This paper clarifies the chaotic switching operation and elegant derivation of Python

Catalog

1、 Chaotic slicing operation

2、 Elegant derivation

3、 summary

Because it's not used much at work python, So for python I don't know enough about , Only learn when you use it , In the previous examples, little was used python The special grammar of , Based on the principle of giving priority to completing tasks, there is no in-depth , But when reading other people's code, I find that some special syntax is not very familiar , I can't figure out what the code really means , Let's figure out the use of slicing and derivation today ,OK. Let's start

Remember ： In the positive direction, the first one is 0, In the negative direction, the first one is -1, You can use the list as a replacement , Positive is 1 , Negative yes -1,0 It's the middle point

1、 Chaotic slicing operation

A complete slice expression contains two “:”, Used to separate three parameters （start_index、end_index、step）, When there is only one “:” when , The third parameter is the default step=1.

 Basic expression of slice operation ：object[start_index : end_index : step]

step： Either positive or negative , Its absolute value determines the “ step ”, and The sign determines “ Cut the direction ”, It means “ From left to right ” Value , Negative representation “ From right to left ” Value . When step When omitted , The default is 1, That is, from left to right in increments 1 Value .“ Cutting direction is very important ！”“ Cutting direction is very important ！”“ Cutting direction is very important ！”, Important things are to be repeated for 3 times ！

start_index： Indicates the starting index （ Include the index itself ）; When this parameter is omitted , To represent a subordinate “ Endpoint ” Start taking value , As for from “ The starting point ” Or from the “ End ” Start , By step The positive and negative determination of parameters ,step From “ The starting point ” Start , For the negative from “ End ” Start .

end_index： Indicates that the index is terminated （ Does not contain the index itself ）; When this parameter is omitted , It means to fetch data all the way to ” Endpoint “, As for the arrival of ” The starting point “ Or to ” End “, By the same step The positive and negative determination of parameters ,step Set the timing until ” End “, It's negative until ” The starting point “.

In a word , final step Determines the direction , from startIndex To end_index Intercept .

for instance ：

l = [1,2,3,4,5,6,7,8,9]
# 1、 Single index 
print(l[2])
​
# 2、 Two indexes , positive 
l2 = l[1:3]
print(l2)
​
# 3、 Two indexes , reverse 
l3 = l[1:3:-1]
print(l3)
​
l4 = l[-1:-3:-1]
print(l4)

2、 Elegant derivation

#  The syntax format of the list derivation is as follows ：
# [ expression  for  Iterative variable  in  Iteratable object  [if  Conditional expression ] ]
​
# ==>  for  Iterative variable  in  Iteratable object 
#           expression 

One sentence summary ： What type of result is indicated by the outer parentheses , hinder for The previous statement runs during traversal

for instance ：

#  The series of values obtained by calculating the expression form a list .
list = [i for i in range(10) if i > 3]
print(list)
​
dict_b = {key: key * key for key in range(6)}
print(dict_b)
​
set_a = {value for value in ' There are people who are indifferent , Someone's carrying a load '}
print(set_a)

3、 summary

Slicing and derivation are python Common grammar in English , If it can be used flexibly, it can be used to force , You can also make the code simpler