raise TypeError on form instantiation if data/files parameter isn't a dict.

[Adi J. Sieker]

No error is thrown if a form is passed a non dict value to it's data or files parameter on instantiation.
When the form is validated a AttributeError: 'type' object has no attribute 'get' is raised.
I found it confusing that the form instantiation was ok and but somewhere in my template everything broke.

The attached patch checks that the data/files are instances of dict or django.utils.datastructures.MergeDict
if not it raises a TypeError.

Tests are included.

There are 2 issues that need to be looked at:

1. passing an instance of something that behaves like a dict but isn't a subclass of dict or MergeDict, won't work with this patch applied, which I'm not really happy about but my python-fu isn't strong enough to come up with a better solution. Rewriting the patch to use getattr is a possibility.
2. It might be considered an backwards incompatible change since now TypeError can be raised.

[Alex Gaynor]

I'm marking this as DDN, however my opinion is this should be wontfixed. This is a violation of a fairly basic python principle that if it walks like a duck and talks like a duck, we'll treat it like a duck. Further, as you've said it already raises an expcetion when whatever you pass ceases to be sufficiently "duck-like", so I don't see this as assisting anything.

[Adi J. Sieker]

I think it assists in that the exception gives you the "correct" reason. In the current situation you get an Exception telling you that object doesn't have an attribute get. Which IMHO isn't really helpful.

On the other hand it's going to be difficult recognizing everything that looks like a duck.

[Alex Gaynor]

Replying to ajs:

On the other hand it's going to be difficult recognizing everything that looks like a duck.

That's why we don't actually check under the feathers to make sure it's a duck, so long as it quacks ok.

[Malcolm Tredinnick]

This is indeed a wontfix. Forms should be passed things that can obey the right protocol, essentially. Which is what is being reported by that exception (it doesn't have a method to retrieve the information). Doing strong type restriction goes against the flow and inhibits usage a bit only for the benefit of cases of programmer error, which are by far the minority situation (sure, it doesn't seem like that when you're developing, but restricting normal use-cases because it makes it 30 seconds easier to spot a typo isn't long-term helpful0.