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As shown in Listing 1 , tuples support the concatenation (+) operator. You can concatenate two or more tuples to produce a new tuple. This program creates twosimple tuples, and then concatenates them to create a third tuple.
Figure 1 shows the output produced by the code in Listing 1 . By now, the creation and display of simple tuples should be very familiar to you based onearlier modules. Therefore, I won't discuss this part of the program further.
| Figure 1 . Output from the code in Listing 1. |
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(1, 2, 'A', 'B') |
The code in Listing 2 is not quite so straightforward. In fact, it looks rather strange if you come from a conventional C, C++, or Java programming background.
| Listing 2 . Unpack the tuple and print individual elements. |
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# Unpack the tuple and print individual elements
w,x,y,z = t3print(w)
print(x)print(y)
print(z) |
Figure 2 shows the output produced by the code in Listing 2 .
| Figure 2 . Output from the code in Listing 2. |
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1
2A
B |
If you compare this output with the original tuple in Listing 1 , or with the previous output in Figure 1 , you will see that each of the individual items in the tuple (in left-to-right order) were assigned respectively to the variables named w , x , y , and z .
Thus, the lines of output produced by printing these four variables in Figure 2 match the items in the original tuple that was created in Listing 1 and displayed in Figure 1 .
Version 3 of The Python Tutorial -- 5.3. Tuples and Sequences refers to the first, second, and third statements in Listing 1 as tuple packing using the following example:
The statement t = 12345, 54321, 'hello!' is an example of tuple packing: the values 12345, 54321 and 'hello!' are packed together in a tuple. The reverse operation is also possible
An early version of The Python Tutorial -- 5.3 Tuples and Sequences refers to the first statement in Listing 2 as tuple unpacking .
The current version of The Python Tutorial -- 5.3. Tuples and Sequences (September 2014) refers to that operation more generally as sequence unpacking .
That tutorial continues by telling us
"... sequence unpacking requires that there are as many variables on the left side of the equals sign as there are elements in the sequence."
If you try to run a program that doesn't meet these criteria, you will get an error similar to the following:
ValueError: too many values to unpack (expected 3)
Just to make things a little more interesting, I decided to combine the use of a tuple (an immutable list) and a regular mutable list in this program.
Listing 3 contains the code to create a mutable list populated with five string characters.
| Listing 3 . Unpack the tuple into the mutable list. |
|---|
# Create and print a list
L1 = ["a","b","c","d","e"]# Unpack tuple into the list and print it
print(L1)L1[0],L1[1],L1[2],L1[3]= t3
print(L1) |
Then the list is displayed, as shown by the first line of text in Figure 3 . The first line of output in Figure 3 shows the contents of the list just after it is created and populated.
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