contex 1.1

The problem with our abstractions

I’ll present two “problems” that this library attempts to solve. The first one is rather contrived, the second one is more realistic. Afterwards I will show how contex can help.

>>> re.sub(r'([0-9]+)', lambda match: '{:0>3}'.format(int(match.group(1)) - 1),  'Photo034.jpg')
'Photo033.jpg'

>>> regex = r'(?<={})([0-9]+)(?={})'.format('Vacation2008Photo_', r'\.jpg')
>>> regex
'(?<=Vacation2008Photo_)([0-9]+)(?=\\.jpg)'
>>> re.sub(regex, lambda match: '{:0>3}'.format(int(match.group(1)) - 1), 'Vacation2008Photo_034.jpg')
'Vacation2008Photo_033.jpg'

Contex to the rescue

It is my thesis that our abstractions aren’t fit for this sort of problem. The problems above “hit it where it hurts” so to speak, because in order to be solved elegantly they require context, and in the one dimensional world of strings this means: what came before? what came after? Which part of the string are we focusing on right now? This is exactly what StringContext is. It contains 3 parts: before, focus, after:

>>> import contex
>>> contex.T('Hello')
StringContext('', 'Hello', '')
>>> contex.T('abcde')[2:]
StringContext('ab', 'cde', '')
>>> contex.T('abcde')[2:][0]
StringContext('', 'a', 'bcde')
>>> contex.T('abcde')[2]
StringContext('ab', 'c', 'de')
>>> view = contex.T('abcde')[2]
>>> view.before, view.focus, view.after
('ab', 'c', 'de')
>>> view.replace(lambda focus: '({})'.format(focus))
StringContext('ab', '(c)', 'de')
>>> str(view.replace(lambda focus: '({})'.format(focus)))
'ab(c)de'

As you can see, slicing has the function of shifting the focus of the string. “I want to look at this part now”. These points are true of StringContext and MatchContext:

• They are treated as immutable objects: methods that “change” stuff doesn’t mutate but returns a new version.

• All methods operates on the full string, not merely the focus point. So StringContext.reverse doesn’t reverse the focus only, it reverses everything; StringContext.search searches everything, you get the picture.

• The 3 composite parts are normal strings. I rejected the idea of a tree of StringContext objects because it seemed too complicated, more confusing than useful.

• Methods that needs str arguments also accept StringContext arguments: it will be converted with str automatically.

>>> contex.match('Vacation2008Photo_034.jpg', r'Vacation2008Photo_(?P<number>[0-9]+)\.jpg')
MatchContext('', 'Vacation2008Photo_034.jpg', '')
>>> m = contex.match('Vacation2008Photo_034.jpg', r'Vacation2008Photo_(?P<number>[0-9]+)\.jpg')
>>> m.group("number")
MatchContext('Vacation2008Photo_', '034', '.jpg')
>>> m.group('number').replace(lambda num: '{:0>3}'.format(int(num) - 1))
MatchContext('Vacation2008Photo_', '033', '.jpg')
>>> result = m.group('number').replace(lambda num: '{:0>3}'.format(int(num) - 1))
>>> str(result)
'Vacation2008Photo_033.jpg'

Conclusion

This is not to say that there’s anything wrong with strings as we use them now, or that these abstractions can serve as a replacement. It’s rather to say that in solving certain problems they make you do dirty things, like fiddling around with indices, consequently making 1-off bugs, and so on. I’ve shown that contex can solve some problems like those above nicely. How often can this contextual abstraction be of use? I don’t really know.

Using Contex

The contex package contains 4 functions: T(string), search(string, pattern, flags=0), match(string, pattern, flags=0) and find(string, substring). T is for bringing a string into the world of contex by converting it into a StringContext object; search and match are for regex searches; find is for normal string search. contex also contains the StringContext and MatchContext classes.

Installing

Install with $ pip3 install contex. If Python3 is the default python on your system, you may replace pip3 with pip.

Developing

Contex is documented and tested. Run $ nosetests or $ python3 setup.py test to run the tests. The code is hosted at https://notabug.org/Uglemat/Contex

License

The library is licensed under the GNU General Public License 3 or later. This README file is public domain.