What are the ways to handle exceptions in python

Usage of exception handling: In order to ensure the robustness and fault tolerance of the program, that is, the program will not crash when an error is encountered, we need to handle exceptions,

1. If the conditions under which the error occurs are predictable, we need to use if to handle it and prevent it before the error occurs.

AGE=10
while True:
    age=input('>>: ').strip()
    if age.isdigit(): #只有在age为字符串形式的整数时,下列代码才不会出错,该条件是可预知的
        age=int(age)
        if age == AGE:
            print('you got it')
            break

2. If the conditions under which the error occurs are unpredictable , you need to use try..except: handle after the error occurs

#The basic syntax is

try:

Detected code block

except exception type:

Once an exception is detected in try, the logic at this location will be executed

#举例
try:
    f=open('a.txt')
    g=(line.strip() for line in f)
    print(next(g))
    print(next(g))
    print(next(g))
    print(next(g))
    print(next(g))
except StopIteration:
    f.close()

5. Detailed usage of try...except...

We put statements that may cause errors in the try module and use except to handle exceptions. except can handle a specific exception or a set of exceptions in parentheses. If no exception is specified after except, all exceptions will be handled by default. Each try must have at least one except

1. The exception class can only handle the specified exception. If the exception is not specified, it cannot be handled.

s1 = 'hello'
try:
    int(s1)
except IndexError as e: # 未捕获到异常，程序直接报错
    print e

2, Multi-branch

s1 = 'hello'
try:
    int(s1)
except IndexError as e:
    print(e)
except KeyError as e:
    print(e)
except ValueError as e:
    print(e)

3, universal exception Exception

s1 = 'hello'
try:
    int(s1)
except Exception as e:
    print(e)

4, multi-branch +Exception

s1 = 'hello'
try:
    int(s1)
except IndexError as e:
    print(e)
except KeyError as e:
    print(e)
except ValueError as e:
    print(e)
except Exception as e:
    print(e)

5, other mechanisms of exception (try...finally syntax)

try. The ..finally statement will execute the last code regardless of whether an exception occurs. The syntax is as follows:

try:
<语句>
finally:
<语句>    #退出try时总会执行
raise

Example:

s1 = 'hello'
try:
    int(s1)
except IndexError as e:
    print(e)
except KeyError as e:
    print(e)
except ValueError as e:
    print(e)
#except Exception as e:
#    print(e)
else:
    print('try内代码块没有异常则执行我')
finally:
    print('无论异常与否,都会执行该模块,通常是进行清理工作')

6, actively trigger exceptions (raise statement)

We can use the raise statement to trigger exceptions ourselves. The raise syntax format is as follows:

raise [Exception [, args [, traceback]]]

The Exception in the statement is the type of exception (for example, NameError) and the parameter is an exception parameter value. This parameter is optional, if not provided, the exception parameter is "None".

The last parameter is optional (rarely used in practice) and, if present, is the trace exception object.

Example:

An exception can be a string, class or object. Most of the exceptions provided by the Python kernel are instantiated classes, which are parameters of an instance of a class.

Defining an exception is very simple, as follows:

def functionName( level ):
    if level < 1:
        raise Exception("Invalid level!", level)
        # 触发异常后，后面的代码就不会再执行

try:
    raise TypeError('类型错误')
except Exception as e:
    print(e)

7, Custom exception

By creating a new exception class, programs can name their own exceptions. Exceptions should typically inherit from the Exception class, either directly or indirectly.

The following is an example related to BaseException. A class is created in the example. The base class is BaseException, which is used to output more information when an exception is triggered.

In the try statement block, the except block statement is executed after the user-defined exception. The variable e is used to create an instance of the Networkerror class.

class Networkerror(BaseException):
    def __init__(self,msg):
        self.msg=msg
    def __str__(self):
        return self.msg
 
try:
    raise Networkerror('类型错误')
except Networkerror as e:
    print(e)

8, assert: assert condition

assert 1 == 1 
assert 1 == 2

9, summarize try...except

1, separate error handling from real work

2, the code is easier to organize and clearer, and complex tasks are easier to achieve

3, there is no doubt that it is safer and will not be caused by some small The program crashed unexpectedly due to negligence

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