Python基本语法

Python基本语法

基本运算

• 除法运算/总是返回浮点数，如果要得到一个整数结果可以使用 // 运算符，要计算余数可以使用%，

  而且python全面支持浮点数，混合类型运算数的运算会把整数转化为浮点数

  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  # floored quotient * divisor + remainder
  17
  

• python使用**运算符计算乘方

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

• 交互模式下，上一次输出的表达式会赋值给变量_。把python当作计算器使用时，用该变量实现下一步计算更为简单

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

字符串

• 字符串可以使用 + 合并在一起，也可以使用*进行重复

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

• 相邻两个或多个字符串字面值会自动合并

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

• 字符串支持索引（下标访问），第一个字符串的索引是0。单字符没有专门的类型，就是长度为1的字符串

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

• Python使用负数是倒着进行访问的因为-0和0都是第一个所以倒数第一个是-1

• Python切片

  word[0:2]  # characters from position 0 (included) to 2 (excluded)
  'Py'
  word[2:5]  # characters from position 2 (included) to 5 (excluded)
  'tho'
  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'
  word[:2] + word[2:]
  'Python'
  word[:4] + word[4:]
  'Python'
  

• 还可以这样理解切片，就是i~j的一段的字符

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

• 索引越界会进行报错，但是切片会自动处理索引越界，而且切片与索引都不能对字符串进行修改，如果要生成不同的字符串应该新建一个字符串

  word[42]  # the word only has 6 characters
  Traceback (most recent call last):File "<stdin>", line 1, in <module>
  IndexError: string index out of rangeword[4:42]
  'on'
  word[42:]
  ''word[0] = 'J'
  Traceback (most recent call last):File "<stdin>", line 1, in <module>
  TypeError: 'str' object does not support item assignment
  word[2:] = 'py'
  Traceback (most recent call last):File "<stdin>", line 1, in <module>
  TypeError: 'str' object does not support item assignment'J' + word[1:]
  'Jython'
  word[:2] + 'py'
  'Pypy'
  

列表

• Python 支持多种 复合 数据类型，可将不同值组合在一起。最常用的 列表 ，是用方括号标注，逗号分隔的一组值。列表 可以包含不同类型的元素，但一般情况下，各个元素的类型相同：

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

• 列表和其他内置的squence类型一样，列表支持切片，索引和合并操作

• 但是列表与字符串不同，列表是mutable就是可以修改的

  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]
  

• python中的简单赋值绝对不会复制数据，当你将一个链表赋值给一个变量时，该变量将引用现有的列表，你通过一个变量对列表的任何更改都会被引用它的所有变量看到

  rgb = ["Red", "Green", "Blue"]
  rgba = rgb
  id(rgb) == id(rgba)  # they reference the same object
  True
  rgba.append("Alph")
  rgb
  ["Red", "Green", "Blue", "Alph"]
  

• 切片操作返回包含请求元素的新列表，以下切片操作会返回列表的浅拷贝，为切片赋值可以改变列表的大小，甚至可以清空整个列表

  correct_rgba = rgba[:]
  correct_rgba[-1] = "Alpha"
  correct_rgba
  ["Red", "Green", "Blue", "Alpha"]
  rgba
  ["Red", "Green", "Blue", "Alph"]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
  []
  

• 列表还可以嵌套列表

  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'