[PEAK] FYI: Generic function API change

Phillip J. Eby pje at telecommunity.com
Mon Jan 10 02:14:22 EST 2005

Just a heads-up; I checked in a change to the generic function API that 
allows you to use chained methods (i.e., call the next-most-specific 
method) without doing anything special to the generic function definition.

But, you do have to change the signature of the individual method if you 
want to be able to call the next method, e.g.:

     def foo(bar,baz):
         """Foo bar and baz"""

     [foo.when("bar>1 and baz=='spam'")]
     def foo_one_spam(next_method, bar, baz):
         return bar + next_method(bar,baz)

     def foo_spam(bar, baz):
         return 42

In this example, calling 'foo(2,"spam")' would return '44'.  Notice that 
only methods that need to call the next less-specific method should add the 
'next_method' parameter, as this allows the method combination machinery to 
stop chaining methods at that point.

This new mechanism replaces the need for using 
'dispatch.generic(strategy.chained_methods)' and 'strategy.next_method' to 
call the next method.  The only reason that there were two different 
strategies for "single best" and "chained methods" was because the latter 
was more resource intensive.  The new implementation incurs no additional 
resource overhead when you don't have a 'next_method' parameter, so you no 
longer have to decide in advance if you need it.  And, it should be faster 
than the old mechanism even when you do use it.

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