Mapping None. Why?

[Terry Reedy]

"David C. Ullrich" <dullrich at sprynet.com> wrote in message 
news:dullrich-571EC3.12043113062008 at text.giganews.com...
| In article
| <d5897630-558e-428a-9bab-e0c9c1de7ba6 at j22g2000hsf.googlegroups.com>,
| Paddy <paddy3118 at googlemail.com> wrote:
|
| > True, but None is not a function. It's a sentinel value to turn on the
| > functionality.
|
| Uh, thanks. I think I knew that - I was just suggesting why
| the way map works makes sense.

filter(None, iterable) works the same way: None-> identity function,
The immediate reason is the Python has no builtin id().
But apparently there is also historical precedent in the functional 
community for this convention.