exceptions

Example Python program exceptions.py Python version 3.x or newer. To check the Python version use:

python --version

Modules

• import sys

Classes

• class CustomValueError(ValueError):

Methods

• def init(self, arg):

Code

Python examples of various exceptions

Generic exception handling

try:
    Value = int(input("Type a number between 1 and 10: "))
except:
    print("You must type a number between 1 and 10!")
else:
    if (Value > 0) and (Value <= 10):
        print("You typed: ", Value)
    else:
        print("The value you typed is incorrect!")

'''
Type a number between 1 and 10: a
You must type a number between 1 and 10!

Type a number between 1 and 10: a
You must type a number between 1 and 10!
-----------------------------------------------------------------
'''

Basic exception handling

try:
    Value = int(input("Type a number between 1 and 10: "))
except ValueError:
    print("You must type a number between 1 and 10!")
else:
    if (Value > 0) and (Value <= 10):
        print("You typed: ", Value)
    else:
        print("The value you typed is incorrect!")

'''
Type a number between 1 and 10: 1
You typed:  1

Type a number between 1 and 10: a
You must type a number between 1 and 10!
-----------------------------------------------------------------
'''

Multiple exceptions with single except clause

try:
    Value = int(input("Type a number between 1 and 10: "))
except (ValueError, KeyboardInterrupt):
    print("You must type a number between 1 and 10!")
else:
    if (Value > 0) and (Value <= 10):
        print("You typed: ", Value)
    else:
        print("The value you typed is incorrect!")

'''
Type a number between 1 and 10: 1
You typed:  1

Type a number between 1 and 10: a
You must type a number between 1 and 10!
-----------------------------------------------------------------
'''

Multiple exceptions with multiple except clauses no.1

try:
    Value = int(input("Type a number between 1 and 10:"))
except ValueError:
    print("You must type a number between 1 and 10!")
except KeyboardInterrupt:
    print("You pressed Ctrl+C!")
else:
    if (Value > 0) and (Value <= 10):
        print("You typed: ", Value)
    else:
        print("The value you typed is incorrect!")

'''
Type a number between 1 and 10:1
You typed:  1
>>> 
Type a number between 1 and 10:a
You must type a number between 1 and 10!
>>> 
Type a number between 1 and 10:
You pressed Ctrl+C! => (when you press Cancel button)
-----------------------------------------------------------------
'''

Exception with argument

import sys
try:
    File = open('myfile.txt')
except IOError as e:
    print("Error opening file!\r\n" +
     "Error Number: {0}\r\n".format(e.errno) +
     "Error Text: {0}".format(e.strerror))
else:
    print("File opened as expected.")
    File.close();

'''
Error opening file!
Error Number: 2
Error Text: No such file or directory
-----------------------------------------------------------------
'''

List of arguments

import sys
try:
    File = open('myfile.txt')
except IOError as e:
    for Arg in e.args:
        print(Arg)
else:
    print("File opened as expected.")
    File.close();

'''
2
No such file or directory
-----------------------------------------------------------------
'''

List of arguments with names

import sys
try:
    File = open('myfile.txt')
except IOError as e:
    for Entry in dir(e):
        if (not Entry.startswith("_")):
            try:
                print(Entry, " = ", e.__getattribute__(Entry))
            except AttributeError:
                print("Attribute ", Entry, " not accessible.")
else:
    print("File opened as expected.")
    File.close();

'''
args  =  (2, 'No such file or directory')
Attribute  characters_written  not accessible.
errno  =  2
filename  =  myfile.txt
filename2  =  None
strerror  =  No such file or directory
winerror  =  None
with_traceback  =  <built-in method with_traceback of FileNotFoundError object at 0x00000000045FABC0>
-----------------------------------------------------------------
'''

Multiple exceptions with multiple except clauses no.2

try:
    Value1 = int(input("Type the first number: "))
    Value2 = int(input("Type the second number: "))
    Output = Value1 / Value2
except ValueError:
    print("You must type a whole number!")
except KeyboardInterrupt:
    print("You pressed Ctrl+C!")
except ZeroDivisionError:
    print("Attempted to divide by zero!")
except ArithmeticError:
    print("An undefined math error occurred.")
else:
    print(Output) #if no errors then print Value1/Value2

'''
Type the first number: 1
Type the second number: 1
1.0

Type the first number: 
You pressed Ctrl+C!

Type the first number: 1
Type the second number: 0
Attempted to divide by zero!
-----------------------------------------------------------------
'''

Nested exceptions with 'try again' input

TryAgain = True
while TryAgain: #Loop until you entered integer number
    try:
        Value = int(input("Type a whole number. "))
    except ValueError:
        print("You must type a whole number!")
        try:
            DoOver = input("Try again (y/n)? ")  #Try again y=Yes, n=No
        except:
            print("OK, see you next time!")
            TryAgain = False
        else:
            if (str.upper(DoOver) == "N"):
                TryAgain = False
    except KeyboardInterrupt:
        print("You pressed Ctrl+C!")
        print("See you next time!")
        TryAgain = False
    else:
        print(Value)
        TryAgain = False
'''
-----------------------------------------------------------------
'''

Raise exception

try:
    raise ValueError
except ValueError:
    print ("ValueError Exception Raised!")

'''
ValueError Exception Raised!
-----------------------------------------------------------------
'''

Raise excweption with passing user error information

try:
    Ex = ValueError()
    Ex.userMessage = "Value Error"
    raise ValueError
except ValueError:
    print(Ex.userMessage)

'''
Value Error
-----------------------------------------------------------------
'''

Custom raise exception

class CustomValueError(ValueError):
    def __init__(self, arg):
        self.strerror = arg
        self.args = {arg}


try:
    raise CustomValueError("Value must be within 1 and 10.")
except CustomValueError as e:
    print("CustomValueError Exception!", e.strerror)

'''
CustomValueError Exception! Value must be within 1 and 10.
-----------------------------------------------------------------
'''

Raise exception with finally clause

import sys
try:
    raise ValueError
    print("Raising an exception.")
except ValueError:
    print("ValueError Exception!")
    sys.exit()
finally:
    print("Taking care of last minute details.")

'''
ValueError Exception!
Taking care of last minute details.
'''

Useful Links

• Articles: https://python-commandments.org/
• Python shell: https://bsdnerds.org/learn-python/
• Tutorial: https://pythonprogramminglanguage.com/