[Python-Dev] PEP 484 proposal: don't default to Optional if argument default is None (original) (raw)

Guido van Rossum guido at python.org
Tue May 9 13:28:41 EDT 2017

There's a proposal to change one detail of PEP 484. It currently says:

An optional type is also automatically assumed when the default value is None, for example::

def handle_employee(e: Employee = None): ...

This is equivalent to::

def handle_employee(e: Optional[Employee] = None) -> None: ...

Now that we've got some experience actually using Optional with mypy (originally mypy ignored Optional), we're beginning to think that this was a bad idea. There's more discussion at https://github.com/python/typing/issues/275 and an implementation of the change (using a command-line flag) in https://github.com/python/mypy/pull/3248.

Thoughts? Some function declarations will become a bit more verbose, but we gain clarity (many users of annotations don't seem to be familiar with this feature) and consistency (since this rule doesn't apply to variable declarations and class attribute declarations).

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