First look at the following code
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, y = y, xThe meaning behind this code is deconstruction and encapsulation
In [1]: t =1,2
In [2]: t
Out[2]:(1,2)
In [3]:type(t)
Out[3]: tuple #The 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)
So the result of encapsulation must be a tuple.
x, y = y, xThe right side of this code will be encapsulated as (y, x)
In [8]: lst =[1,2]
In [9]: first, second = lst
In [10]:print(first, second)12
According to the order of the elements, assign the elements of the linear structure lst to the variables first, second
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 an asterisked variable 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 multiple asterisks, only one
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)
Summarized as the following rules:
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 #Can be deconstructed in one step
In [29]: val
Out[29]:3
In [30]: _,[*_, val],*_ = lst #Deconstruct the middle part into a list
In [31]: val
Out[31]:3
In [32]: _, _, val,*_ = lst # (2,3)Resolve to the second_
In [33]: val
Out[33]:5
Using a single underscore _ means discarding the variable, which is a Python convention. A single underscore is also a valid identifier in Python, but if you don't want to discard a variable, you usually don't use a single underscore to indicate a meaningful variable. It can be understood as a convention.
Very complex data structure, multi-layer nested linear structure, you can use deconstruction to quickly extract the value, which is very convenient
For example, the following method of use
In [1]: key, _, value ='I love Python'.partition(' love ')
In [2]: key
Out[2]:'I'
In [3]: value
Out[3]:'Python'
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