字典

字典是将键(key)映射到值(value)的无序数据结构。值可以是任何值（列表，函数，字符串，任何东西）。键(key)必须是不可变的，例如，数字，字符串或元组。

示例

字典：我们查找的单词是key，查找的定义是值

In [111]

# Defining a dictionary
webstersDict = {'person': 'a human being, whether an adult or child', 'marathon': 'a running race that is about 26 miles', 'resist': ' to remain strong against the force or effect of (something)', 'run': 'to move with haste; act quickly'}

In [112]

webstersDict

{'marathon': 'a running race that is about 26 miles','person': 'a human being, whether an adult or child','resist': ' to remain strong against the force or effect of (something)','run': 'to move with haste; act quickly'}

访问字典中的值

In [113]

# Finding out the meaning of the word marathon
# dictionary[key]
webstersDict['marathon']

'a running race that is about 26 miles'

更新字典

In [114]

# add one new key value pair to dictionary
webstersDict['shoe'] = 'an external covering for the human foot'# return the value for the 'shoe' key
webstersDict['shoe']

'an external covering for the human foot'

In [115]

# update method, update or add more than key value pair at a time
webstersDict.update({'shirt': 'a long- or short-sleeved garment for the upper part of the body', 'shoe': 'an external covering for the human foot, usually of leather and consisting of a more or less stiff or heavy sole and a lighter upper part ending a short distance above, at, or below the ankle.'})
webstersDict

{'marathon': 'a running race that is about 26 miles','person': 'a human being, whether an adult or child','resist': ' to remain strong against the force or effect of (something)','run': 'to move with haste; act quickly','shirt': 'a long- or short-sleeved garment for the upper part of the body','shoe': 'an external covering for the human foot, usually of leather and consisting of a more or less stiff or heavy sole and a lighter upper part ending a short distance above, at, or below the ankle.'}

In [116]

# Removing key from dictionary
del webstersDict['resist']
webstersDict

{'marathon': 'a running race that is about 26 miles','person': 'a human being, whether an adult or child','run': 'to move with haste; act quickly','shirt': 'a long- or short-sleeved garment for the upper part of the body','shoe': 'an external covering for the human foot, usually of leather and consisting of a more or less stiff or heavy sole and a lighter upper part ending a short distance above, at, or below the ankle.'}

不是所有东西都可以当作Key

下方是错误用法示例

In [117]

webstersDict[['sock']] = 'a short stocking usually reaching to the calf or just above the ankle.'

---------------------------------------------------------------------------TypeError Traceback (most recent call last)<ipython-input-117-95ef7f324c79> in <module>() ----> 1 webstersDict[['sock']] = 'a short stocking usually reaching to the calf or just above the ankle.' TypeError: unhashable type: 'list'

使用get（）方法返回给定键的值

你会明白为什么这在字数统计任务中如此有价值

In [118]

# incorporate into get example and such below.
storyCount = {'is': 100, 'the': 90, 'Michael': 12, 'runs': 5}

In [119]

storyCount

{'Michael': 12, 'is': 100, 'runs': 5, 'the': 90}

In [120]

# key error for keys that do not exist
storyCount['run']

---------------------------------------------------------------------------KeyError Traceback (most recent call last)<ipython-input-120-7457d9dbff5f> in <module>() 1 # key error for keys that do not exist ----> 2 storyCount['run'] KeyError: 'run'

In [121]

# if key doesnt exist,
# specify default value for keys that dont exist.
# returns value for key you enter if it is in dictionary
# else it returns the value you have for default
storyCount.get('Michael', 0)

In [122]

# When you dont set default value for key that doesnt exist,
# it defaults to none
print(storyCount.get('run'))

None

In [123]

# Making default value for key that doesn't exist 0.
print(storyCount.get('run', 0))

删除键，但同时可以返回值

In [124]

count = storyCount.pop('the')
print(count)

遍历字典

In [125]

# return keys in dictionary
print(storyCount.keys())# return values in dictionary
print(storyCount.values())

['is', 'runs', 'Michael']
[100, 5, 12]

In [126]

# iterate through keys
for key in storyCount: print(key)

is
runs
Michael

In [127]

# iterate through keys and values
for key, value in webstersDict.items():print(key, value)

('person', 'a human being, whether an adult or child')
('run', 'to move with haste; act quickly')
('shoe', 'an external covering for the human foot, usually of leather and consisting of a more or less stiff or heavy sole and a lighter upper part ending a short distance above, at, or below the ankle.')
('marathon', 'a running race that is about 26 miles')
('shirt', 'a long- or short-sleeved garment for the upper part of the body')

元组

元组是一种序列，就像列表一样。元组和列表之间的区别在于，与列表（可变）不同，元组不能更改（不可变）。元组使用括号，而列表使用方括号。

初始化一个元组

有两种方法可以初始化空元组。您可以通过让（）没有值来初始化空元组

In [69]

# Way 1
emptyTuple = ()

您还可以使用元组函数初始化空元组。

In [70]

# Way 2
emptyTuple = tuple()

可以通过用逗号分隔值的序列来初始化具有值的元组。

In [72]

# way 1
z = (3, 7, 4, 2)# way 2 (tuples can also can be created without parenthesis)
z = 3, 7, 4, 2

重要的是要记住，如果要创建仅包含一个值的元组，则需要在项目后面添加一个逗号。

In [73]

# tuple with one value
tup1 = ('Michael',)# tuple with one value
tup2 = 'Michael', # This is a string, NOT a tuple.
notTuple = ('Michael')

访问元组内的值

元组中的每个值都有一个指定的索引值。值得注意的是，python是一种基于零索引的语言。所有这些意味着元组中的第一个值是索引0。

In [75]

# Initialize a tuple
z = (3, 7, 4, 2)# Access the first item of a tuple at index 0
print(z[0])

Python还支持负索引。负索引从元组结束开始。使用负索引来获取元组中的最后一项有时会更方便，因为您不必知道元组的长度来访问最后一项。

In [76]

# print last item in the tuple
print(z[-1])

提醒一下，您也可以使用正索引访问相同的项目（如下所示）。

In [77]

print(z[3])

切分元组

切分操作返回包含所请求项的新元组。切分很适合在元组中获取值的子集。对于下面的示例代码，它将返回一个元组，其中包含索引0的对象，而不包括索引2的对象。

In [78]

# Initialize a tuple
z = (3, 7, 4, 2)# first index is inclusive (before the :) and last (after the :) is not.
print(z[0:2])

(3, 7)

In [80]

# everything up to but not including index 3
print(z[:3])

(3, 7, 4)

负索引也OK

In [81]

print(z[-4:-1])

(3, 7, 4)

元组是不可改变的

元组是不可变的，这意味着在初始化元组之后，不可能更新元组中的单个项。正如您在下面的代码中所看到的，您无法更新或更改元组项的值（这与可变的Python列表不同）。

下方有错误示例

In [83]

z = (3, 7, 4, 2)z[1] = "fish"

---------------------------------------------------------------------------TypeError Traceback (most recent call last)<ipython-input-83-1ba53bfc5f04> in <module>() 1 z = (3, 7, 4, 2) 2 ----> 3 z[1] = "fish" TypeError: 'tuple' object does not support item assignment

即使元组是不可变的，也可以采用现有元组的一部分来创建新的元组，如下例所示。

In [85]

# Initialize tuple
tup1 = ('Python', 'SQL')# Initialize another Tuple
tup2 = ('R',)# Create new tuple based on existing tuples
new_tuple = tup1 + tup2;
print(new_tuple)

('Python', 'SQL', 'R')

Tuple方法

在开始本节之前，让我们首先初始化一个元组

In [86]

# Initialize a tuple
animals = ('lama', 'sheep', 'lama', 48)

index 方法(索引)

index方法返回对应值的第一个索引

In [87]

print(animals.index('lama'))

count 方法(计数)

count方法返回值在元组中出现的次数。

In [88]

print(animals.count('lama'))

遍历元组

您可以使用for循环遍历元组的项目

In [89]

for item in ('lama', 'sheep', 'lama', 48):print(item)

lama
sheep
lama
48

元组拆包

元组对序列解包非常有用

In [91]

x, y = (7, 10);
print("Value of x is {}, the value of y is {}.".format(x, y))

Value of x is 7, the value of y is 10.

枚举

枚举函数返回一个元组，其中包含每次迭代的计数（从默认为0的开始）和迭代序列获得的值

In [93]

friends = ('Steve', 'Rachel', 'Michael', 'Monica')
for index, friend in enumerate(friends):print(index,friend)

(0, 'Steve')
(1, 'Rachel')
(2, 'Michael')
(3, 'Monica')

元组相对列表的优势

列表和元组是标准Python数据类型，用于在序列中存储值。元组是不可变的，而列表是可变的。以下是元组列表的一些其他优点

组比列表更快。如果你要定义一组常量值，那么你将要做的就是迭代它，使用元组而不是列表。可以使用timeit库部分测量性能差异，该库允许您为Python代码计时。下面的代码为每个方法运行代码100万次，并输出所花费的总时间（以秒为单位）。

In [1]

import timeit
print('Tuple time: ', timeit.timeit('x=(1,2,3,4,5,6,7,8,9,10,11,12)', number=1000000))
print('List time: ', timeit.timeit('x=[1,2,3,4,5,6,7,8,9,10,11,12]', number=1000000))

('Tuple time: ', 0.0192110538482666)
('List time: ', 0.16498994827270508)

元组可以用作字典键

一些元组可以用作字典键（特别是包含不可变值的元组，如字符串，数字和其他元组）。列表永远不能用作字典键，因为列表不是不可变的

In [98]

bigramsTupleDict = {('this', 'is'): 23,('is', 'a'): 12,('a', 'sentence'): 2}print(bigramsTupleDict)

{('is', 'a'): 12, ('this', 'is'): 23, ('a', 'sentence'): 2}

列表不可以用作字典键

In [99]

bigramsListDict = {['this', 'is']: 23,['is', 'a']: 12,['a', 'sentence']: 2}print(bigramsListDict)

---------------------------------------------------------------------------TypeError Traceback (most recent call last)<ipython-input-99-834f1c969506> in <module>() ----> 1 bigramsListDict = {['this', 'is']: 23, 2 ['is', 'a']: 12, 3 ['a', 'sentence']: 2} 4 5 print(bigramsListDict) TypeError: unhashable type: 'list'

元组可以是集合中的值

In [50]

graphicDesigner = {('this', 'is'),('is', 'a'),('a', 'sentence')}
print(graphicDesigner)

set([('is', 'a'), ('this', 'is'), ('a', 'sentence')])

列表不可以是集合中的值

In [49]

graphicDesigner = {['this', 'is'],['is', 'a'],['a', 'sentence']}
print(graphicDesigner)

---------------------------------------------------------------------------TypeError Traceback (most recent call last)<ipython-input-49-008c17adfe8f> in <module>() 1 graphicDesigner = {['this', 'is'], 2 ['is', 'a'], ----> 3 ['a', 'sentence']} 4 print(graphicDesigner) TypeError: unhashable type: 'list'

Task: 用Python生成斐波那契序列

Fibonacci序列是一个整数序列，其特征在于前两个之后的每个数字是前两个数字的总和。根据定义，Fibonacci序列中的前两个数字是1和1，或0和1，具体取决于所选择的序列起点，以及每个后续数字是前两个数字的总和。

In [2]

print(1, 1, 2, 3, 5, 8, 13, 21, 34, 55)

1 1 2 3 5 8 13 21 34 55

1.使用循环，编写一个Python程序，打印出前10个Fibonacci数

In [24]

# Note, there are better ways to code this which I will go over in later videos
a,b = 1,1
for i in range(10):print("Fib(a): ", a, "b is: ", b)a,b = b,a+b

('Fib(a): ', 1, 'b is: ', 1)
('Fib(a): ', 1, 'b is: ', 2)
('Fib(a): ', 2, 'b is: ', 3)
('Fib(a): ', 3, 'b is: ', 5)
('Fib(a): ', 5, 'b is: ', 8)
('Fib(a): ', 8, 'b is: ', 13)
('Fib(a): ', 13, 'b is: ', 21)
('Fib(a): ', 21, 'b is: ', 34)
('Fib(a): ', 34, 'b is: ', 55)
('Fib(a): ', 55, 'b is: ', 89)

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字典

示例