Write a Python program that shows the importance of the order of exception handlers. Write two programs, one with the correct order of except handlers and another with an order that causes a logic error. If you attempt to catch a base-class exception type before a derived-class type, the pro- gram may produce a logic error.
What will be an ideal response?
```
# Correct order for exception handlers.
import time
class MenuError( ValueError ):
"""User selected an invalid menu choice"""
pass
# print menu instructions
def printMenu():
print """Menu:
[1] Print seconds since epoch
[2] Print formatted time
[3] Quit"""
# get a menu selection from user
# raise MenuError on incorrect choice
# raise ValueError if user does not enter integer
# raise SystemExit if user wants to quit
def getSelection():
selection = int( raw_input( "Choice: " ) )
# handle user input
if selection == 1:
print time.time()
# format time
elif selection == 2:
print time.ctime( time.time() )
# quit
elif selection == 3:
raise SystemExit
# incorrect input
else:
raise MenuError, "Value %d not in range [1, 3]" % selection
# main program
printMenu()
# keep trying until user selects a proper menu item
while 1:
try:
getSelection()
break
# incorrect menu choice
except MenuError:
print "Invalid menu choice!\n"
printMenu()
# not integer choice
except ValueError, exception:
print "***%s***" % exception
# user wants to quit
except SystemExit:
print "Goodbye!"
break
```
Menu:
[1] Print seconds since epoch
[2] Print formatted time
[3] Quit
Choice: -1
Invalid menu choice!
Menu:
[1] Print seconds since epoch
[2] Print formatted time
[3] Quit
Choice: a
***invalid literal for int(): a***
Choice: 1
1010071347.53
Menu:
[1] Print seconds since epoch
[2] Print formatted time
[3] Quit
Choice: 3
Goodbye!
```
# Incorrect order for exception handlers--menu not printed
# on incorrect choice.
import time
class MenuError( ValueError ):
"""User selected an invalid menu choice"""
pass
# print menu instructions
def printMenu():
print """Menu:
[1] Print seconds since epoch
[2] Print formatted time
[3] Quit"""
# get a menu selection from user
# raise MenuError on incorrect choice
# raise ValueError if user does not enter integer
# raise SystemExit if user wants to quit
def getSelection():
selection = int( raw_input( "Choice: " ) )
# handle user input
if selection == 1:
print time.time()
# format time
elif selection == 2:
print time.ctime( time.time() )
# quit
elif selection == 3:
raise SystemExit
# incorrect input
else:
raise MenuError, "Value %d not in range [1, 3]" % selection
# main program
printMenu()
# keep trying until user selects a proper menu item
while 1:
# get user input
try:
getSelection()
break
# incorrectly placed before MenuError
# not integer choice
except ValueError, exception:
print "***%s***" % exception
# incorrect menu choice
except MenuError:
print "Invalid menu choice!\n"
printMenu()
# user wants to quit
except SystemExit:
print "Goodbye!"
break
```
Menu:
[1] Print seconds since epoch
[2] Print formatted time
[3] Quit
Choice: a
***invalid literal for int(): a***
Choice: 5
***Value 5 not in range [1, 3]***
Choice: 3
Goodbye!
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