Reopening of issue 4181.

Evaluating default parameter values when the function definition is
executed is a design bug.

Even the documentation of this behaviour (see
http://docs.python.org/reference/compound_stmts.html#index-754) makes
this clear by stating: "This is generally not what was intended." It
then makes a suggestion for a workaround ("A way around this ..."). Only
bugs need workarounds.

An interface or behaviour should fulfil a users expectation for the
"normal" case, not for some "special" case. For a default parameter
value a user expects that whenever this function gets called a new
instance of a mutable object is created. This is much closer to the
current behaviour for non-mutable objects.

Usage cases for the current default behaviour are less common.