Beginning Python - From Novice to Professional
Beginning Python - From Novice to Professional Beginning Python - From Novice to Professional
CHAPTER 3 ■ WORKING WITH STRINGS 63 split A very important string method, split is the inverse of join, and is used to split a string into a sequence: >>> '1+2+3+4+5'.split('+') ['1', '2', '3', '4', '5'] >>> '/usr/bin/env'.split('/') ['', 'usr', 'bin', 'env'] >>> 'Using the default'.split() ['Using', 'the', 'default'] Note that if no separator is supplied, the default is to split on all runs of consecutive whitespace characters (spaces, tabs, newlines, and so on). See also: join. In Appendix B: rsplit, splitlines. strip The strip method returns a string where whitespace on the left and right (but not internally) has been stripped (removed): >>> ' internal whitespace is kept '.strip() 'internal whitespace is kept' As with lower, strip can be useful when comparing input to stored values. Let’s return to the user name example from the section on lower, and let’s say that the user inadvertently types a space after his name: >>> names = ['gumby', 'smith', 'jones'] >>> name = 'gumby ' >>> if name in names: print 'Found it!' ... >>> if name.strip() in names: print 'Found it!' ... Found it! >>> You can also specify which characters are to be stripped, by listing them all in a string parameter: >>> '*** SPAM * for * everyone!!! ***'.strip(' *!') 'SPAM * for * everyone' Stripping is only performed at the ends, so the internal asterisks are not removed. In Appendix B: lstrip, rstrip.
64 CHAPTER 3 ■ WORKING WITH STRINGS translate Similar to replace, translate replaces parts of a string, but unlike replace, translate only works with single characters. Its strength lies in that it can perform several replacements simultaneously, and can do so more efficiently than replace. There are quite a few rather technical uses for this method (such as translating newline characters or other platform-dependent special characters), but let’s consider a simpler (although slightly more silly) example. Let’s say you want to translate a plain English text into one with a German accent. To do this, you must replace the character “c” with “k,” and “s” with “z.” Before you can use translate, however, you must make a translation table. This translation table is a full listing of which characters should be replaced by which. Because this table (which is actually just a string) has 256 entries, you won’t write it out yourself: You’ll use the function maketrans from the string module. The maketrans function takes two arguments: two strings of equal length, indicating that each character in the first string should be replaced by the character in the same position in the second string. Got that? In the case of our simple example, the code would look like the following: >>> from string import maketrans >>> table = maketrans('cs', 'kz') WHAT’S IN A TRANSLATION TABLE? A translation table is a string containing one replacement letter for each of the 256 characters in the ASCII character set: >>> table = maketrans('cs', 'kz') >>> len(table) 256 >>> table[97:123] 'abkdefghijklmnopqrztuvwxyz' >>> maketrans('', '')[97:123] 'abcdefghijklmnopqrstuvwxyz' As you can see, I’ve sliced out the part of the table that corresponds to the lowercase letters. Take a look at the alphabet in the table and that in the empty translation (which doesn’t change anything). The empty translation has a normal alphabet, while in the preceding code, the letter “c” has been replaced by “k,” and “s” has been replaced by “z.” Once you have this table, you can use it as an argument to the translate method, thereby translating your string: >>> 'this is an incredible test'.translate(table) 'thiz iz an inkredible tezt'
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64 CHAPTER 3 ■ WORKING WITH STRINGS<br />
translate<br />
Similar <strong>to</strong> replace, translate replaces parts of a string, but unlike replace, translate only works<br />
with single characters. Its strength lies in that it can perform several replacements simultaneously,<br />
and can do so more efficiently than replace.<br />
There are quite a few rather technical uses for this method (such as translating newline<br />
characters or other platform-dependent special characters), but let’s consider a simpler<br />
(although slightly more silly) example. Let’s say you want <strong>to</strong> translate a plain English text in<strong>to</strong><br />
one with a German accent. To do this, you must replace the character “c” with “k,” and “s” with “z.”<br />
Before you can use translate, however, you must make a translation table. This translation<br />
table is a full listing of which characters should be replaced by which. Because this table (which is<br />
actually just a string) has 256 entries, you won’t write it out yourself: You’ll use the function<br />
maketrans from the string module.<br />
The maketrans function takes two arguments: two strings of equal length, indicating that<br />
each character in the first string should be replaced by the character in the same position in the<br />
second string. Got that? In the case of our simple example, the code would look like the following:<br />
>>> from string import maketrans<br />
>>> table = maketrans('cs', 'kz')<br />
WHAT’S IN A TRANSLATION TABLE?<br />
A translation table is a string containing one replacement letter for each of the 256 characters in the ASCII<br />
character set:<br />
>>> table = maketrans('cs', 'kz')<br />
>>> len(table)<br />
256<br />
>>> table[97:123]<br />
'abkdefghijklmnopqrztuvwxyz'<br />
>>> maketrans('', '')[97:123]<br />
'abcdefghijklmnopqrstuvwxyz'<br />
As you can see, I’ve sliced out the part of the table that corresponds <strong>to</strong> the lowercase letters. Take a look<br />
at the alphabet in the table and that in the empty translation (which doesn’t change anything). The empty<br />
translation has a normal alphabet, while in the preceding code, the letter “c” has been replaced by “k,” and<br />
“s” has been replaced by “z.”<br />
Once you have this table, you can use it as an argument <strong>to</strong> the translate method, thereby<br />
translating your string:<br />
>>> 'this is an incredible test'.translate(table)<br />
'thiz iz an inkredible tezt'
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