List derivation (List Comprehensions) Also from functional programming languages , You can filter and process a sequence of data gracefully to get a result list .
such as , Put all the items in a list that are greater than 0 The number of squares after the production of a new list .
num = [-2, 3, 5, -4, 10, 7]As usual , We will
num = [-2, 3, 5, -4, 10, 7]
result = []
for x in num:
if x > 0:
result.append(x**2)
print resultthen , In the form of list derivation , It only needs
print [x**2 for x in num if x > 0]How to interpret this formula ？
Divide the equation into three parts ： Output processing 、for loop 、if Judge . among if The judgment section is optional .
if The judgment part is used to filter .
The output processing part is flexible , Do whatever you want .
For example, there is a list A = ‘1’, ‘2’, ‘3’, We can int(x) for x in A, Convert all the elements of the list into int type .
The above is the most basic form . There are also slightly more complex forms , How to interpret it ？
The method is , Apart from “ Output processing ” part , The others are from left to right .
1） Multiple for loop
[(x, y) for x in range(3) for y in range(5)]ad locum , The processing part is (x, y), Press other buttons from left to right . Equivalent to
for x in range(3):
for y in range(5):
# processing section 2） Nested list derivation
[[0 for y in range(3)] for x in range(5)]ad locum , The processing part is 0 for y in range(3), Press other buttons from left to right . Equivalent to
for x in range(5):
# processing section 3） Multiple if Judge
[(x, y) for x in range(3) if x>1 for y in range(5) if y>2]ad locum , The processing part is (x, y), Press other buttons from left to right . Equivalent to
for x in range(3):
if x>1:
for y in range(5):
if y>2:
# processing section 1） Other derivation
Except for list derivation . And the generator derivation 、 Set derivation (python2.7 And above )、 Dictionary derivation (python2.7 And above ).
[x for x in range(3)] #-> List derivation
(x for x in range(3)) #-> Generator derivation
{x for x in range(3)} #-> Set derivation
{x:None for x in range(3)} #-> Dictionary derivation such as
>>> d = {'a':1, 'b':2, 'c':3}
>>> d2 = {v:k for k,v in d.items()}# Reverse mapping
>>> d2
{1: 'a', 2: 'b', 3: 'c'}
>>> d3 = {k:v.upper() for k,v in d2.items()}# Capitalize all values
>>> d3
{1: 'A', 2: 'B', 3: 'C'}2）Python3 base64 Encoding and decoding
In the blue whale project , Some interface messages are sent through base64 Encrypted transmission , So in the process of interface automation , It is necessary to modify the transmitted parameters base64 code , Decode the received response message ;
for example ： Operation platform fast_execute_script( Execute scripts quickly ) Of script_content
python3.x All Chinese characters are unicode code , and b64encode The argument to the function is byte type , So we must transcode first .
import base64
# encode code
# decode decode
s ='adsvsdega15s1dasda'
encodestr = base64.b64encode(s.encode('utf-8'))
print(encodestr)
# b'YWRzdnNkZWdhMTVzMWRhc2Rh'
print(str(encodestr, 'utf-8'))
# YWRzdnNkZWdhMTVzMWRhc2Rh
decodestr = base64.b64decode(encodestr)
print(decodestr)
# b'adsvsdega15s1dasda'
print(str(decodestr, 'utf-8'))
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