0x17 Dictionaries 字典
- 字典是一种数据结构，map keys to values.list 可以理解为一种特殊的字典，字典与list有相同的索引方式,[ ]
>>> ages = {"Dave":24,"Mary":42,"John":58} >>> print(ages["Dave"]) 24 >>> print(ages["John"]) 58 - 键入一个没有的index keys，会报KeyError
>>> primary = { ... "red":[255,0,0], ... "green":[0,255,0], ... "blue":[0,0,255] ... } >>> print(primary[red]) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'red' is not defined #key有"" >>> print(primary["red"]) [255, 0, 0] >>> print(primary["yellow"]) Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 'yellow'>>> test = {} >>> print(test[0]) Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 0 - 只有不可变的对象才能当作key使用。
>>> bad_dict = { ... [1,2,3]:"one two three" ... } Traceback (most recent call last): File "<stdin>", line 2, in <module> TypeError: unhashable type: 'list' - 字典中的操作，替换值添加值
>>> squares = {1:1,2:4,3:"error",4:16} >>> squares[8] = 125 >>> squares[3] = 68 >>> print(squares) {8: 125, 1: 1, 2: 4, 3: 68, 4: 16}>>> print(squares) {8: 125, 1: 1, 2: 4, 3: 68, 4: 16} >>> print(squares[squares[1]]) 1 - 使用in或not in 来确定key是否在字典中
>>> nums = { ... 1:"one", ... 2:'two', ... 3:'three' ... } >>> print(1 in nums) True >>> print(5 in nums) False >>> print(5 not in nums) True - 字典中一个常用的方法是get，它像index一样，但出现不存在的key的时候会返回特殊的值，默认为None
>>> pairs = {1:'apples', ... 'orange':[1,2,4], ... True:False, ... None:'True' ... } >>> print(pairs.get('orange')) [1, 2, 4] >>> print(pairs.get(7)) None >>> print(pairs.get('apples')) None >>> print(pairs.get(12345,"not in dictionary")) not in dictionary>>> fib = {1:1,2:1,3:2,4:3} >>> print(fib.get(4,0) + fib.get(7,5)) 8 #get(4,0) return 3 get(7,5) because 7 not exist so return 5 - 字典中的每一个元素都是由 key:value来表示
- 创建空字典用{}
0x18 tuples 元祖
- tuples与list极其相似，tuples是不可变的。它用()创建
>>> words = ('spam','eggs','sausages') >>> print(words[0]) spam >>> words[0] = 'good' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment - tuples可以不用()创建,只要将values用,隔开即可
>>> words = 'spam','eggs','sausages' >>> print(words[0]) spam - 创建一个空的tuple用()
tpl = ()>>> tpl = (1,(1,2,3)) >>> print(tpl[1]) (1, 2, 3) >>> print(tpl[1][0]) 1 >>> print(tpl[1][1]) 2 >>> print(tpl[1][2]) 3 >>> print(tpl[1][2][0]) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'int' object is not subscriptable - tuples are faster than list,but they cannot be change
- tuples与list dictionary 一样都能够被互相嵌套
0x19 list slices 切片
- list slices是一种高级的list操作。写法是中括号中用:隔开的两个整数
>>> squares = [0,1,4,9,16,25,36,47,64,81] >>> print(squares[2:6]) [4, 9, 16, 25] >>> print(squares[3:8]) [9, 16, 25, 36, 47] >>> print(squares[0:1]) [0] # 第一个index的数包含在里面，第二个没有。和range一样 - 第一个数被省略的话从头开始算，第二个省略的话从数到尾
>>> squares = [0,1,4,9,16,25,36,47,64,81] >>> print(squares[:4]) [0, 1, 4, 9] >>> print(squares[4:]) [16, 25, 36, 47, 64, 81] - slicing 在tuple上面也能使用
- slices 可添加3个参数，第3个参数代表step
>>> squares = [0,1,4,9,16,25,36,47,64,81] >>> print(squares[::2]) [0, 4, 16, 36, 64] >>> print(squares[2:8:2]) [4, 16, 36] >>> print(squares[2:8:3]) [4, 25] >>> print(squares[2:8:4]) [4, 36] >>> print(squares[2:8:5]) [4, 47] >>> print(squares[2:8:6]) [4] - slices中的负数代表从list的尾部算
>>> squares = [0,1,4,9,16,25,36,47,64,81] >>> print(squares[1:-1]) [1, 4, 9, 16, 25, 36, 47, 64] >>> print(squares[1:-2]) [1, 4, 9, 16, 25, 36, 47] >>> print(squares[-5:-2]) [25, 36, 47] - slice中step的负数，代表向后完成，通常用[::-1]反转list
>>> squares = [0,1,4,9,16,25,36,47,64,81] >>> print(squares[::-1]) [81, 64, 47, 36, 25, 16, 9, 4, 1, 0] >>> print(squares[::-2]) [81, 47, 25, 9, 1] >>> print(squares[::-5]) [81, 16] >>> print(squares[7:2:-2]) [47, 25, 9] >>> print(squares[7:2:-3]) [47, 16]
