In my code, I commonly use a NOT_SET singleton used as default value. I like this name for the test: if arg is NOT_SET: ... ;-) I use that when I want to behave differently when None is passed. And yes, I have such code. Victor Le 2 mars 2017 9:36 AM, "M.-A. Lemburg" <mal@egenix.com> a écrit : 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 Professional Python Services directly from the Experts (#1, Mar 02 2017)

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