list 是一个可变的集合，其中的元素按_顺序_排列。和大多数集合一样（参见内置的 tuple、dict 和 set），数组可以存放对任意一种（或多种）数据类型的引用，其中也包括其他数组。和任何序列一样，可以用从左侧开始的 0-based index 编号或从右侧开始的 -1-based index 编号来访问元素。数组可以通过切片写法或<list>.copy()整体或部分复制。
数组同时支持通用和可变两类序列操作，例如min()/max()、<list>.index()、<list>.append()和<list>.reverse()。使用for item in <list>结构可以遍历数组中的元素。当同时需要元素下标和元素值时，可以使用for index, item in enumerate(<list>)。
在底层，lists 以动态数组实现，类似于 Java 的ArrayList类型，最常用于存储长度未知、成组的相似数据（字符串、数字、集合等）。数组是一种极其灵活、实用的数据结构，Python 中许多内置方法和操作都会以数组作为输出。
构造
list 可以用方括号[]写成_字面量_，元素之间用逗号分隔：
>>> no_elements = []
>>> no_elements
[]
>>> one_element = ["Guava"]
>>> one_element
['Guava']
>>> elements_separated_with_commas = ["Parrot", "Bird", 334782]
>>> elements_separated_with_commas
['Parrot', 'Bird', 334782]
为了提高可读性，当 list 中元素很多或含有嵌套数据结构时，可以换行书写：
>>> lots_of_entries = [
"Rose",
"Sunflower",
"Poppy",
"Pansy",
"Tulip",
"Fuchsia",
"Cyclamen",
"Lavender"
]
>>> lots_of_entries
['Rose', 'Sunflower', 'Poppy', 'Pansy', 'Tulip', 'Fuchsia', 'Cyclamen', 'Lavender']
# Each data structure is on its own line to help clarify what they are.
>>> nested_data_structures = [
{"fish": "gold", "monkey": "brown", "parrot": "grey"},
("fish", "mammal", "bird"),
['water', 'jungle', 'sky']
]
>>> nested_data_structures
[{'fish': 'gold', 'monkey': 'brown', 'parrot': 'grey'}, ('fish', 'mammal', 'bird'), ['water', 'jungle', 'sky']]
list() 构造函数可以不带参数调用，也可以接收一个_可迭代对象_作为参数。构造函数会依次遍历该可迭代对象中的元素，并按顺序加入 list：
>>> no_elements = list()
>>> no_elements
[]
# The tuple is unpacked and each element is added.
>>> multiple_elements_from_tuple = list(("Parrot", "Bird", 334782))
>>> multiple_elements_from_tuple
['Parrot', 'Bird', 334782]
# The set is unpacked and each element is added.
>>> multiple_elements_from_set = list({2, 3, 5, 7, 11})
>>> multiple_elements_from_set
[2, 3, 5, 7, 11]
用 list 构造函数处理 string 或 dict 时，结果可能会出人意料：
# String elements (Unicode code points) are iterated through and added *individually*.
>>> multiple_elements_string = list("Timbuktu")
>>> multiple_elements_string
['T', 'i', 'm', 'b', 'u', 'k', 't', 'u']
# Unicode separators and positioning code points are also added *individually*.
>>> multiple_code_points_string = list('अभ्यास')
>>> multiple_code_points_string
['अ', 'भ', '्', 'य', 'ा', 'स']
# The iteration default for dictionaries is over the keys, so only key data is inserted into the list.
>>> source_data = {"fish": "gold", "monkey": "brown"}
>>> multiple_elements_dict_1 = list(source_data)
['fish', 'monkey']
由于 list 构造函数只接受_可迭代对象_（或不接受任何参数）作为参数，_不可迭代_的对象会抛出类型错误。因此，用字面量方式创建只含一个元素的 list 要容易得多。
# Numbers are not iterable, and so attempting to create a list with a number passed to the constructor fails.
>>> one_element = list(16)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
# Tuples *are* iterable, so passing a one-element tuple to the constructor does work, but it's awkward
>>> one_element_from_iterable = list((16,))
>>> one_element_from_iterable
[16]
访问元素
数组中的元素（以及其他序列类型 str 和 tuple 中的元素）可以通过 0-based index 和_方括号写法_来访问。下标可以从**left** --> right（从零开始），也可以从**right** --> left（从 -1 开始）。
从左数下标 ⟹
|
0
👇🏾 |
1
👇🏾 |
2
👇🏾 |
3
👇🏾 |
4
👇🏾 |
5
👇🏾 |
| P |
y |
t |
h |
o |
n |
👆🏾
-6 |
👆🏾
-5 |
👆🏾
-4 |
👆🏾
-3 |
👆🏾
-2 |
👆🏾
-1 |
|
⟸ 从右数下标 |
>>> breakfast_foods = ["Oatmeal", "Fruit Salad", "Eggs", "Toast"]
# Oatmeal is at index 0 or index -4.
>>> breakfast_foods[0]
'Oatmeal'
>>> breakfast_foods[-4]
'Oatmeal'
# Eggs are at index -2 or 2
>>> breakfast_foods[-2]
'Eggs'
>>> breakfast_foods[2]
'Eggs'
# Toast is at -1
>>> breakfast_foods[-1]
'Toast'
数组内部的一段元素可以通过_切片写法_（<list>[start:stop]）访问。_切片_定义为位置 index 上的元素序列，满足 start <= index < stop。_切片_返回被“切”出来的元素的副本，不会修改原 list。
还可以使用 step 参数 [start:stop:step] 来“跳过”或筛选数组元素（例如，step 为 2 时会每隔一个元素选取一个）：
>>> colors = ["Red", "Purple", "Green", "Yellow", "Orange", "Pink", "Blue", "Grey"]
# If there is no step parameter, the step is assumed to be 1.
>>> middle_colors = colors[2:6]
>>> middle_colors
['Green', 'Yellow', 'Orange', 'Pink']
# If the start or stop parameters are omitted, the slice will
# start at index zero, and will stop at the end of the list.
>>> primary_colors = colors[::3]
>>> primary_colors
['Red', 'Yellow', 'Blue']
使用数组
对数组使用内置的 sum() 函数，会返回数组中所有数字之和：
>>> number_list = [1, 2, 3, 4]
>>> sum(number_list)
10
也可以用 len() 函数获取数组的_长度_：
>>> long_list = ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J"]
>>> len(long_list)
10
数组还可以用多种方式组合：
# Using the plus + operator unpacks each list and creates a new list, but it is not efficient.
>>> new_via_concatenate = ["George", 5] + ["cat", "Tabby"]
>>> new_via_concatenate
['George', 5, 'cat', 'Tabby']
# Likewise, using the multiplication operator * is the equivalent of using + n times.
>>> first_group = ["cat", "dog", "elephant"]
>>> multiplied_group = first_group * 3
>>> multiplied_group
['cat', 'dog', 'elephant', 'cat', 'dog', 'elephant', 'cat', 'dog', 'elephant']
数组带有_迭代器_，可以像其他_序列类型_一样被遍历。
# Looping through the list and printing out each element.
>>> colors = ["Orange", "Green", "Grey", "Blue"]
>>> for item in colors:
... print(item)
...
Orange
Green
Grey
Blue
想更深入地了解 loops 和 iterators，请完成 loops 概念练习。
学习练习
