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Multiple Exception Handling in Python

  • Difficulty Level : Hard
  • Last Updated : 12 Jun, 2019

Given a piece of code that can throw any of several different exceptions, and one needs to account for all of the potential exceptions that could be raised without creating duplicate code or long, meandering code passages.

If you can handle different exceptions all using a single block of code, they can be grouped together in a tuple as shown in the code given below :

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Code #1 :






try:
    client_obj.get_url(url)
except (URLError, ValueError, SocketTimeout):
    client_obj.remove_url(url)

The remove_url() method will be called if any of the listed exceptions occurs. If, on the other hand, if one of the exceptions has to be handled differently, then put it into its own except clause as shown in the code given below :

Code #2 :




try:
    client_obj.get_url(url)
except (URLError, ValueError):
    client_obj.remove_url(url)
except SocketTimeout:
    client_obj.handle_url_timeout(url)

Many exceptions are grouped into an inheritance hierarchy. For such exceptions, all of the exceptions can be caught by simply specifying a base class. For example, instead of writing code as shown in the code given below –

Code #3 :




try:
    f = open(filename)
except (FileNotFoundError, PermissionError):
    ...

Except statement can be re-written as in the code given below. This works because OSError is a base class that’s common to both the FileNotFoundError and PermissionError exceptions.

Code #4 :




try:
    f = open(filename)
except OSError:
    ...

Although it’s not specific to handle multiple exceptions per se, it is worth noting that one can get a handle to the thrown exception using them as a keyword as shown in the code given below.

Code #5 :




try:
    f = open(filename)
  
except OSError as e:
    if e.errno == errno.ENOENT:
        logger.error('File not found')
    elif e.errno == errno.EACCES:
        logger.error('Permission denied')
    else:
        logger.error('Unexpected error: % d', e.errno)

The e variable holds an instance of the raised OSError. This is useful if the exception has to be invested further, such as processing it based on the value of the additional status code. The except clauses are checked in the order listed and the first match executes.

Code #6 : Create situations where multiple except clauses might match




f = open('missing')

Output :

Traceback (most recent call last):
File "", line 1, in 
FileNotFoundError: [Errno 2] No such file or directory: 'miss




try:
    f = open('missing')
    except OSError:
        print('It failed')
    except FileNotFoundError:
        print('File not found')

Output :

Failed

Here the except FileNotFoundError clause doesn’t execute because the OSError is more general, matches the FileNotFoundError exception, and was listed first.




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