[Python-Dev] nodef (original) (raw)

Martin Blais blais at furius.ca
Thu May 24 02:08:56 CEST 2007

Hi I often have the need for a generic object to use as the default value for a function parameter, where 'None' is a valid value for the parameter. For example:

_sentinel = object()

def first(iterable, default=_sentinel):
"""Return the first element of the iterable, otherwise the default value (if
specified).
"""
for elem in iterable:   # thx to rhettinger for optim.
  return elem
if default is _sentinel:
  raise StopIteration
return default

Here, 'default' could legally accept None, so I cannot use that as the default value, nor anything else as far as I'm concerned (I don't know what lives in the iterable, so why should I make assumptions?). Sometimes in the past I've create generic objects, declared a class, e.g.:

class NoDef: pass def foo(default=NoDef): ...

and lately I've even started using the names of builtin functions (it bothers me a little bit though, that I do that).

I think Python needs a builtin for this very purpose. I propose 'nodef', a unique object whose sole purpose is to serve as a default value. It should be unique, in the same sense that 'None' is unique.

Comments or alternative solutions?

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