Beginning Python - From Novice to Professional
Beginning Python - From Novice to Professional Beginning Python - From Novice to Professional
CHAPTER 2 ■ LISTS AND TUPLES 41 The list Function Because strings can’t be modified in the same way as lists, often it can be useful to create a list from a string. You can do this with the list function: 1 >>> list('Hello') ['H', 'e', 'l', 'l', 'o'] Note that list works with all kinds of sequences, not just strings. ■Tip To convert a list of characters such as the preceding code back to a string, you would use the following expression: ''.join(somelist) where somelist is your list. For an explanation of what this really means, see the section about join in Chapter 3. Basic List Operations You can perform all the standard sequence operations on lists, such as indexing, slicing, concatenating, and multiplying; but the interesting thing about lists is that they can be modified. In this section, you see some of the ways you can change a list: item assignments, item deletion, slice assignments, and list methods. (Note that not all list methods actually change their list.) Changing Lists: Item Assignments Changing a list is easy. You just use ordinary assignment as explained in the first chapter. However, instead of writing something like x = 2, you use the indexing notation to assign to a specific, existing position, such as x[1] = 2. >>> x = [1, 1, 1] >>> x[1] = 2 >>> x [1, 2, 1] ■Note You cannot assign to a position that doesn’t exist; if your list is of length 2, you cannot assign a value to index 100. To do that, you would have to make a list of length 101 (or more). See the section “None, Empty Lists, and Initialization,” earlier in this chapter. 1. It’s actually a type, not a function, but the difference isn’t important right now.
42 CHAPTER 2 ■ LISTS AND TUPLES Deleting Elements Deleting elements from a list is easy too; you can simply use the del statement: >>> names = ['Alice', 'Beth', 'Cecil', 'Dee-Dee', 'Earl'] >>> del names[2] >>> names ['Alice', 'Beth', 'Dee-Dee', 'Earl'] Notice how Cecil is completely gone, and the length of the list has shrunk from five to four. The del statement may be used to delete things other than list elements. It can be used with dictionaries (see Chapter 4) or even variables. For more information, see Chapter 5. Assigning to Slices Slicing is a very powerful feature, and it is made even more powerful by the fact that you can assign to slices: >>> name = list('Perl') >>> name ['P', 'e', 'r', 'l'] >>> name[2:] = list('ar') >>> name ['P', 'e', 'a', 'r'] So you can assign to several positions at once. You may wonder what the big deal is. Couldn’t you just have assigned to them one at a time? Sure, but when you use slice assignments, you may also replace the slice with a sequence whose length is different from that of the original: >>> name = list('Perl') >>> name[1:] = list('ython') >>> name ['P', 'y', 't', 'h', 'o', 'n'] Slice assignments can even be used to insert elements without replacing any of the original ones: >>> numbers = [1, 5] >>> numbers[1:1] = [2, 3, 4] >>> numbers [1, 2, 3, 4, 5] Here, I basically “replaced” an empty slice, thereby really inserting a sequence. You can do the reverse to delete a slice: >>> numbers [1, 2, 3, 4, 5] >>> numbers[1:4] = [] >>> numbers [1, 5] As you may have guessed, this last example is equivalent to del numbers[1:4].
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42 CHAPTER 2 ■ LISTS AND TUPLES<br />
Deleting Elements<br />
Deleting elements from a list is easy <strong>to</strong>o; you can simply use the del statement:<br />
>>> names = ['Alice', 'Beth', 'Cecil', 'Dee-Dee', 'Earl']<br />
>>> del names[2]<br />
>>> names<br />
['Alice', 'Beth', 'Dee-Dee', 'Earl']<br />
Notice how Cecil is completely gone, and the length of the list has shrunk from five <strong>to</strong> four.<br />
The del statement may be used <strong>to</strong> delete things other than list elements. It can be used<br />
with dictionaries (see Chapter 4) or even variables. For more information, see Chapter 5.<br />
Assigning <strong>to</strong> Slices<br />
Slicing is a very powerful feature, and it is made even more powerful by the fact that you can<br />
assign <strong>to</strong> slices:<br />
>>> name = list('Perl')<br />
>>> name<br />
['P', 'e', 'r', 'l']<br />
>>> name[2:] = list('ar')<br />
>>> name<br />
['P', 'e', 'a', 'r']<br />
So you can assign <strong>to</strong> several positions at once. You may wonder what the big deal is. Couldn’t<br />
you just have assigned <strong>to</strong> them one at a time? Sure, but when you use slice assignments, you<br />
may also replace the slice with a sequence whose length is different from that of the original:<br />
>>> name = list('Perl')<br />
>>> name[1:] = list('ython')<br />
>>> name<br />
['P', 'y', 't', 'h', 'o', 'n']<br />
Slice assignments can even be used <strong>to</strong> insert elements without replacing any of the<br />
original ones:<br />
>>> numbers = [1, 5]<br />
>>> numbers[1:1] = [2, 3, 4]<br />
>>> numbers<br />
[1, 2, 3, 4, 5]<br />
Here, I basically “replaced” an empty slice, thereby really inserting a sequence. You can do<br />
the reverse <strong>to</strong> delete a slice:<br />
>>> numbers<br />
[1, 2, 3, 4, 5]<br />
>>> numbers[1:4] = []<br />
>>> numbers<br />
[1, 5]<br />
As you may have guessed, this last example is equivalent <strong>to</strong> del numbers[1:4].
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