[Python-ideas] Optional parameters without default value

[Victor Stinner]

I dislike the try/except NameError test to chevk if the parameter is set.
Catching NameError is slow.

What is the root issue? Function signature in help(func)? If yes, the
solution can be a special value understood by inspect.signature().

Should it be possible to pass explicitly the special value?
func(optional=special_value)? I think it's ok to allow it. The expected
behaviour is to behave as func().

Victor

Le 2 mars 2017 9:04 AM, "Serhiy Storchaka" <storchaka at gmail.com> a écrit :

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

Alternative syntaxes:

* "=" not followed by an expression: "def get(store, key, default=)".

* The "del" keyword: "def get(store, key, del default)".

This feature is orthogonal to supporting positional-only parameters.
Optional parameters without default value can be positional-or-keyword,
keyword-only or positional-only (if the latter is implemented).

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