catalogue

Exception - exception mechanism

Brief description of Python exception handling mechanism

try...except... Multiple structures included

try... An except ion structure

try... Multiple except structures

try...except...else structure

try...except...finally structure

return statement and exception handling

Summary of common exceptions

with context management

traceback module

Custom exception class

Exception - exception mechanism

python introduces many classes to describe and handle exceptions

Brief description of Python exception handling mechanism

 

Everything in Python is an object, and exceptions are handled in the way of objects. Process:

When an exception occurs, the interpreter will throw an exception to the current method. 1. If an exception occurs, the interpreter will stop the execution of the method

2. Catch exception: after getting the exception, the interpreter looks for relevant code to handle the exception

The key to the interpreter: positioning

When an exception occurs, the interpreter will report the relevant error information and print the relevant error information on the console. You can trace the process of the error from top to bottom

try...except... Multiple structures included

try... An except ion structure

try... Exception is a common exception handling structure. The structure is as follows:

try:

Monitored statement blocks that may throw exceptions

except BaseException [as e]:

Exception handling statement block

# Test try... except... Structure
# Enter a number in the loop. If it is not a number, an exception will be thrown, and enter 21 to end the loop

while True:
    try:
        a = int(input("Please enter a number:"))
        print(a)
        if a == 21:
            break
    except BaseException as e:
        print(e)
        print("Input exception!!!")

print("End of cycle!!!")

'''
Please enter a number: 23
 Output: 23
 Please enter a number: g
 Output: invalid literal for int() with base 10: 'g'
Output: input exception!!!
Please enter a number: 21
 Output: 21
 Output: end of cycle!!!
'''

During execution, if no exception is thrown in the try block, skip the except statement block and continue to execute the subsequent code; If an exception is found in the try during execution, skip the subsequent code in the try statement and jump to the corresponding exception block to handle the exception. After the exception is handled, continue to handle other codes after exception

try... Multiple except structures

The BaseException above can catch all exceptions, but from the perspective of classical theory, it is generally recommended to catch multiple exceptions that may occur (in the order of subclass first and then parent class) and write exception handling code specifically. In order to avoid omitting possible situations, BaseException can be added at the end. The structure is as follows:

try:

Monitored statement blocks that may throw exceptions

except Exception1 :

Statement block handling Exception1

except Exception2 :

Statement block handling Exception2

......

except BaseException :

A statement block that handles exceptions that may be missing

# Test try... Basic usage of multiple except

try:
    a = int(input("Please enter a divisor:"))
    b = int(input("Please enter a divisor:"))
    c = float(a)/float(b)
    print(c)
except ZeroDivisionError :
    print("Exception, 0 cannot be used as divisor")
except ValueError :
    print("Exception. The entered value is not a number and cannot be divided")
except NameError :
    print("Argument does not exist")
except BaseException as e:
    print(e)

print("Output end!!!")
'''
Please enter a divisor: 5
 Please enter a divisor: 0
 Output: exception, 0 cannot be used as divisor
 Output: end of output!!!

Please enter a divisor: g
 Output: exception. The input value is not a number and cannot be divided
 Output: end of output!!!
'''

try...except...else structure

try... except... An "else block" is added to the else structure. If no exception is thrown in the try, the else statement block will be executed. If an exception is thrown in the try, the except statement block will be executed instead of the else statement block. The two are parallel and competitive. The structure is as follows:

try:

Monitored statement blocks that may throw exceptions

except Exception1 :

Statement block handling Exception1

except Exception2 :

Statement block handling Exception2

......

except BaseException :

A statement block that handles exceptions that may be missing

else:

try the statement block executed after normal execution

# Test try... except... else structure

try:
    f = open("d:\\a.txt","r")
    content = f.readline()
    print(content)
except BaseException as e:
    print(e)
else:
    print("Successfully found{0}file".format(f))
f.close()
'''
Output:
1234566

Successfully found<_io.TextIOWrapper name='d:\\a.txt' mode='r' encoding='cp936'>file
'''

try...except...finally structure

try... except... In the finally structure, the finally block will be executed regardless of whether an exception occurs. It is usually used to release the resources applied in the try. The structure is as follows:

try:

Monitored statement blocks that may throw exceptions

except BaseException [as e]:

Exception handling statement block

finally:

Statement block

# Test the try... except... finally statement
import os
try:
    f = open("e:\\a.txt", "r")
    content = f.readline()
    print(content)
except BaseException as e:
    print(e)
finally:
    if os.path.exists("e:\\a.txt"):
        f.close()
    print("End of program execution!!!")
'''
Output:
[Errno 2] No such file or directory: 'e:\\a.txt'
End of program execution!!!
'''

return statement and exception handling

Because return has two functions: ending the operation of a method and returning a value.

Generally, return is not placed in the exception handling structure, but at the end of the method

Summary of common exceptions

Exceptions in Python are derived from BaseException class. Exception summary:

Exception name	interpretative statement
ArithmeticError	Base class for all numeric errors
AssertionError	Judgment statement failed
AttributeError	Object does not have this property
BaseException	Base class for all exceptions
DepressionWarning	Warning about deprecated features
EnvironmentError	Base class for operating system error
EOFError	No built-in input, reaching eof mark
Exception	Base class for general errors
FloatingPointError	Floating point calculation error
FutureWarning	Warnings about future semantic changes in constructs
GeneratorExit	An exception occurred in the generator and the generator was notified to exit
ImportError	Failed to import module object
IndentationError	Indent error
IndexError	There is no such index in the sequence
IOError	Input \ output operation failed
KeyboardInderrupt	User interrupts execution (usually access ^ c)
KeyError	This key is not in the map
LookupError	Base class for invalid data query
MemoryError	Memory overflow error (not fatal for Python interpreter)
NameError	Undeclared \ initialized object (no properties)
NotImplementedError	Methods not yet implemented
OSError	Operating system error
OverflowError	Numerical calculation exceeds the maximum limit
PendingDeprecationWarning	Warning that features will be discarded
OverflowWarning	Old warning about automatic promotion to long integer
ReferenceError	A weak reference attempts to access an object that has been garbage collected
RuntimeError	General runtime error
RuntimeWarning	Warning of suspicious runtime behavior
StandardError	Base class for all built-in standard exceptions
StopIteration	The iterator has no more values
SyntaxError	Python syntax error
SyntaxWarning	Warning of suspicious syntax
SystemError	General interpreter system error
SystemExit	Interpreter requests exit
TabError	Mixed use of Tab and space
TypeError	Invalid operation for type
UNboundLocalError	Accessing uninitialized local variables
UnicodeDecodeError	Error in Unicode decoding
UnicodeEncodeError	Error encoding Unicode
UnicodeError	Unicode related errors
UnicodeTranslateError	Error converting Unicode
UserWarning	Warnings generated by user code
ValueError	Invalid parameter passed in
Warning	Warning base class
WindowsError	system call filed
ZeroDivisionError	Divide by (modulo) 0 (all data types)

with context management

try... In a series of structures, the finally block will be executed whether there is an exception or not. It is usually used to release resources

Structure:

with context_expr [as var]:

Statement block

With context management can automatically manage resources, automatically restore the scene or context before entering the code after the execution of the with code block, and always ensure the normal release of resources when jumping out of the with block for any reason, whether there are exceptions or not. It is often used in occasions related to file operation and network communication

# Test with statement (not to replace try... except... finally, but to be used in file operation management and network communication)
with open("E:\\codeblock..cc\\1.cpp\\a.txt", "r") as f:
    content = f.readline()
    print(content)
'''
Output:
full name=Xiao Wang, age=20，Gender=male
'''

traceback module

Use traceback module to print exception information

import traceback

try:

print("xxxxxx")

num = 1/0

except:

traceback.print_exc()

# Test traceback module usage
import traceback

try:
    print("x"*10)
    a = 1/0
except BaseException:
    with open("E:\\codeblock..cc\\1.cpp\\a.txt","a") as f:
        traceback.print_exc(file=f)
'''
Output:
XXXXXXXXXX
'''

Custom exception class

In program development, you sometimes need to define your own Exception class. Generally, the runtime Exception usually inherits Exception or its subclass, and the name is generally suffixed with Error and Exception

The user-defined exception is actively thrown with raise statement

# Test custom exception class
class AgeError(Exception):
    def __init__(self, errorinfo):
        Exception.__init__(self)
        self.errorinfo = errorinfo
    def __str__(self):
        return str(self.errorinfo)+"，Wrong age, age should be 1-150 between"

##########Test code#############


if __name__ =="__main__":  # If TRUE, the file is run as a stand-alone module and the test code can be executed
    age = int(input("Please enter an age:"))
    if age < 0 or age > 150:
        raise AgeError(age)
    else:
        print(age)
'''
Please enter an age: 200
 Output: Traceback (most recent call last):
 	 File "E:\python project\py project 01\mypy20.py", line 15, in <module>
   	 raise AgeError(age)
	__main__.AgeError: 200，Wrong age, age should be 1-150 between
'''