[Phil] >>> I found the documentation confusing regarding Optional. Intuitively it >>> seems to be the way to specify arguments with default values. However it is >>> explained in terms of (for example) Union[str, None] and I (intuitively but >>> incorrectly) read that as meaning "a str or None" as opposed to "a str or >>> nothing". [me] >> But it *does* mean 'str or None'. The *type* of an argument doesn't >> have any bearing on whether it may be omitted from the argument list >> by the caller -- these are orthogonal concepts (though sadly the word >> optional might apply to both). It's possible (though unusual) to have >> an optional argument that must be a str when given; it's also possible >> to have a mandatory argument that may be a str or None. [Phil] > In the case of Python wrappers around a C++ library then *every* optional > argument will have to have a specific type when given.
IIUC you're saying that every argument that may be omitted must still have a definite type other than None. Right? In that case just don't use Optional[]. If a signature has the form def foo(a: str = 'xyz') -> str: ... then this means that str may be omitted or it may be a str -- you cannot call foo(a=None). You can even (in a stub file) write this as: def foo(a: str = ...) -> str: ... (literal '...' i.e. ellipsis) if you don't want to commit to a specific default value (it makes no difference to mypy). > So you are saying that a mandatory argument that may be a str or None would > be specified as Union[str, None]? Or as Optional[str], which means the same. > But the docs say that that is the underlying implementation of Option[str] - > which (to me) means an optional argument that should be a string when given. (Assuming you meant Option*al*.) There seems to be an utter confusion of the two uses of the term "optional" here. An "optional argument" (outside PEP 484) is one that has a default value. The "Optional[T]" notation in PEP 484 means "Union[T, None]". They mean different things. >> Can you help improve the wording in the docs (preferably by filing an issue)? > > When I eventually understand what it means... -- --Guido van Rossum (python.org/~guido) _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
