The text and pictures of this article come from the Internet and are only for learning and communication. They do not have any commercial use. The copyright belongs to the original author. If you have any questions, please contact us in time for handling

The following article comes from Tencent cloud, written by brother Liang in the workplace

(want to learn Python? Python learning exchange group: 1039649593, meet your needs. All materials have been uploaded to the group file stream and can be downloaded by yourself! There are also a large number of the latest 2020 Python learning materials.)

Python deconstruction and encapsulation

Ask questions

Look at the following code first

x = 1
y = 2

tmp = x
x = y
y = tmp

print(x, y)

The output of the code is: 2 1

Look at the following code:

x = 1
y = 2

x, y = y, x
print(x, y)

The output of the code is: 2 1

x. The meaning behind the code y = y, X is deconstruction and encapsulation

Python encapsulation

In [1]: t = 1, 2

In [2]: t
Out[2]: (1, 2)

In [3]: type(t)
Out[3]: tuple			# Parentheses can be omitted when defining tuples

In [4]: t1 = (1, 2)

In [5]: t2 = 1, 2
						# t1 and t2 are equivalent
In [6]: t1
Out[6]: (1, 2)

In [7]: t2
Out[7]: (1, 2)

Therefore, the encapsulated result must be tuples.

x. Y = y, x the right side of the code will be encapsulated as (y, x)

Python deconstruction

Basic deconstruction

In [8]: lst = [1, 2]

In [9]: first, second = lst

In [10]: print(first, second)
1 2

Assign the elements of the linear structure lst to the variables first and second according to the element order

Asterisk deconstruction

In [11]: lst = list(range(5))

In [12]: head, *tail = lst

In [13]: head
Out[13]: 0

In [14]: tail
Out[14]: [1, 2, 3, 4]

In [15]: *lst2 = lst	# There must be a variable with an asterisk on the left
  File "<ipython-input-15-98211a44ccfb>", line 1
    *lst2 = lst
               ^
SyntaxError: starred assignment target must be in a list or tuple


In [16]: *head, tail = lst

In [17]: head
Out[17]: [0, 1, 2, 3]

In [18]: lst
Out[18]: [0, 1, 2, 3, 4]

In [19]: tail
Out[19]: 4

In [20]: head, *m1, *m2, tail = lst		# There cannot be more than one asterisk. There can only be one asterisk
  File "<ipython-input-20-1fc1a52caa8e>", line 1
    head, *m1, *m2, tail = lst
                              ^
SyntaxError: two starred expressions in assignment


In [21]: v1, v2, v3, v4, v5, v6, v7 = lst	# The number of variables on the left cannot exceed the number of elements on the right
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-21-9366cfb498a1> in <module>()
----> 1 v1, v2, v3, v4, v5, v6, v7 = lst

ValueError: not enough values to unpack (expected 7, got 5)

In [22]: v1, v2 = lst						#The number of variables on the left cannot be less than the number of elements on the right
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-22-d7b0a4e7871e> in <module>()
----> 1 v1, v2 = lst

ValueError: too many values to unpack (expected 2)

It can be summarized as the following rules:

• Elements are assigned to variables in order
• Variables and elements must match
• Asterisk variables can accept any number of elements
• Variables with asterisks cannot appear alone

Multilevel deconstruction

Deconstruction supports multiple levels

In [23]: lst = [1, (2, 3), 5]

In [24]: _, v, *_ = lst		# v resolves to (2, 3)

In [25]: v
Out[25]: (2, 3)

In [26]: _, val = v			# v can be further deconstructed

In [27]: val
Out[27]: 3

In [28]: _, (_, val), *_ = lst		# It can be deconstructed in one step

In [29]: val
Out[29]: 3

In [30]: _, [*_, val], *_ = lst		# Middle part solution composition list

In [31]: val
Out[31]: 3

In [32]: _, _, val, *_ = lst		# (2, 3) parse into the second_

In [33]: val
Out[33]: 5

Use of Python underscores

Use single underline_ Indicates that the variable is discarded, which is a python convention. A single underscore is also a legal identifier for Python, but if you don't want to discard a variable, you usually don't use a single underscore to represent a meaningful variable. It can be understood as a convention.

Use of deconstruction and encapsulation

For very complex data structures and multi-layer nested linear structures, deconstruction can be used to quickly extract the values, which is very convenient

For example, the following usage methods

In [1]: key, _, value = 'I love Python'.partition(' love ')

In [2]: key
Out[2]: 'I'

In [3]: value
Out[3]: 'Python'