Detailed explanation of Python built-in functions/methods - list list

A list is an ordered and mutable collection, and is the most commonly used Python data type. In Python, lists are []written .

1. Create a list

In Python, the data types of elements in a list can be different, including integers, floating-point numbers, and complex numbers. Of course, lists, tuples, dictionaries, and sets can also be included.

1.1 Use [ ] to create a list

To create a list, just enclose the different data items separated by commas with square brackets [] .

• create empty list

list0 = []

• create non-empty list

list1 = ['Baidu', 'Alibaba', 'Tencent']

1.2 Use the list() function to create (convert to) a list

• Lists can be created using the list() constructor:

this_list = list(('apple', 'banana', 'cherry'))

Note: When using the list() function to create a list, be sure to pay attention to the double brackets .

• In Python, we can convert other similar objects like strings, tuples, dictionaries, and sets into lists using the list() function. See the following built-in functions for specific usage:

2. Access list

Like lists, we can either use subscripts to access an element in the list (to get the value of an element), or use slices to access a set of elements in the list (to get a sublist).

2.1 Subscript index access

Subscript index access tuples are divided into two categories, namely forward index and reverse index . The format is list_name[i], where list_name represents the list name, i represents the index value, and i can be a positive number (forward index) or a negative number ( inverted index).

It can be known that the first element of list_name[0]the list represents the last element of the list .list_name[-1]

list_name = ['wzq', 'lgl', 'gz', 'whl', 'sj', 'hxw']
print(list_name[0])
print(list_name[-1])

wzq
hxw

Forward index: start from the first (subscript 0), second (subscript 1)...

Reverse index: from the first to last (subscript -1), second to last (subscript -2)...

2.2 Slice access

If you don't understand the above description, you can refer to the following table:

The format of using a slice to access the list is list_name[strat : end : step], where start indicates the start index, end indicates the end index, and step indicates the step size.

list_name = ['wzq', 'lgl', 'gz', 'whl', 'sj', 'hxw']
print(list_name[1:5:2])
print(list_name[-6:-1:3])

['lgl', 'whl']
['wzq', 'whl']

When using slices to access list elements, list_name[strat : end : step][start: end] is a left-closed right-open interval, that is, the element represented by end cannot be accessed.

2.3 for loop to traverse the list

The items in a list can be iterated over using a for loop:

fruit_list = ['apple', 'pear', 'cherry']
for i in fruit_list:
    print(i)

apple
pear
cherry

2.4 Check if the project exists

To determine whether a specified item exists in the list, we can use the in keyword:

# 检查列表中是否存在'apple'
fruit_list = ['apple', 'pear', 'cherry']
print('apple' in fruit_list)

True

When using the in keyword to check whether the specified item exists in the list, if it exists, it returns True ; otherwise, it returns False .

2.5 Changing List Values

When we create a list, we can modify or update the data items in the list. Of course, we can also use the append() method to add list items.

fruit_list = ['apple', 'pear', 'cherry']
fruit_list[2] = 'banana'
print(fruit_list)

['apple', 'pear', 'banana']

Note: Once a tuple is created, its value cannot be changed, but there are other workarounds.

2.6 List join (merge)/copy

Like strings, tuples can +be concatenated and copied* between lists using the and signs , which means they can generate a new list.

1. +Connection (merge)

x = [1, 2, 3]
y = [4, 5, 6]
print(x + y)

[1, 2, 3, 4, 5, 6]

2. *Copy

x = ['Hello']
print(x * 5)

['Hello', 'Hello', 'Hello', 'Hello', 'Hello']

2.7 Nested lists

Use nested lists to create other lists inside lists.

x = [1, 2, 3]
y = ['a', 'b', 'c']
z = [x, y]
print(z)

[[1, 2, 3], ['a', 'b', 'c']]

2.8 List comparison

List comparison needs to introduce the eq method of the operator module .

# 导入 operator 模块
import operator

a = [1, 2]
b = [2, 3]
c = [2, 3]
print("operator.eq(a, b):", operator.eq(a, b))
print("operator.eq(b, c):", operator.eq(b, c))

operator.eq(a, b): False
operator.eq(b, c): True

3. Built-in functions

3.1 Print output print()

1. print()Function

We are already very familiar with the function of the print() function, which is to print the output.

my_list = ['pink', True, 1.78, 65]
print(my_list)

['pink', True, 1.78, 65]

3.2 Determine list item len()

2. len()Function

Functions are used when we want to determine how many items (elements) a list has len().

fruit_list = ['apple', 'banana', 'cherry']
print(len(fruit_list))

3

3.3 return variable type type()

3. type()Function

Use the type() function to determine what type a variable is (string, list, tuple, dictionary, or set).

info_list = ['name', 'gender', 'age', 'height', 'weight']
print(type(info_list))

<class 'list'>

When info_listusing type() to determine the variable type, it will return <class 'list'>, indicating that this is a list .

3.4 Convert to list list()

4. tuple()Function

The function of the tuple() function is to convert other types into tuple types. The detailed usage is as follows:

• convert string to list

str1 = 'Hello Python'
print(list(str1))

['H', 'e', 'l', 'l', 'o', ' ', 'P', 'y', 't', 'h', 'o', 'n']

• convert tuple to list

tuple1 = ('Hello', 'Python')
print(list(tuple1))

['Hello', 'Python']

• convert dictionary to list

dict1 = {'Hello': 'Python', 'name': 'pink'}
print(list(dict1))

['Hello', 'name']

• convert collection to list

set1 = {'Hello', 'Python', 'name', 'pink'}
print(list(set1))

['Python', 'name', 'pink', 'Hello']

• convert range to list

range1 = range(1, 6)
print(list(range1))

[1, 2, 3, 4, 5]

3.5 The maximum/minimum value of tuple elements max(), min()

5. max()Functions and min()functions

The function of max() is to return the maximum value of the elements in the list . The function of min() is to return the minimum value of the elements in the list .

list1 = [4, 6, 2, 0, -5]
print(max(list1))
print(min(list1))

list2 = ['a', 'z', 'A', 'Z']
print(max(list2))
print(min(list2))

6
-5
z
A

3.6 delete list del

• delete a single element

We can use del list_name[i]to delete a specified element, where list_name represents the list name, and i represents the index value of the specified value.

list_de = ['Baidu', 'Alibaba', 'Tencent', 'Bytedance']
del list_de[1]
print(list_de)

['Baidu', 'Tencent', 'Bytedance']

• delete list

The del function can not only delete a certain element, but also delete the entire list.

list_de = ['Baidu', 'Alibaba', 'Tencent', 'Bytedance']
del list_de

When we use the del function to delete a list, and then use the print() function to print out, an error will be reported NameError: name 'list_de' is not defined, indicating that the list is not defined.

4. Built-in methods

4.1 Add elements append(), insert(), extend()

1. append()Method

The append() method is used to add new objects at the end of the list.

grammar

list.append(element)

parameter value

example

• add element

fruit_list = ['apple', 'banana', 'cherry']
fruit_list.append('pear')
print(fruit_list)

['apple', 'banana', 'cherry', 'pear']

• add list

x = [1, 2, 3]
y = ['A', 'B', 'C']
x.append(y)
print(x)

[1, 2, 3, ['A', 'B', 'C']]

2. insert()Method

The insert() method is used to insert the specified object into the specified position of the list.

grammar

list.insert(position, element)

parameter value

example

• Insert the value "orange" as the second element into the fruits list:

fruits = ['apple', 'banana', 'cherry']
fruits.insert(1, "orange")
print(fruits)

['apple', 'orange', 'banana', 'cherry']

• insert list y into list x

x = [1, 2, 3]
y = ['a', 'c']
x.insert(0, y)
print(x)

[['a', 'c'], 1, 2, 3]

append() can only add elements or lists at the end , and insert() can add elements or lists at any position .

3. extend()Method

The extend() method is used to append multiple values ​​from another sequence at the end of the list at once (extending the original list with the new list).

grammar

list.extend(iterable)

parameter value

example

aver = ['A', 'B', 'C']

• Adds string elements to the end of the list

str1 = 'Hello'
aver.extend(str1)
print(aver)

['A', 'B', 'C', 'H', 'e', 'l', 'l', 'o']

• Add list element to the end of the list

list1 = [1, 2, 3]
aver.extend(list1)
print(aver)

['A', 'B', 'C', 1, 2, 3]

• Add tuple elements to the end of the list

tuple1 = (1, 2, 3)
aver.extend(tuple1)
print(aver)

['A', 'B', 'C', 1, 2, 3]

• Add dictionary elements to the end of the list

dict1 = {'name': 'pink', 'gender': True}
aver.extend(dict1)
print(aver)

['A', 'B', 'C', 'name', 'gender']

• Add collection elements to the end of the list

set1 = {1, 2, 3}
aver.extend(set1)
print(aver)

['A', 'B', 'C', 1, 2, 3]

• add range to end of list

range1 = range(1,10)
aver.extend(range1)
print(aver)

['A', 'B', 'C', 1, 2, 3, 4, 5, 6, 7, 8, 9]

4.2 Element occurrence count()

The count() method is used to count the number of times an element appears in the list.

grammar

list.count(value)

parameter value

example

num = [1, 4, 2, 9, 7, 8, 9, 3, 1]
print(num.count(9))

2

4.3 Specify the value index index()

The index() method is used to find the index position of the first occurrence of a value from the list.

grammar

list.index(element)

parameter value

example

num = [4, 55, 64, 32, 16, 32]
print(num.index(32))

3

When the searched value occurs multiple times in the list, only the first occurrence is returned.

4.4 Sorting a list sort()

The sort() method is used to sort the original list. If parameters are specified, the comparison function specified by the comparison function is used.

grammar

list.sort(reverse=True|False, key=myFunc)

parameter value

example

• Sort the list alphabetically:

words = ['Name', 'Gender', 'Age', 'Height', 'Weight']
words.sort()
print(words)

['Age', 'Gender', 'Height', 'Name', 'Weight']

• 对列表进行降序排序：

words = ['Name', 'Gender', 'Age', 'Height', 'Weight']
words.sort(reverse=True)
print(words)

['Weight', 'Name', 'Height', 'Gender', 'Age']

• 按照值的长度对列表进行排序：

# 返回值的长度的函数：
def myfunc(e):
    return len(e)

words = ['a', 'bb', 'ccc', 'dddd', '']

words.sort(key=myfunc)
print(words)

['', 'a', 'bb', 'ccc', 'dddd']

• 根据字典的 “year” 值对字典列表进行排序：

# 返回 'year' 值的函数：
def myfunc(e):
    return e['year']

words = [
    {'char': 'a', 'year': 1963},
    {'char': 'b', 'year': 2010},
    {'char': 'c', 'year': 2019}
]

words.sort(key=myfunc)
print(words)

[{'char': 'a', 'year': 1963}, {'char': 'b', 'year': 2010}, {'char': 'c', 'year': 2019}]

• 按照值的长度对列表进行降序排序：

# 返回值的长度的函数：
def myfunc(e):
    return len(e)

words = ['aa', 'b', 'ccc', 'dddd']

words.sort(reverse=True, key=myfunc)
print(words)

['dddd', 'ccc', 'aa', 'b']

• 指定列表中的元素排序来输出列表：

# 获取列表的第二个元素
def takeSecond(elem):
    return elem[1]

# 列表
random = [(2, 2), (3, 4), (4, 1), (1, 3)]

# 指定第二个元素排序
random.sort(key=takeSecond)

# 输出类别
print('排序列表：', random)

排序列表： [(4, 1), (2, 2), (1, 3), (3, 4)]

4.5 复制列表 copy()

copy() 方法用于复制列表，类似于 a[:]。

语法

list.copy()

参数值

无参数

实例

fruits = ['apple', 'banana', 'cherry', 'orange']
x = fruits.copy()
print(x)

['apple', 'banana', 'cherry', 'orange']

复制（制作副本）的另一种方法是使用内置函数 list() ，如下：

list1 = ['apple', 'banana', 'cherry']
list_2 = list(list1)

4.6 颠倒列表顺序 reverse()

reverse() 方法用于反向列表中元素。

语法

list.reverse()

参数值

无参数

实例

fruits = ['apple', 'banana', 'cherry']
fruits.reverse()
print(fruits)

['cherry', 'banana', 'apple']

4.7 删除元素 pop()、remove()、clear()

1、pop()方法

pop() 方法用于移除列表中的一个元素（默认最后一个元素），并且返回该元素的值。

语法

list.pop(pos)

参数值

实例

• 当 pos 未指定时，默认删除最后的元素

fruits = ['apple', 'banana', 'cherry']
fruits.pop()
print(fruits)

['apple', 'banana']

• pos 指定要删除元素的位置

fruits = ['apple', 'banana', 'cherry']
fruits.pop(1)
print(fruits)

['apple', 'cherry']

2、remove()方法

remove() 方法用于移除列表中某个值的第一个匹配项。

语法

list.remove(element)

参数值

实例

num = [1, 3, 2, 8, 3]
num.remove(3)
print(num)

[1, 2, 8, 3]

当被删除的元素在列表中存在多个时，默认删除首次出现的那个。

3、clear()方法

clear() 方法用于清空列表，类似于 del a[:]。

语法

list.clear()

参数值

无参数

实例

word = ['A', 'B', 'C']
word.clear()
print(word)

[]

The function of the clear() method is to clear the list. When the print() is used to print the output after the execution, it will output [], indicating that the list still exists, but it is just an empty list .

The role of the del function is to delete the list. When the print() is used to print the output after the execution, an error will be reported NameError: name 'word' is not defined..

5. Summary

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Last edited at: 2023/1/4 20:22