[Python-Dev] AC Derby and accepting None for optional positional arguments (original) (raw)

Guido van Rossum guido at python.org
Thu Jan 16 19:57:39 CET 2014

On Thu, Jan 16, 2014 at 12:31 AM, Serhiy Storchaka <storchaka at gmail.com> wrote:

getattr(foo, 'bar', None) is not the same as getattr(foo, 'bar'). So None can't be used as universal default value.

Not universal, but I still think that most functions don't need to have such a subtle distinction.

E.g. in the case of sha1() I still believe that it's totally fine to switch the default to b''. In that particular case I don't see the need to also accept None as a way to specify the default.

Basically, my philosophy about this is that anytime you can't easily reimplement the same signature in Python (without reverting to manually parsing the args using *args and **kwds) it is a pain, and you should think twice before canonizing such a signature.

Also, there are two somewhat different cases:

(a) The default can easily be expressed as a value of the same type that the argument normally has. This is the sha1() case. In this case I see no need to also accept None as an argument (unless it is currently accepted, which it isn't for sha1()). Another example is .read() -- here, passing in a negative integer means the same as not passing an argument.

(b) The default has a special meaning that does something different than any valid value. A good example is getattr(), which must forever be special.

To me, most functions should fall in (a) even if there is currently ambiguity, and it feels like repeat() was meant to be in (a).

I'm not sure how AC should deal with (b), but I still hope that true examples are rare enough that we can keep hand-coding them.

-- --Guido van Rossum (python.org/~guido)

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