第191页 | Learning Python | 阅读 ‧ 电子书库

String Method Examples: Parsing Text

Another common role for string methods is as a simple form of text parsing—that is, analyzing structure and extracting substrings. To extract substrings at fixed offsets, we can employ slicing techniques:

>>> line = 'aaa bbb ccc'
>>> col1 = line[0:3]
>>> col3 = line[8:]
>>> col1
'aaa'
>>> col3
'ccc'

Here, the columns of data appear at fixed offsets and so may be sliced out of the original string. This technique passes for parsing, as long as the components of your data have fixed positions. If instead some sort of delimiter separates the data, you can pull out its components by splitting. This will work even if the data may show up at arbitrary positions within the string:

>>> line = 'aaa bbb  ccc'
>>> cols = line.split()
>>> cols
['aaa', 'bbb', 'ccc']

The string split method chops up a string into a list of substrings, around a delimiter string. We didn’t pass a delimiter in the prior example, so it defaults to whitespace—the string is split at groups of one or more spaces, tabs, and newlines, and we get back a list of the resulting substrings. In other applications, more tangible delimiters may separate the data. This example splits (and hence parses) the string at commas, a separator common in data returned by some database tools:

>>> line = 'bob,hacker,40'
>>> line.split(',')
['bob', 'hacker', '40']

Delimiters can be longer than a single character, too:

>>> line = "i'mSPAMaSPAMlumberjack"
>>> line.split("SPAM")
["i'm", 'a', 'lumberjack']

Although there are limits to the parsing potential of slicing and splitting, both run very fast and can handle basic text-extraction chores.