Securing a future for anonymous functions in Python

Scott David Daniels Scott.Daniels at Acm.Org
Fri Dec 31 19:24:21 CET 2004

David Bolen wrote:
> So for example, an asynchronous sequence of operations might be like:
>     d = some_deferred_function()
>     d.addCallback(lambda x: next_function())
>     d.addCallback(lambda blah: third_function(otherargs, blah))
>     d.addCallback(lambda x: last_function())
> which to me is more readable (in terms of seeing the sequence of
> operations being performed in their proper order), then something like:
>     def cb_next(x):
>         return next_function()
>     def cb_third(blah, otherargs):
>         return third_function(otherargs, blah)
>     def cb_last(x):
>         return last_function()
>     d = some_deferred_function()
>     d.addCallback(cb_next)
>     d.addCallback(cb_third, otherargs)
>     d.addCallback(cb_next)
> which has an extra layer of naming (the callback functions), and
> requires more effort to follow the flow of what is really just a simple
> sequence of three functions being called.

But this sequence contains an error of the same form as the "fat":

     while test() != False:

The right sequence using lambda is:
      d = some_deferred_function()
      d.addCallback(lambda blah: third_function(otherargs, blah))

And I would write it as:

      def third_function_fixed_blah(blah):
          def call_third(otherargs):
              return third_function(otherargs, blah)
          return call_third

      d = some_deferred_function()
      d.addCallback(third_function_fixed_blah, otherargs)

The name gives you the chance to point out that the argument order is
tweaked.  In many such cases, I use curry (ASPN recipe #52549), which
should show up in Python as "partial" in the "functional" module
according to PEP 309 <>
(accepted but not included).  I suppose it will show up in Python 2.5.

Programming is a quest is for clear, easy-to-read code, not quick,
easy-to-write code.  Choosing a name is a chance to explain what you
are doing.  lambda is used too often in lieu of deciding what to write.

--Scott David Daniels
Scott.Daniels at Acm.Org

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