use variable_decode to add automagical formparsing like tg1

[mramm]

Add formencode variable_decode stuff to get tg1 like automatic nested dictionary generation in tg2. We will also need a decorator that allows us to skip this behavior, since there are some forms that variable_decode doesn't like.

[rick446]

The following trivial dictionaries will crash variable_decode:

>>> from formencode.variabledecode import variable_encode, variable_decode
>>> variable_decode({'a-a':0})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/FormEncode-0.7.1-py2.5.egg/formencode/variabledecode.py", line 49, in variable_decode
    new_keys.append(int(index))
ValueError: invalid literal for int() with base 10: 'a'
>>> variable_decode({'a-0-0':0})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/FormEncode-0.7.1-py2.5.egg/formencode/variabledecode.py", line 46, in variable_decode
    key, index = key.split(list_char)
ValueError: too many values to unpack

The second error is especially troubling because it means variable_decode is not the inverse of variable_encode:

>>> variable_decode(variable_encode({'a':[[0]]}))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/FormEncode-0.7.1-py2.5.egg/formencode/variabledecode.py", line 46, in variable_decode
    key, index = key.split(list_char)
ValueError: too many values to unpack

[mramm]

I'm actually a little bit hesitant about this, as it's kind of magical -- and because of the failure cases outlined above.

[mramm]

We can always add this later if people ask for it back, and I'm a bit hesitant to do it if variable_decode isn't cleaned up a bit first.

[nickmurdoch]

First time I accidentally encountered this in TG1, it was a bit bewildering as I wasn't expecting my form names to be played around with; however once I knew about it, it was very useful for contructing forms that could be easily worked with in the controllers.

A decorator (or argument for expose()?) to optionally provide this functionality would be nice to have, although I suspect it'd be trivial to write one myself! Although it did take me a good few hours to work out what in TG1 was actually responsible for causing this behaviour before being able to even find this ticket -- it's quite a difficult concept to put simply into words.

[percious]

ftr, im -1 on this idea. If people want to decode their vars that way, they can using formencode directly.

[rick446]

Fixed in d1bb21aa4f1b by creating a @variable_decode decorator