Exception overview

Exception refers to the error caused when the program is running. There are many reasons for the error, such as syntax error, logic error, etc. if these errors are not handled, the program will terminate
In the program, when python detects an error, the interpreter will point out that the current process cannot continue, and will give relevant prompts for the error, as shown in the following example:
The above prompt tells us that strings and integers cannot be added. TypeError is a built-in exception class in python. There are many exception classes like this in python
Exception class

Exception class	describe
Exception	Base class for all exceptions
ZeroDivisionError	Divisor is 0
TypeError	The incoming object type is not acceptable
NameError	Variable not declared
IndexError	Index out of sequence range
KeyError	A dictionary keyword that does not exist was requested
ValueError	The parameter type passed into the function is incorrect
AttributeError	An attempt was made to access an unknown object property

Exception capture and handling

In order to prevent the program from terminating unexpectedly due to exceptions, the possible exceptions should be caught and handled during development,

Try except statement

Syntax format:

try:
	Statements with possible exceptions
except Exception class name:
	Statement handling exception
except Exception class name:
	Statement handling exception	
except ...
	...	

When an error occurs in a statement under try, the program will directly jump to the except statement. Let's see an example of the program

Case 1: program exception capture

try:
	choice=''
	while choice!='f':
		a=int(input('Please enter an integer:'))
		b=int(input('Please enter another integer:'))
		print('a/b The result of integer division is:',a/b)
		choice=input('Continue: (yes) y no f):')
except ZeroDivisionError:
	print('Divisor cannot be zero!')	
except ValueError:
	print('Invalid parameter entered')	

Operation results

From the above program, we can see that the program did not execute the print statement under try, because the program caught an error with divisor 0 and processed it, so the program can end normally

Get exception information using as

as statement converts the captured exception class into an exception object, and the exception information can be accessed through the exception object name
Single exception class information

Syntax format

try:
	Statements with possible exceptions
except Exception class name as Exception object name:
	Statement handling exception	

Multiple exception class information
Syntax format:

try:
	Possible exception classes
except (Exception class name 1,,Exception class name 2,..) as Exception object name:
	Statement handling exception	

Case 2: use the as statement to return the caught exception

try:
	choice=''
	while choice!='f':
		a=int(input('Please enter an integer:'))
		b=int(input('Please enter another integer:'))
		print('a/b The result of integer division is:',a/b)
		choice=input('Continue: (yes) y no f):')
except (ZeroDivisionError，ValueError) as e:
	print(type(e),e)	

Operation results

Try except else statement

It is often used to handle cases where exceptions are not caught. When exceptions are caught successfully, else statements are not executed
:
Syntax format:

try:
	Statements with possible exceptions
except Exception class name as Exception object name:
	The statement that handles the exception after the exception is caught	
else:
	No abnormally executed statements were caught	

Case 3: use of else statement during exception capture

try:
	a=int(input('Please enter an integer:'))
	b=int(input('Please enter another integer:'))
	c=a/b
except	BaseException as e:
	print('Please check whether the entered data is legal',e)	
else:
	print('The program execution result is:',c)		

Operation results

Try finally statement

The finally statement block is executed regardless of whether an exception occurs in the try statement. It is often used to clean up the operations performed in the try statement, such as releasing the memory occupied by it
Syntax format:

try:
	Statements with possible exceptions
finally:
	Other statements	

Case 4: use of finally statement during exception capture

try:
	file1=open('123.txt')
	line=file1.readline()
	print(line)
finally:
	file1.close()	

Trigger exception

There are two conditions for triggering exceptions:

1. An exception is automatically triggered due to an error during program execution
2. Explicitly use raise or assert statements to manually trigger exceptions

raise statement

1. Trigger exception by class name
   You only need to specify the exception class to create an instance object of the exception class and trigger the exception
   Syntax format: raise exception class name
   

2. Exception triggered by instance object
   You only need to specify the instance object of the exception class to trigger the exception
   Syntax format: instance object of raise exception class ('specify exception information ')
   

assert statement

assert statement, also known as assertion statement, is mainly used to express some judgment conditions of an object in the program
Syntax format: assert expression [, parameter]
If the expression is true, no exception will be triggered. If the expression is false, an AssertError exception will be triggered. Optional parameters can be thrown as part of the exception information

Custom exception

python's built-in exception class is limited after all. It can be used to set other exceptions according to requirements

Case 5: customize a class that can only take positive numbers

class NumberError(Exception):	# Inherit parent class
	def __init__(self,data):
		super().__init__(self,data)	# Inherit the parent class construction method and add a parameter
		self.data=data
	def __str__(self):
		return self.__class__.__name__+':'+self.data+'illegal value(negative)'	# __Class_ represents the class of the object under current operation,
try:
	num=input('Please enter a positive number:')
	if int(num)<=0:
		raise NumberError(num)	
	print('The positive number entered is:',num)
except BaseException as e:
	print(e)			

Operation results