Beginning Python - From Novice to Professional
Beginning Python - From Novice to Professional Beginning Python - From Novice to Professional
CHAPTER 2 ■ LISTS AND TUPLES 51 So What’s the Point? By now you are probably wondering why anyone would ever want such a thing as an immutable (unchangeable) sequence. Can’t you just stick to lists and leave them alone when you don’t want them to change? Basically, yes. However, there are two important reasons why you need to know about tuples: • They can be used as keys in mappings—lists can’t be. (You may remember that mappings were mentioned in the chapter introduction. You’ll learn more about them in Chapter 4.) • They are returned by some built-in functions and methods, which means that you have to deal with them. As long as you don’t try to change them, “dealing” with them most often means treating them just like lists (unless you need methods such as index and count, which tuples don’t have). In general, lists will probably be adequate for all your sequencing needs. A Quick Summary Let’s review some of the most important concepts covered in this chapter: Sequences. A sequence is a data structure in which the elements are numbered (starting with zero). Examples of sequence types are lists, strings, and tuples. Of these, lists are mutable (you can change them), whereas tuples and strings are immutable (once they’re created, they’re fixed). Parts of a sequence can be accessed through slicing, supplying two indices, indicating the starting and ending position of the slice. To change a list, you assign new values to its positions, or use assignment to overwrite entire slices. Membership. Whether a value can be found in a sequence (or other container) is checked with the operator in. Using in with strings is a special case—it will let you look for substrings. Methods. Some of the built-in types (such as lists and strings, but not tuples) have many useful methods attached to them. These are a bit like functions, except that they are tied closely to a specific value. Methods are an important aspect of object-oriented programming, which we look at later, in Chapter 7.
52 CHAPTER 2 ■ LISTS AND TUPLES New Functions in This Chapter Function cmp(x, y) len(seq) list(seq) max(args) min(args) reversed(seq) sorted(seq) tuple(seq) Description Compares two values Returns the length of a sequence Converts a sequence to a list Returns the maximum of a sequence or set of arguments Returns the minimum of a sequence or set of arguments Lets you iterate over a sequence in reverse Returns a sorted list of the elements of seq Converts a sequence to a tuple What Now? Now that you’re acquainted with sequences, let’s move on to character sequences, also known as strings.
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52 CHAPTER 2 ■ LISTS AND TUPLES<br />
New Functions in This Chapter<br />
Function<br />
cmp(x, y)<br />
len(seq)<br />
list(seq)<br />
max(args)<br />
min(args)<br />
reversed(seq)<br />
sorted(seq)<br />
tuple(seq)<br />
Description<br />
Compares two values<br />
Returns the length of a sequence<br />
Converts a sequence <strong>to</strong> a list<br />
Returns the maximum of a sequence or set of arguments<br />
Returns the minimum of a sequence or set of arguments<br />
Lets you iterate over a sequence in reverse<br />
Returns a sorted list of the elements of seq<br />
Converts a sequence <strong>to</strong> a tuple<br />
What Now?<br />
Now that you’re acquainted with sequences, let’s move on <strong>to</strong> character sequences, also known<br />
as strings.
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