[Python-ideas] Optional parameters without default value

[M.-A. Lemburg]

On 02.03.2017 10:06, Serhiy Storchaka wrote:
> On 02.03.17 10:36, M.-A. Lemburg wrote:
>> Why a new syntax ? Can't we just have a pre-defined sentinel
>> singleton NoDefault and use that throughout the code (and also
>> special case it in argument parsing/handling)?
>>
>> def get(store, key, default=NoDefault):
>>     if store.exists(key):
>>         return store.retrieve(key)
>>     ...
> 
> This means adding a new syntax. NoDefault should be a keyword (we can
> reuse existing keyword couldn't be used in expression), and it should be
> accepted only in the specific context of declaring function parameter.

This is not new syntax, nor is it a keyword. It's only a
new singleton and it is well usable outside of function
declarations as well, e.g. for class attributes which are
not yet initialized (and which can accept None as value).

The only special casing would be in function call
parameter parsing to signal errors when the parameter
is used as keyword parameter.

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