Series articles ：

file

File read and write

file open

General format for file opening

with open(" File path ", " Turn on mode ", encoding= " Character encoding of operation file ") as f:
	" Read and write the file accordingly "
 Use  with Benefits of block ： After execution , Automatically update files close operation 

• example 1 Read a simple file

with open("C:/Users/ytm/Desktop/ The test file .txt", "r", encoding="utf-8") as  f: # The first step is to open the file 
    text = f.read() #  Step 2 read the file 
    print(text)

1、 Turn on mode
“r”   read only mode , If the file does not exist , Report errors
“w”  Overwrite write mode , If the file does not exist , Create ; If the file exists , Then completely overwrite the original file
“x”  Create write mode , If the file does not exist , Create ; If the file exists , Report errors
“a”   Append write mode , If the file does not exist , Create ; If the file exists , Then add content after the original document
“b” Binary mode , Not to be used alone , It needs to be used together, such as "rb",“wb”,“ab”, This mode does not need to be specified encoding
“t” Text file mode , The default value is , To be used together Such as "rt",“wt”,“at”, Generally omit , Abbreviated as "r",“w”,“a”
“+”, And "r",“w”,“x”,"a" In combination with , Based on the original function , Add read and write function
Open mode default , The default is read-only mode

2、 File path
Absolute path or relative path

3、 Character encoding

• unicode utf-8 Contains characters needed in all countries of the world
• Chinese code gbk Specially solve the problem of Chinese coding .
  windows Under the system , If the default , The default is gbk( Code of the region )
  For the sake of clarity , In addition to dealing with binary files , It is recommended not to default encoding

File reading

1、 Read the whole content f.read()

with open("C:/Users/ytm/Desktop/ The test file .txt", "r", encoding="utf-8") as  f: # The first step is to open the file 
    text = f.read() #  Step 2 read the file 
    print(text)
 Linjiang fairy · The Yangtze River flows east 
 The Yangtze River flows east , The waves wash away the heroes .
 Right or wrong, turn around .
 Green mountain is still in , Several sunsets are red .
 White haired fisherman and woodcutter on the river , Used to watch the autumn moon and spring wind .
 A pot of wine likes to meet .
 How many things in ancient and modern times , All of them are laughing .

Decoding mode does not match

with open("C:/Users/ytm/Desktop/ The test file .txt", "r" ) as  f: # encoding default windows The system defaults to gbk
    text = f.read() #  Step 2 read the file 
    print(text)
UnicodeDecodeError: 'gbk' codec can't decode byte 0x80 in position 50: illegal multibyte sequence

2、 Read line by line - f.readline()

with open("C:/Users/ytm/Desktop/ The test file .txt", "r", encoding="utf-8" ) as  f: # encoding default windows The system defaults to gbk
    for i in range(3):
        text = f.readline()
        print(text)
 Linjiang fairy · The Yangtze River flows east 

 The Yangtze River flows east , The waves wash away the heroes .

 Right or wrong, turn around .

with open("C:/Users/ytm/Desktop/ The test file .txt", "r", encoding="utf-8" ) as  f: # encoding default windows The system defaults to gbk
    while True:
        text = f.readline()
        if not text:
            break
        else:
            print(text, end="") ##  Keep the newline of the original , send print() Line feed of does not work 
 Linjiang fairy · The Yangtze River flows east 
 The Yangtze River flows east , The waves wash away the heroes .
 Right or wrong, turn around .
 Green mountain is still in , Several sunsets are red .
 White haired fisherman and woodcutter on the river , Used to watch the autumn moon and spring wind .
 A pot of wine likes to meet .
 How many things in ancient and modern times , All of them are laughing .

3、 Read in all lines , Form a list with each behavior element f.readlines()

with open("C:/Users/ytm/Desktop/ The test file .txt", "r", encoding="utf-8" ) as  f: # encoding default windows The system defaults to gbk
    text = f.readlines()
    print(text)
[' Linjiang fairy · The Yangtze River flows east \n', ' The Yangtze River flows east , The waves wash away the heroes .\n', ' Right or wrong, turn around .\n', ' Green mountain is still in , Several sunsets are red .\n', ' White haired fisherman and woodcutter on the river , Used to watch the autumn moon and spring wind .\n', ' A pot of wine likes to meet .\n', ' How many things in ancient and modern times , All of them are laughing .']

with open("C:/Users/ytm/Desktop/ The test file .txt", "r", encoding="utf-8" ) as  f: # encoding default windows The system defaults to gbk
    for text in f.readlines():
        print(text,end="")
 Linjiang fairy · The Yangtze River flows east 
 The Yangtze River flows east , The waves wash away the heroes .
 Right or wrong, turn around .
 Green mountain is still in , Several sunsets are red .
 White haired fisherman and woodcutter on the river , Used to watch the autumn moon and spring wind .
 A pot of wine likes to meet .
 How many things in ancient and modern times , All of them are laughing .

4、 Text file reading summary
When the file is large ,read() and readlines() Too much memory , Not recommended
readline It's not convenient to use

5、 Binary

with open("test.jpg", "rb") as f:     
        print(len(f.readlines()))

File is written to

1、 Write a string or byte stream to the file （ Binary system ）f.write()

with open(" Love songs 1980.txt", "w", encoding="utf-8") as f:                      
        f.write(" You once said to me \n")        #  If the file does not exist, create one immediately 
        f.write(" You always love me \n")        # If the file exists , It will cover ！！！
        f.write(" I understand love \n") 
        f.write(" But what is forever \n") #  If you need a new line , Final newline \n

2、 Append mode ——“a”
3、 Write a list whose element is a string to the file as a whole ——f.writelines()

ls = [" The wind blows in spring ", " It rains in autumn ", " How many vows of eternal love go with the wind in spring and autumn "]
with open("C:/Users/ytm/Desktop/ The test file .txt", "w", encoding="utf-8" ) as  f: # encoding default windows The system defaults to gbk
    f.writelines(ls)

exception handling

Common exceptions occur

1. except 0 operation ZeroDivisionError
2. Cannot find readable file FileNotFoundError
3. Wrong value ValueError
   Pass in a value that the caller doesn't expect , Even if the type of the value is correct
   s = “1.3”
   n = int(s)
4. Index error IndexError
   The subscript crossing the line
5. Type error TypeError
   NameError Use an undefined variable
   KeyError Trying to access a key that doesn't exist in the dictionary

When an exception occurs , If the processing method is not preset , The program will break

exception handling

try_except

• If try The inner code block executes smoothly ,except Not triggered
• If try An error occurred in the inner code block , Trigger except, perform except Internal code block
• Omnipotent abnormality Exception （ All the wrong ancestors ）
• Catch the abnormal value as
• If try Module execution , be else The module also performs ry_except_else
• Can be else regard as try The bonus of success
• Regardless of try Whether the module performs ,finally At the end of the day try_except_finally

x = 10
y = 0
ls = []
d = {
    "name":" Big hero "}
try:
    z = x/y
except ZeroDivisionError:
    print("0 Not divisible ！！！")
except NameError:
    print(" Variable name does not exist ")
except KeyError:
    print(" The key doesn't exist ")
except Exception as e: # Omnipotent abnormality  Exception （ All the wrong ancestors ）
    print(" Error ！")
    print(e) # Capture exception 
else:
    print("else  regard as try The bonus of success ")
finally:
    print(" Whether touching or not triggers an exception , Will execute ")

modular

Has been encapsulated , No need to “ To build the wheels ”, Declare after import , Use immediately

Generalized module classification

1. Python built-in
   Time bank time Random library random Container data type collection iterator function itertools
2. Third party Library
   Data analysis numpy 、pandas Data visualization matplotlib machine learning scikit-learn Deep learning Tensorflow
3. Customization files
   A separate py file
   package ---- Multiple py file

Module import

1. Import the entire module ----- import Module name

import time
start = time.time()
time.sleep(3)
end = time.time()
print(" Program running time ：{}".format(end-start))

import fun1
fun1.f1()

2. Import a class or function from a module ---- from modular import Class name or function name

from itertools import product
ls = list(product("AB","123"))
print(ls)
[('A', '1'), ('A', '2'), ('A', '3'), ('B', '1'), ('B', '2'), ('B', '3')]

from function.fun1 import f1         #  Pay attention to this usage 
f1()

Import more than one at a time

from function import fun1, fun2
fun1.f1()
fun2.f2()

3. Import all classes and functions in the module —from modular import *

from random import * 
print(randint(1,100))       #  Produce a [1,100] Random integer between 
print(random())             #  Produce a [0,1) Between random decimals 

Module lookup path

Module search order ：

• Priority is given to importing modules from memory
• Built-in module

# Python  Startup time , The interpreter will load some by default  modules  Store in sys.modules in 
# sys.modules  The variable contains one loaded by the current ( Complete and successful import ) A dictionary of modules to the interpreter ,  Module name as key ,  Their positions are taken as values 
import math
import sys
# print(sys.modules)

print("math" in  sys.modules) #True
print("numpy" in sys.modules) #False

sys.path Modules contained in path

import sys
print(sys.path)

• sys.path The first path of is the folder where the current execution file is located
• If you need to import modules that are not in this folder , You need to add the path of the module to sys.path

import sys

sys.path.append("C:\\Users\\ibm\\Desktop")    #  Notice the double slash 

import fun3

fun3.f3()