[Python-ideas] Optional parameters without default value

[M.-A. Lemburg]

On 02.03.2017 11:22, Stephan Houben wrote:
> In cases like this I would recommend creating the sentinel yourself:
> 
> NoDefault = object()
> 
> def get(store, key, default=NoDefault):
>    if default is NoDefault:
>         # do something
> 
> You can arrange to not export NoDefault so that the client code cannot even
> access
> the sentinel value.

Yes, I know... I've been using the mxTools NotGiven since 1998.

> This is strictly preferable over having yet another global
> value meaning "no value", since that just moves the goal posts:
> clients will complain they cannot pass in a default=NoDefault and get back
> NoDefault.

Not really. NoDefault would mean: no value provided, not that
you don't want a value. As a result, passing NoDefault would
not be allowed, since then you'd be providing a value :-)

> Stephan
> 
> 
> 2017-03-02 11:04 GMT+01:00 M.-A. Lemburg <mal at egenix.com>:
> 
>> 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.
>>
>> --
>> Marc-Andre Lemburg
>> eGenix.com
>>
>> Professional Python Services directly from the Experts (#1, Mar 02 2017)
>>>>> Python Projects, Coaching and Consulting ...  http://www.egenix.com/
>>>>> Python Database Interfaces ...           http://products.egenix.com/
>>>>> Plone/Zope Database Interfaces ...           http://zope.egenix.com/
>> ________________________________________________________________________
>>
>> ::: We implement business ideas - efficiently in both time and costs :::
>>
>>    eGenix.com Software, Skills and Services GmbH  Pastor-Loeh-Str.48
>>     D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg
>>            Registered at Amtsgericht Duesseldorf: HRB 46611
>>                http://www.egenix.com/company/contact/
>>                       http://www.malemburg.com/
>>
>> _______________________________________________
>> Python-ideas mailing list
>> Python-ideas at python.org
>> https://mail.python.org/mailman/listinfo/python-ideas
>> Code of Conduct: http://python.org/psf/codeofconduct/
>>
> 
> 
> 
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
> 

-- 
Marc-Andre Lemburg
eGenix.com

Professional Python Services directly from the Experts (#1, Mar 02 2017)
>>> Python Projects, Coaching and Consulting ...  http://www.egenix.com/
>>> Python Database Interfaces ...           http://products.egenix.com/
>>> Plone/Zope Database Interfaces ...           http://zope.egenix.com/
________________________________________________________________________

::: We implement business ideas - efficiently in both time and costs :::

   eGenix.com Software, Skills and Services GmbH  Pastor-Loeh-Str.48
    D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg
           Registered at Amtsgericht Duesseldorf: HRB 46611
               http://www.egenix.com/company/contact/
                      http://www.malemburg.com/