What data type should my widgets accept/return?
While simply passing strings around seems like a useful idea, I think you're going to discover it doesn't work as well as you might hope.
Think about the date example—instead of passing around a date object, instead you pass around a str of the format "2010-01-01".  In order to work with that data, every user of the class needs to know not only that it's a str which represents a date, but what the format of that string is.  In other words, you haven't gained anything.  Worse, you lose the ability to pass a datetime object into the widget (unless you take extra steps to deal with that case).
The validator or formatter issue isn't as big a deal as you might think; how often are you going to want to validate a string which doesn't represent a date as if it were a date?
I'm building a form class in python for producing and validating HTML forms. Each field has an associated widget which defines how the field is rendered.
When the widget is created, it is passed in a (default) value so that it knows what to display the first time it is rendered. After the form is submitted, the widget is asked for a value. I delegate this to the widget rather than just nabbing it from the POST data because a widget may consist of several HTML inputs (think of a month/day/year selector). Only the widget knows how to mash these together into one value.
Problem is, I don't know if I should have the widget always accept a string, and always return a string for consistency, or accept and return a data type consistent with its purpose (i.e., a date selector should probably return a DateTime object).
The philosophy behind my form class is "mix and match". You choose what widget you want, and what validators/formatters/converters you want to run on it. Which I guess lends itself towards "use strings" and let the developer decide on the data type afterwords, but... I can't think of a good but. Do you anticipate any problems with this approach?