Professional IronPython

Working with Tuples, Lists, and Arrays
 
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The main difference between a 
tuple
and a 
list
is that you can change the content of a 
list
, but you 
can’t change the content of a 
tuple
. If you create a 
tuple
and try to change one of its elements, you 
get an error message. Listing 4-2 shows what happens when you try to change a 
tuple
and a 
list
.
lISTINg 4-2: 
 Changing tuples and lists
# Create the tuple 
MyTuple = ‘Red’, ‘Green’, ‘Blue’, ‘Yellow’
# Create the list 
MyList = [‘Red’, ‘Green’, ‘Blue’, ‘Yellow’]
# Attempt to change the tuple, which will result in an error.
try:
MyTuple[1] = ‘Orange’
except TypeError:
print ‘Couldn\‘t change the tuple.\n’
# Attempt to change the list.
try:
MyList[1] = ‘Orange’
except TypeError:
print ‘Couldn\‘t change the list.\n’
# Verify that the change worked.
print ‘Displaying the list content.’
for TheString in MyList:
print TheString
# Pause after the debug session.
raw_input(‘Press any key to continue...’)
The example attempts to change both a 
tuple
and a 
list
. However, when the example tries to 
change the 
tuple
, it generates an exception, as shown in Figure 4-2. Notice how the code uses a 
try...except
structure to catch the exception and display an error message onscreen. The 
except
clause can stand alone, but it’s better to provide a particular kind of exception, which is 
TypeError
in this case. The code does successfully modify the 
list
and displays the results in Figure 4-2.
FIguRE 4-2: 
Lists are mutable; tuples aren’t.
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2/24/10 12:47:34 PM
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