Python exception handling (Python learning memo ⑥)

Handling exceptions

point

• Execute try first, and if no exception occurs, exit as it is
• When an exception occurs, the rest of the try clause is skipped, and if the corresponding type is defined in except, the except clause is executed.
• If the exception type does not match the name defined in except, the exception is passed to the outer try statement.
• If no handler is found, an exception message is displayed and execution ends.
• The except clause can cover all exceptions when omitting the exception name (not a very good way)

import sys

try:
    f = open('myfile.txt')
    s = f.readline()
except OSError as err:
    print("OS error: {0}".format(err))
except ValueError:
    print("The data cannot be converted to an integer.")
except:
    print("unexpected error: ", sys.exc_info()[0])
    raise #Rethrow the exception so that the caller can catch it

• Tuples allow you to specify multiple exceptions in one except clause

Catch multiple types of exceptions

except(RuntimeError, TypeError, NameError):
    pass

• By using the else clause, you can write a process to be executed only when an exception does not occur in the try clause.
• Also, if you write ʻexcept exception name as err, an exception instance will be passed to ʻerr.
• Exception names can be referenced with ʻerr.str () `

Added else clause

import sys

try:
    f = open('myfile.txt')
    s = f.readline()
except OSError as err:
    print("OS error: {0}".format(err))
except ValueError:
    print("The data cannot be converted to an integer.")
except:
    print("unexpected error: ", sys.exc_info()[0])
    raise #Rethrow the exception so that the caller can catch it
else:
    print(s)
    f.close()

Make an exception

point

• You can force an exception with raise exception name (argument)
• You can rethrow an exception by simply writing raise in the except clause

Define an exception

point

• You can define your own exception by creating an exception class
• Normally, it is a class derived from the Exception class

Exception class

class MyError(Exception):
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return repr(self.value)
    
try:
    raise MyError(2*2)
except MyError as e:
    print('My exception occurred, value:', e.value)
#output
# My exception occurred, value: 4

raise MyError('oops!')
#output
# Traceback (most recent call last):
#   File ".\exception_class.py", line 12, in <module>
#     raise MyError('oops!')
# __main__.MyError: 'oops!'

Cleanup process with try-finally

point

• The finally clause is always executed when exiting try, regardless of whether an exception has occurred.
• Executed when exiting from other clauses in try statement by break, continue, return statement

finally example

def divide(x, y):
    try:
        result = x / y
    except ZeroDivisionError:
        print("Division by zero error")
    else:
        print('answer: ', result)
    finally:
        print('in finally')

divide(2, 1)
#answer:  2.0
# in finally

divide(2, 0)
#Division by zero error
# in finally

divide("2", "1")
# in finally
# Traceback (most recent call last):
#   File ".\divide.py", line 15, in <module>
#     divide("2", "1")
#   File ".\divide.py", line 3, in divide
#     result = x / y
# TypeError: unsupported operand type(s) for /: 'str' and 'str'

Cleanup process with with

point

• Some objects have standard cleanup behavior defined (eg file objects)
• The default cleanup process is automatically executed by using the with statement.

with open('workfile', 'r') as f:
    read_data = f.read()

print(f.closed)
# True