1. Function

function

2. Lists and Tuples

Lists and Tuples

3. Dictionary

dictionary

4. Documentation

4.1 What is a file?

Variables save data in memory. If the program is restarted/powered off, the data in memory will be lost.
To achieve persistent storage of data, you need to store the data , which can be stored in the hard disk, that is, in a file.
Some common file types :

文本文件 txt
可执行文件 exe class 
图片文件 jpg png
视频文件 mp4 mov
office文件 ppt docx

This article focuses primarily on text files .

4.2 File Path

There are a lot of files in the system. In order to organize these files more comprehensively, many folders are often used , which are also called directories in Linux
. In fact, a file often has many nested directories. In order to easily determine the location of a file, it is very convenient to use a file path to describe it.
Take the path where I store Python code as an exampleD:\code\python

• D: represents the drive letter, not case sensitive.
• Each \represents a first-level directory. The current Python is placed in the code directory under the D drive.
• The separator between directories can be used \or it can be used . It is generally more convenient /to use when writing code . Absolute path and relative path:/
  
• The path that starts with a drive letter is called an absolute path.
• A relative path requires specifying a base directory first, and then using the base directory as a reference point to indirectly find the target file.
  When describing a file path, you can use either an absolute path or a relative path.

4.3 File Operations

To use files, you need to save data through files and retrieve the saved data later. In order to read and write files, you must first open the file and close the file after completing the operation.

4.3.1 Opening a File

Use built-in functions opento open the file.

f = open('d:/test.txt','r')

• The first parameter is a string representing the path to open.
• The second parameter is a string indicating the opening method, where r' means opening in read mode, ' wmeans opening in write mode, and 'a' means opening in append mode.
• If the file is opened successfully, a file object is returned, and subsequent read and write file operations are carried out around this file object.
• If opening the file fails, such as if the specified path does not exist, an exception will be thrown.

4.3.2 Closing a File

Use closemethod to close an opened file.

f.close()

Remember to close the files you have used.
There is a limit to the number of files a program can open at the same time.

flist = []
count = 0
while True:
	f = open('d:/tmp/test.txt','r')
	flist.append(f)
	count+=1
	print(f'count = {count}')
'''
...
count = 8187
count = 8188
count = 8189
Traceback (most recent call last):
  File "d:\code\python\python_test1\test.py", line 158, in <module>
OSError: [Errno 24] Too many open files: 'd:/tmp/test.txt'
'''

From the code, it can be seen that a program can open about 8,000 files. If files are opened in a loop without being closed, the above problem will occur. When a program opens more files than the upper limit, an exception will be thrown.

In the above code, a list is used to save all file objects. If it is not saved, Python's built-in garbage collection mechanism will automatically close the file when the file is destroyed.
However, since the garbage collection mechanism is not necessarily timely, we still need to manually recycle when writing code to avoid relying on automatic recycling.

4.3.3 Writing Files

Once the file is open, you can write to it.

• To write a file, use the following method to open it, and openthe second parameter is w.
• Write to the file using writemethod.

f = open('d:/tmp/test.txt','w')
f.write('i am yui')
f.close()

Open test.txt and find that it has been written.
If ryou write in this way, the program will throw an exception.

After wsuccessfully opening the file, the data in the original file will be cleared.
To aimplement appending, the original content remains unchanged and the written content will exist at the end of the previous file content.

f = open('d:/tmp/test.txt','w')
f.write('i am yui')
f.close()

f = open('d:/tmp/test.txt','a')
f.write(',hello')
f.close()

• If you try to write to a closed file object, the program will throw an exception.

4.3.4 Reading Files

• Reading file contents requires ropening the file using the method.
• Use readthe method to complete the read operation, and the parameters are indicated 读取几个字符.

f = open('d:/tmp/test.txt','r')
result = f.read(10)
print(result)
f.close()
'''
i am yui,h
'''

• If the file is a multi-line text file, you can use a for loop to read one line at a time.
  Let's first construct a multi-line text file.

111111
222222
333333
444444

Let's print the contents of this file.

f = open('d:/tmp/test.txt','r')
for line in f:
	print(f'line = {line}')
f.close()
'''
line = 111111

line = 222222

line = 333333

line = 444444

'''

Since there is a line break at the end of each line in the file, a line break will be added by default when printing, so there will appear to be spaces between the printed results
.

print(f'line = {line}',end='')

It can replace the line break that comes with print.

• Use to readlinedirectly read the entire content of the file and return a list with each element being a line.

f = open('d:/tmp/test.txt','r')
lines = f.readlines()
print(lines)
f.close()
'''
['111111\n', '222222\n', '333333\n', '444444\n']
'''

4.4 About Chinese processing

When Chinese characters appear in the file content, errors may occur when reading the file content. That is, garbled characters appear.
Why does this happen?
When computers represent Chinese characters, they use a certain encoding method, which is called字符集

The so-called "encoding method" is essentially to use numbers to represent Chinese characters.
As we all know, computers can only represent binary data. If you want to represent English letters, Chinese characters or other text characters, you have to use encoding.
The simplest character encoding ASCII uses a simple integer to represent English letters and numbers, but if you want to represent Chinese characters, you need a larger code table.
The commonly used Chinese character encoding methods are mainly: GBK and UTF-8.

To do this, it is necessary to ensure that the encoding method of the file itself is consistent with the encoding method used to read the file in the Python code to avoid errors.

By default, python3 opens files in the same character set as the system, and the character set of Windows Simplified Chinese version uses GBK, so if the file itself is encoded in GBK, it can be processed correctly. If not, an error will occur.

4.5 Using context managers

After opening a file, the easiest thing to forget is closing it. Python provides a context manager to help us close files automatically.

• Use the with statement to open the file.
• When the code inside with is executed, the close method will be called automatically.

with open('d:/tmp/test.txt','r') as f:
	lines = f.readlines()
	print(lines)
'''
['111111\n', '222222\n', '333333\n', '444444\n']
'''