Python(2)-第二天

除法

>>> 8 / 5 

1
>>> 8 / 5.0
1.6
>>> 8.0 / 5
1.6
>>> 8 // 5.0
1.0

余数

>>> 8 % 5 

2

**运算符

>>> 5 ** 2 

25

=运算符

>>> width = 20 

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

_变量，表示最近一次表达式的值

>>> _ 

900

字符串输出

>>> 'spam eggs'

'spam eggs'
>>> "span eggs"

'spam eggs'

>>> '"Yes," he said.'

'"Yes," he said.'

转义

说明：输出的字符串会用引号引起来，特殊字符会用反斜杠转义。

虽然可能和输入看上去不太一样（外围的引号会改变），但是两个字符串是相等的。

如果你前面带有\的字符被当作特殊字符，你可以使用原始字符串，方法是在第一个引号前面加上一个r:

>>>  'doesn\'t'

"doesn't"
>>> "\"Yes,\" he said."

'"Yes," he said.'
>>> r'doesn\'t'

"doesn\'t"
>>> 8 // 5.0
1.0

字符串跨多行(""")

>>> print """\
Usage: thingy [OPTIONS]
     -h                        

     -H hostname
"""

Usage: thingy [OPTIONS]
     -h
     -H hostname

字符串连接

>>> 'Py' + 'thon'

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

'Python'

>>> 'Py' * 3

'PyPyPy'

字符串的索引

>>> word = 'Python'

>>> word[0]

'P'

>>> word[5]

'n'

索引为负数时，此时从右侧开始计数（因为-0和0是一样的，索引索引从-1开始）

>>> word = 'Python'

>>> word[-1]

'n'

>>> word[-6]

'P'

切片（截取字符串）

>>> word = 'Python'

>>> word[0:2]

'Py'

>>> word[2:5]

'tho'

包含起始的字符，不包含末尾的字符。这使得s[:i] + s[i:]永远等于s：

>>> word = 'Python'

>>> word[:2] + word[2:]

'Python'

切片的索引有非常有用的默认值；省略的第一个索引默认为零，省略的第二个索引默认为切片的字符串的大小

>>> word = 'Python'

>>> word[:2]

'Py'

>>> word[4:]

'on'

>>> word[-2:]

'on'

当用于切片时，超出范围的切片索引会被优雅地处理：

>>> word = 'Python'

>>> word[4:42]

'on'

>>> word[42:]

''

内置函数len()

>>> word = 'Python'

>>> len(word)

6

Unicode字符串

创建Unicode字符串与创建普通字符串一样简单，u表示内置函数unicode()

>>> u'Hello World !'

u'Hello World !'

如果字符串中包含特殊字符，可以使用Unicode转义编码

>>> u'Hello\u0020World !'

u'Hello World !'

Unicode字符串的raw模式

>>> ur'Hello\\u0020World !'

u'Hello\\\\u0020World !'

列表

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

>>> squares

[1, 4, 9, 16, 25]

和字符串(以及其他所有内置的序列类型)一样，列表可以索引和切片

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

>>> squares[0] 

1

>>> squares[-1] 

25

>>> squares[-3:] 

[9, 16, 25]

列表也支持连接

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

>>> squares + [36, 49, 64, 81, 100]

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

列表是可变的(字符串是不可变的)

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

>>> cubes[3] = 33

>>> cubes

[1, 4, 9, 33, 25]

append()方法

>>> cubes= [1, 4, 9]

>>> cubes.append(16)

>>> cubes

[1, 4, 9, 16]

给切片赋值，此操作甚至可以改变列表的大小或者清空它

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']

>>> letters[2:5] = ['C', 'D', 'E']

>>> letters

['a', 'b', 'C', 'D', 'E', 'f', 'g']

>>> letters[2:5] = []

>>> letters

['a', 'b', 'f', 'g']

>>> letters

['a', 'b', 'C', 'D', 'E', 'f', 'g']

len函数

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']

>>> len(letters)

4

嵌套列表

>>> 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'