3. Python 的非正式介绍

在下面的例子中，通过提示符的出现与否来区分输入和输出(>>> and …)：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。

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。

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. Python 作为计算器使用

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. 数字

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

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。

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

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

等号（=）用于给一个变量赋值。然后在喜爱个交互命令之前不会有结果显示出来:

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

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 "<stdin>", line 1, in <module>
NameError: name 'n' is not defined

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

在交互模式下，最后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

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。

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. 字符串

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," he said.'
'"Yes," he said.'
>>> "\"Yes,\" he said."
'"Yes," he said.'
>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'

在交互式解释器中，输出字符串是在引号中的，特殊字符是前置了反斜杠的。虽然看起来有可能不一样（单双引号可能不一样），两种字符串其实是一样的。如果字符串中有单引号而没有双引号，字符串就显示在双引号中，否则就显示在单引号中。函数 print() 会产生更方便阅读的输出，就是不使用引号，也不转义特殊字符：

>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'
>>> print('"Isn\'t," she said.')
"Isn't," she 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.

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

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
""")

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

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'

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'

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.'

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
  ...
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
  ...
SyntaxError: invalid syntax

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'

字符串是可以被 索引 （脚标访问）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'

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'

注意 -0 和 0 是一样的，所以负数索引从 -1 开始。

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:

>>> 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.

>>> 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:

 +---+---+---+---+---+---+
 | 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.

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.

Attempting to use an index that is too large will result in an error:

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

However, out of range slice indexes are handled gracefully when used for slicing:

>>> 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:

>>> word[0] = 'J'
  ...
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
  ...
TypeError: 'str' object does not support item assignment

If you need a different string, you should create a new one:

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

The built-in function len() returns the length of a string:

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

参见

Text Sequence Type — str

Strings are examples of sequence types, and support the common operations supported by such types.

String Methods

Strings support a large number of methods for basic transformations and searching.

Formatted string literals

String literals that have embedded expressions.

Format String Syntax

Information about string formatting with str.format().

printf-style String Formatting

The old formatting operations invoked when strings are the left operand of the % operator are described in more detail here.

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.

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

Like strings (and all other built-in sequence type), lists can be indexed and sliced:

>>> 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 new (shallow) copy of the list:

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

Lists also support operations like concatenation:

>>> 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:

>>> 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):

>>> 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:

>>> 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:

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

It is possible to nest lists (create lists containing other lists), for example:

>>> 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

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:

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

This example introduces several new features.

• 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.

• The while loop executes as long as the condition (here: b < 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).

• 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.

• 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:

  >>> 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:

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

脚注

[1]	Since ** has higher precedence than -, -3**2 will be interpreted as -(3**2) and thus result in -9. To avoid this and get 9, you can use (-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.