Handling lines with something-separated fields for Python

March 4, 2007

As a system administrator, I spend a bunch of my time dealing with files made up of lines that are composed of fields separated by some character. A classical example is /etc/passwd, with colon-separated fields. These file formats are ordered lists with named fields, which should sound familiar, but they don't show up as Python lists, they show up as lines of text and they want to be output as text; that we use lists to represent them is just an implementation detail.

This only takes a little bit of extra work to implement on top of our previous SetMixin class:

class FieldLine(SetMixin, list):
    separator = ":"
    def __init__(self, line):
        n = line.split(self.separator)
        super(FieldLine, self).__init__(n)
    def __str__(self):
        return self.separator.join(self)

class PasswdLine(FieldLine):
    fields = gen_fields('name', 'passwd',
                        'uid', 'gid', 'gecos',
                        'dir', 'shell')

(Where gen_fields is basically the dict()-ized version of enum_args from here.)

Now that I've written these entries, I have a confession: this is actually what I started out doing. I didn't first build a general ordered list with named fields class and then realized it could be used to deal with /etc/passwd lines; I started out needing to deal with /etc/passwd lines, decided that I wanted read/write access to named fields, and then built downwards. I just wrote it up backwards because it looks neater that way.

(In fact this is the cleaned up and idealized version of this class. The real one in my program does not subclass list; instead it is a normal class with a private 'field_store' list and everything just directly manipulates that. It also doesn't handle the slicing cases, because I didn't need to. I did the new version for here for various reasons, including that it was a good excuse to play around with subclassing built in types.)

Written on 04 March 2007.
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Last modified: Sun Mar 4 22:44:56 2007
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