Dangerous if_missing in ForEach

Hi,

I was bitten by a formencode default just some minutes ago so I
thought that sharing my experience may help others to avoid this
trap:
If use you ForEach but the key is not present in your input
dictionary, you will get an empty list ([]) by default. This is
somewhat dangerous because the list instance is a mutable object.

So if your program modifies the resulting list AND your process
lives longer than just one request, you will get the old list
instance next time.

This example will show you the problem:
# -----------------------------------------
from formencode import ForEach, Schema
from formencode.validators import Int

class Foo(Schema):
      baz = ForEach(Int)

schema = Foo()
fields = schema.to_python({})
fields["baz"].append(1)
print schema.to_python({})
# -----------------------------------------

Yes, I agree you probably should not modify the result dict in the
first place. However, sometimes it may be convenient to do so.

A solution to this would be to change the default of ForEach to
"()" (tuple) which is not mutable. Unfortunately, this is a backward
incompatible change which would break many apps for sure.

fs

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