Python3 Lists

Sequences are the most basic data structures in Python.

Each value in a sequence has a corresponding position value, called an index. The first index is 0, the second is 1, and so on.

Python has 6 built-in types of sequences, but the most common are lists and tuples.

Operations that can be performed on lists include indexing, slicing, adding, multiplying, and checking membership.

In addition, Python has built-in methods for determining the length of a sequence and for determining the largest and smallest elements.

Lists are the most commonly used Python data type, and they can appear as comma-separated values within square brackets.

The data items in a list do not need to have the same type.

To create a list, simply enclose different comma-separated data items in square brackets. As shown below:

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Accessing Values in a List

Like string indexing, list indices start from0the second index is1and so on.

By using indices, lists can be sliced, combined, and so on.

The above example outputs:

red
green
blue

Indexing can also start from the tail, with the last element's index being-1and the one before it being-2and so on.

The above example outputs:

black
white
yellow

Use subscript indices to access values in a list. Similarly, you can use square brackets[]to slice characters, as shown below:

The above example outputs:

[10, 20, 30, 40]

Slicing with negative index values:

The above example outputs:

list[1]:  Example
list[1:-2]:  ['Example', 'Zhihu']

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Updating Lists

You can modify or update data items in a list, and you can also use the append() method to add list items, as shown below:

Note:We will discuss in the following chaptersappend()the usage of the method.

The above example outputs:

第三个元素为 :  1997
更新后的第三个元素为 :  2001
更新后的列表 :  ['Google', 'Example', 'Taobao', 'Baidu']

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Deleting List Elements

You can use the del statement to delete elements in a list, as in the following example:

The above example outputs:

原始列表 :  ['Google', 'Example', 1997, 2000]
删除第三个元素 :  ['Google', 'Example', 2000]

Note:We will discuss the use of the remove() method in the following chapters

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Python List Script Operators

The + and * operators for lists are similar to those for strings. The + sign is used to combine lists, and the * sign is used to repeat lists.

As shown below:

Python Expression	Result	Description
len([1, 2, 3])	3	Length
[1, 2, 3] + [4, 5, 6]	[1, 2, 3, 4, 5, 6]	Concatenation
['Hi!'] * 4	['Hi!', 'Hi!', 'Hi!', 'Hi!']	Repetition
3 in [1, 2, 3]	True	Whether the element exists in the list
for x in [1, 2, 3]: print(x, end=" ")	1 2 3	Iteration

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Python List Slicing and Concatenation

Python list slicing is similar to string operations, as shown below:

Operation:

Python Expression	Result	Description
L[2]	'Taobao'	Read the third element
L[-2]	'Example'	Read the second-to-last element from the right: count from the right
L[1:]	['Example', 'Taobao']	Output all elements starting from the second element

Lists also support concatenation operations:

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Nested Lists

Using nested lists means creating other lists within a list, for example:

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List Comparison

List comparison requires importing theoperatormodule'seqmethod (see:Python operator module）：

The above code outputs:

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

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Python List Functions & Methods

Python includes the following functions:

No.	Function
1	len(list) Number of elements in a list
2	max(list) Returns the maximum value of list elements
3	min(list) Returns the minimum value of list elements
4	list(seq) Converts a tuple to a list

Python includes the following methods:

No.	Method
1	list.append(obj) Adds a new object at the end of the list
2	list.count(obj) Counts the number of times an element appears in the list
3	list.extend(seq) Appends multiple values from another sequence to the end of the list at once (extends the original list with a new list)
4	list.index(obj) Finds the index position of the first matching item of a value in the list
5	list.insert(index, obj) Inserts an object into the list
6	list.pop([index=-1]) Removes an element from the list (the last element by default), and returns the value of that element
7	list.remove(obj) Removes the first matching item of a value in the list
8	list.reverse() Reverses the elements in the list
9	list.sort( key=None, reverse=False) Sorts the original list
10	list.clear() Clears the list
11	list.copy() Copies the list

Other Extensions