Splitting lists

[Brandon Beck]

Alex Martelli <aleax at aleax.it> wrote in message news:<2Vl7a.329137$AA2.12321414 at news2.tin.it>...

> appenders = tl.append, fl.append
> for e in lst: appenders[not fn(e)](e)
> 
> now isn't THAT more like it...?


When I read this, I was immediately reminded of one of Guido's essays
on optimization (http://www.python.org/doc/essays/list2str.html).  One
of the takeaways I had from reading his optimization anecdote was that
if possible you should attempt to use built-in functions with implied
loops when possible.  So I immediately thought this should be faster:

  func = lambda e, a=[t1.append, f1.append]: a[not fn(e)](e)
  map(func, lst)
  return t1, f1


After rereading Guido's essay, I see that he says that you should only
favor a built-in construct with an implied loop in situations where
the function argument is also a python built-in, and not anything
containing python code.  Now I don't claim to have any knowledge of
the inner workings of the python interpreter, but I was wondering why
this is the case?  I understand that a lambda needs to be interpreted
where as C code doesn't, but what is it about that in combination with
map that makes it slower then your for loop version?


Brandon