[Python-ideas] Optional parameters without default value

[Barry Warsaw]

On Mar 02, 2017, at 10:03 AM, Serhiy Storchaka wrote:

>Currently you need to use the sentinel idiom for implementing this:
>
>_sentinel = object()
>def get(store, key, default=_sentinel):
>     if store.exists(key):
>         return store.retrieve(key)
>     if default is _sentinel:
>         raise LookupError
>     else:
>         return default

The reason for using a special sentinel here is to ensure that there's no way
(other than deliberate subterfuge) for code using this API to pass that
sentinel in.  Normally, None is just fine, but for some cases None is a
possible legitimate value, so it won't do as a sentinel.  Thus you make one up
that you know will mean "wasn't given".

A classic example is dict.get():

missing = object()
if d.get('key', missing) is missing:
    its_definitely_not_in_the_dictionary()

i.e. because it's possible for d['key'] == None.

I don't think this use case is common enough for special syntax, or a
keyword.  I'm -0 for adding a new built-in because while it might serve your
purposes, it's easier to commit subterfuge.

>>> get(store, key, default=NoDefault)
# Whoop!

or

if d.get('key', NoDefault) is NoDefault:
    hopefully_its_not_in_the_dictionary()

Cheers,
-Barry
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