求值了。右手边的表达式是从左到右被求值的。

• The "while" loop executes as long as the condition (here: "a < 10") remains true. In Python, like in C, any non-zero integer value is true; zero is false. The condition may also be a string or list value, in fact any sequence; anything with a non-zero length is true, empty sequences are false. The test used in the example is a simple comparison. The standard comparison operators are written the same as in C: "<" (less than), ">" (greater than), "==" (equal to), "<=" (less than or equal to), ">=" (greater than or equal to) and "!=" (not equal to).
• "while" 循环只要它的条件（这里指： "a < 10"）保持为真就会一直执行。 Python 和 C 一样，任何非零整数都为真；零为假。这个条件也可以是字符串或是列表的值，事实上任何序列都可以；长度非零就为真，空序列就为假。在这个例子里，判断条件是一个简单的比较。标准的比较操作符的写法和 C 语言里是一样： "<" （小于）、 ">" （大于）、 "==" （等于）、"<=" （小于或等于)、 ">=" （大于或等于）以及 "!=" （不等于）。

• The body of the loop is indented: indentation is Python's way of grouping statements. At the interactive prompt, you have to type a tab or space(s) for each indented line. In practice you will prepare more complicated input for Python with a text editor; all decent text editors have an auto-indent facility. When a compound statement is entered interactively, it must be followed by a blank line to indicate completion (since the parser cannot guess when you have typed the last line). Note that each line within a basic block must be indented by the same amount.
• 循环体 是 缩进的 ：缩进是 Python 组织语句的方式。在交互式命令行里，你得给每个缩进的行敲下 Tab 键或者（多个）空格键。实际上用文本编辑器的话，你要准备更复杂的输入方式；所有像样的文本编辑器都有自动缩进的设置。交互式命令行里，当一个组合的语句输入时, 需要在最后敲一个空白行表示完成（因为语法分析器猜不出来你什么时候打的是最后一行）。注意，在同一块语句中的每一行，都要缩进相同的长度。

• The "print()" function writes the value of the argument(s) it is given. It differs from just writing the expression you want to write (as we did earlier in the calculator examples) in the way it handles multiple arguments, floating point quantities, and strings. Strings are printed without quotes, and a space is inserted between items, so you can format things nicely, like this:
• "print()" 函数将所有传进来的参数值打印出来. 它和直接输入你要显示的表达式(比如我们之前在计算器的例子里做的)不一样， print() 能处理多个参数，包括浮点数，字符串。字符串会打印不带引号的内容, 并且在参数项之间会插入一个空格, 这样你就可以很好的把东西格式化, 像这样:

     >>> i = 256*256 
     >>> print('The value of i is', i) 
     The value of i is 65536

The keyword argument end can be used to avoid the newline after the output, or end the output with a different string:
关键字参数 end 可以用来取消输出后面的换行, 或是用另外一个字符串来结尾:

     >>> a, b = 0, 1 
     >>> while a < 1000: 
     ...     print(a, end=',') 
     ...     a, b = b, a+b 
     ... 
     0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,

-[ Footnotes ]-
-[ 脚注 ]-

[1] Since "" has higher precedence than "-", "-32" will be interpreted as "-(32)" and thus result in "-9". To avoid this and get "9", you can use "(-3)*2".
[1] 因为 "" 比 "-" 有更高的优先级, 所以 "-32" 会被解释成"-(32)" ，因此结果是 "-9". 为了避免这个并且得到结果 "9", 你可以用这个式子 "(-3)*2".

[2] Unlike other languages, special characters such as "\n" have the same meaning with both single ("'...'") and double (""..."") quotes. The only difference between the two is that within single quotes you don't need to escape """ (but you have to escape "\'") and vice versa.
[2] 和其他语言不一样的是, 特殊字符比如说 "\n" 在单引号 ("'...'")和双引号 (""..."") 里有一样的意义. 这两种引号唯一的区别是，你不需要在单引号里转义双引号 """ (但是你必须把单引号转义成 "\'") ，反之亦然 .

3. An Informal Introduction to Python 非正式介绍