Common syntax and modules in Python crawler

SunriseCai 2020-11-13 11:31:59
common syntax modules python crawler


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This article is pure and wild , There is no reference to other people's articles or plagiarism . Insist on originality !!

Preface

Hello . Here is Python Reptiles from getting started to giving up series of articles . I am a SunriseCai.

This article is my usual study Python Some knowledge points commonly used in , Don't be Python System learning .

Python Common basic usage

Python Basic data structure

This is simpler , It is recommended to click the link directly to check the usage .

data structure features
list list variable , Orderly
Tuples tuple immutable , Don't add, delete, or alter
Dictionaries dict Exists as a key value pair
aggregate set variable , disorder

Python Placeholders and format()

Definition :

Place holder Indicates the type
%s character string
%d Integers
%f Floating point numbers
format() hold % Replace with {} To achieve formatted output

Place holder % Example :

name = input(' Type in your name :') # SunriseCai
age = input(' Enter your age :') # 20
weight= input(' Enter your weight :') # 65.432
print(' Your name is %s, Age is %d, Height is %.2f kg ' % (name, int(age), float(weight)))
# Your name is SunriseCai, Age is 20, The weight is 65.43 kg
# %.2f It means taking 2 Decimal place

format() Format example :

name = input(' Type in your name :') # SunriseCai
age = input(' Enter your age :') # 20
weight = input(' Enter your salary :') # 65.432
print(' Your name is {}, Age is {}, The weight is {} kg '.format(name, age, weight))
# Your name is SunriseCai, Age is 20, The weight is 65.432 kg

Python Conditional statements

Python Programming if Statement is used to control the execution of a program , The basic form is :

if Judge the condition 1:
Execute statement A
elif Judge the condition 2:
Execute statement B
else:
Execute statement C

To understand is : If meet Conditions 1 execute A, dissatisfaction Conditions 1 But satisfied Conditions 2 execute B, If neither of them is satisfied, they will execute C.
Example :

num = 10
if num > 10: # Judge num
print(' I am bigger than 10')
elif num < 10: # num Less than 10 It outputs
print(' I am smaller than 10')
else: # If the above is not satisfied, output
print(' I am equal to 10')
# Final output I am equal to 10

Python Loop statement

Here's a quote from the rookie tutorial Python Loop statement .

  • Python Provides for Circulation and while loop :
Type of cycle describe
while loop The given judgment condition is true When the loop is executed , Otherwise, push out the circulatory body
for loop Repeat the statement

while Loop example :

  • Always print the input string , Until input end Exit loop .
while True:
string = input(' Input string :')
if string == 'end':
break # Exit loop
print(string)

for Loop example :

  • repeat 100 Print through Hello World.
for i in range(100):
print('Hello World')

Python Of join()、split()、strip()

Method describe
Python join() Used to connect elements in a sequence with specified characters to generate a new string .
Python strip() Used to remove characters specified at the beginning and end of a string ( The default is space or newline ) Or character sequence .
Python split() Specify the separator to slice the string , If parameters num There is a specified value , Then separate num+1 Substring

Example :

  • Python join() :
string = ['SunriseCai', 'SunriseCai', 'SunriseCai']
print('--'.join(string)) # SunriseCai--SunriseCai--SunriseCai
  • Python strip():
string = '%#SunriseCai%#'
print(string.strip('%#')) # SunriseCai
  • Python split():
string = '%#SunriseCai%#'
print(string.split('%#')) # ['', 'SunriseCai', '']

Python exception handling

Python Commonly used try/except Statement fetch catch exception .
Common use :

  • Normal execution A, If the error is reported, execute B,C No matter whether there is an error or not, it will be executed .
try:
Execute statement A
except Exception as e:
Execute statement B
# raise NameError # once raise perform ,raise None of the following statements will be executed
finally:
Execute statement C

Example :

try:
print(1 / 0) # 1 Divide 0 Will report a mistake
except Exception as e:
print(e)
finally:
print('SunriseCai')
# division by zero
# SunriseCai

Common modules

The method called by the module :

Method effect
import module The import module
import module as xx Rename the import module to xx
from module import xx Import the xx function
from module import xx as bb Module functions to be imported xx Renamed bb

time modular

  • Sleep 10 second :
import time
time.sleep(10) # here Python The program will sleep 10 second
  • Get the current timestamp , namely 1970.01.01 How many seconds have passed by now :
import time
print(time.time()) # 1578838874.4708762
  • Get the current date :
import time
stamp = time.time()
print(time.strftime('%Y-%m-%d', time.localtime(stamp))) # 2020-01-12
  • Get the current date Minutes and seconds
import time
stamp = time.time()
print(time.strftime('%Y-%m-%d %H:%M:%S', time.localtime(stamp))) # 2020-01-12 22:20:20

os modular

  • os Module provides a very rich way to handle files and directories . The common methods are shown in the following table :
Method describe
os.listdir(path) return path The specified folder contains a list of files or folder names .
os.path.exists(path) Judge the document ( Folder ) Existence or not
os.makedirs(path) Recursive folder creation function
os.removedirs(path) Recursively delete directories .
os.rmdir(path) Delete path Empty directory specified , If the directory is not empty , Throws an error
os.remove(path) The deletion path is path The file of . If path It's a folder , An error will be thrown

It's usually os These two methods of os.path.exists(path) and os.makedirs(path) Most used . Judge whether the document exists or not , Then judge if the next file needs to be created .

json modular

  • json The module mainly uses two methods :
function describe
json.dumps take Python The object is encoded as JSON character string
json.loads To encode JSON The string is decoded to Python object

above , Is in Python Reptiles Very few knowledge points commonly used in .
There are many knowledge points and modules that will be used , Here, just remember some of the most used .


Finally, I will summarize the content of this chapter :

  1. It introduces Python Basic data structure
  2. It introduces Place holder And format()
  3. It introduces Conditional statements And Loop statement exception handling etc.
  4. It introduces joinsplitstrip Handling sequence and string usage
  5. Finally, it introduces timeosjson Basic usage of module

sunrisecai

  • Thank you for your patience in watching , Focus , Neverlost .
  • For the convenience of chicken pecking each other , Welcome to join QQ Group organization :648696280( There is no learning material in it , Just for questions )

Next article , be known as 《Python Reptiles from getting started to giving up 03 | Python Advanced usage in reptile 》.

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