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Internet & World Wide Web How to Program, 3/e
Internet & World Wide Web How to Program, 3/e

© 2004
pages: 1420

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Python defines several data structures that help programmers store and manipulate data quickly and easily. This tutorial presents Python's built-in data structures for lists (list and tuple) and key-value pairs (dictionary). The tutorial is intended for students and developers who are familiar with basic Python programming, or for people who have read our prior Python articles, Introduction to Python and Python Basic Data Types, Control Statements and Functions.
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[Note: This tutorial is an excerpt (Section 35.3) of Chapter 35, Python, from our textbook Internet & World Wide Web How to Program, 3/e. This tutorial may refer to other chapters or sections of the book that are not included here. Permission Information: Deitel, Harvey M. and Paul J., INTERNET & WORLD WIDE WEB HOW TO PROGRAM, 3/E, 2004, pp.1246-1251. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.]
35.3   Tuples, Lists and Dictionaries (Continued)
Line 5 creates a tuple with elements 1, "a" and 3.0. Tuples are created as a comma-separated list of values inside parentheses. A tuple is used most often to contain combinations of many data types (e.g., strings, integers, other tuples). Lines 6-8 use the [] operator to access specific elements through an index. The first element in a tuple has index 0.
Tuple element contents are immutable-they cannot be modified. So the statement
aTuple[ 0 ] = 0
produces a runtime error similar to
Traceback (innermost last):
File "<interactive input>", line 1, in ?
TypeError: object doesn't support item assignment
Common Programming Error 35.5
 35.5 Attempting to change an immutable data structure is a syntax error.
Attempting to access a value at a nonexistent element is also an error. The statement
print aTuple[ 10 ]
produces a runtime error similar to
Traceback (innermost last):
File "<interactive input>", line 1, in ?
IndexError: tuple index out of range
because aTuple does not have a tenth element.
Common Programming Error 35.6
 35.6 Trying to access an out-of-range element (i.e., an element at an index that does not exist) produces a runtime error.
Line 15 unpacks the items of the tuple into three variables. This statement produces the same results as lines 6-8. Line 21 has the effect of adding an element to the end of variable aTuple. The right-hand side of the += statement must be a tuple; therefore, we must specify a one-element tuple, or singleton, on the right-hand side of the statement. The value ( 4, ) is a one-element tuple. The comma after the tuple element value is mandatory, because the value ( 4 ) is an integer.
Because tuples are immutable, the += statement actually creates a new tuple that combines the tuple on the left side of the += sign (i.e., aTuple) with the tuple on the right side of the += sign (i.e., ( 4, )) to create a new tuple. The new tuple is stored in variable aTuple.
The output of line 26 shows how the print statement handles a variable that is a tuple. Lines 29-30 use a for loop to print each element in variable aTuple.
The statement in line 29 assigns the first element in aTuple (i.e., aTuple[ 0 ]) to variable item. Line 30 then prints the value of variable item to the screen. The for loop iterates over each element in the tuple, assigns the element to variable item and executes the code in line 30.
Other Python Tutorials:
Introduction to Python
Pytthon Basic Data Types, Control Statements and Functions
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