[Python-ideas] Optional parameters without default value

[M.-A. Lemburg]

On 02.03.2017 09:03, Serhiy Storchaka wrote:
> Function implemented in Python can have optional parameters with default
> value. It also can accept arbitrary number of positional and keyword
> arguments if use var-positional or var-keyword parameters (*args and
> **kwargs). But there is no way to declare an optional parameter that
> don't have default value. 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
> 
> There are drawback of this:
> 
> * Module's namespace is polluted with sentinel's variables.
> 
> * You need to check for the sentinel before passing it to other function
> by accident.
> 
> * Possible name conflicts between sentinels for different functions of
> the same module.
> 
> * Since the sentinel is accessible outside of the function, it possible
> to pass it to the function.
> 
> * help() of the function shows reprs of default values. "foo(bar=<object
> object at 0xb713c698>)" looks ugly.
> 
> 
> I propose to add a new syntax for optional parameters. If the argument
> corresponding to the optional parameter without default value is not
> specified, the parameter takes no value. As well as the "*" prefix means
> "arbitrary number of positional parameters", the prefix "?" can mean
> "single optional parameter".
> 
> Example:
> 
> def get(store, key, ?default):
>     if store.exists(key):
>         return store.retrieve(key)
>     try:
>         return default
>     except NameError:
>         raise LookupError

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)
    ...

I added a special singleton NotGiven to our mxTools long ago for
this purpose.

-- 
Marc-Andre Lemburg
eGenix.com

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