What is exception handling in Python?

Exception handling in Python means that when running a program, if an error occurs (such as dividing by zero, the file does not exist, etc.) causing the program to terminate or an exception occurs, the program will automatically jump to the exception handling code block and perform the corresponding processing so that the program does not terminate directly.

Exception handling is very important. It can help programmers output error information, and can help us find the location of the error, reduce the error rate of the program, and improve the stability and maintainability of the program.

Exception handling in Python mainly includes the following keywords: try, except, finally and raise. Their usage and functions are introduced below:

1.try-except

The try-except statement can catch exceptions and handle them to prevent the program from being interrupted due to exceptions. The syntax is as follows:

try:
    #代码块
except [异常类型]:
    #异常处理代码块

The code block below the try statement is the code block to be executed. If an exception occurs, it will jump to the except block for exception handling. If no exception occurs, the code in the except block is skipped.

When except is followed by a specific exception type, the code in the except block will be executed only when the exception of that type is caught; when except is not followed by any exception type, it will be caught. All abnormal.

An example is given below:

try:
    a = 5 / 0  # 这里会抛出一个ZeroDivisionError异常
except ZeroDivisionError:
    print("除数不能为零")

Explanation: The above program will throw a ZeroDivisionError exception when executing 5/0, but due to the use of the try-except statement, the program will not It crashes, but outputs the sentence "The divisor cannot be zero".

2.finally

finally is an optional code block. Regardless of whether there is an exception in the try block, the code in the finally block will be executed. For example:

try:
    # 代码块
except:
    # 异常处理代码块
finally:
    # finally块

In the above program, if an exception occurs in the try block, the except block will be executed for exception handling; if no exception occurs, the code in the finally block will also be executed. If statements such as return, break, or continue appear in the finally block, the value in finally will be returned.

An example is given below:

try:
    f = open('filename.txt', 'r')
    lines = f.readlines()
    result = ""
    for line in lines:
        result += line
except IOError:
    print("文件读取出错")
finally:
    print("关闭文件")
    f.close()

Explanation: The above program uses try-except to read the file. If a reading error occurs, the code in the except block will be executed regardless of whether If an exception occurs, the code in the finally block will be executed to ensure that the file is closed correctly.

3.raise

The raise statement is used to throw exceptions manually. For example:

x = 10
if x > 5:
    raise Exception('x不能大于5。x的值为：{}'.format(x))

In the above program, when the value of x is greater than 5, the program will manually throw an Exception type exception. After the exception is thrown, the program will jump to the except block for exception handling.

4.try-except-else

The code in the else block in the try-except-else statement will only be executed when no exception is thrown in the try block. For example:

try:
    x = int(input("请输入一个整数："))
except ValueError:
    print("无效的输入！")
else:
    print("输入的数字是：", x)

In the above program, when the user input is not an integer, a ValueError exception will be thrown, and the program will execute the code in the except block. When the input is an integer, the program will execute else. Code in a block that outputs the number entered by the user.

Summary

Exception handling is very important for programmers. It can help us find errors in the program, reduce the crash rate of the program, and improve the stability and maintainability of the program. . In Python, the try-except statement can help us catch exceptions and handle them. The finally block can ensure that the code in the finally block will be executed under any circumstances. The raise statement can manually throw exceptions. Please make reasonable use of these keywords to ensure that your program is more stable and reliable.

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