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

Ivan Levkivskyi levkivskyi at gmail.com
Wed May 10 06:31:20 EDT 2017 
There is a discrepancy now between PEP 484 and PEP 526:

def f(x: int = None): ... # OK
x: int = None     # Error

I think the two rules should be "in sync", in view of this I agree with the
proposal.

Concerning verbosity and a long name Optional there are many reasonable
workarounds.
One is already mentioned from typing import Optional as O. Another
(unrelated to `` = None`` pattern)
is https://github.com/python/typing/issues/420 that allows to avoid
Optional altogether in patterns like this:

def func(x: Optional[X]) -> Optional[Y]:
    if x is None:
        return None
    # do some stuff with 'x'

With @maybe decorator proposed in
https://github.com/python/typing/issues/420
this will be simplified to:

@maybe
def func(x: X) -> Y:
    if x is None:
        return None
    # do some stuff with 'x'

Even if @maybe will not make it to typing, one still can define such (or
similar) decorators
(especially with @decorated_type and extended Callable syntax).

In view of this, I think verbosity is not a problem at all.

--
Ivan
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