[Python-Dev] Formatting of positional-only parameters in signatures (original) (raw)

Terry Reedy tjreedy at udel.edu
Tue Jan 21 18:22:13 CET 2014

On 1/21/2014 10:59 AM, Yury Selivanov wrote:

There is one more, hopefully last, open urgent question with the signature object. At the time we were working on the PEP 362, PEP 457 didn’t exist. Nor did we have any function with real positonal-only parameters, since there was no Argument Clinic yet. However, I implemented rudimentary support for them:

“Parameter.POSITIONALONLY” constant; “Signature.bind” and “Signature.bindpartial” fully support functions with positional-only parameters; _“Signature.str_” renders them distinctively from other kinds. _The last point is the troublesome now. "Signature.str_” renders positional-only parameters in ‘<>’ brackets, so in: foo(, , baz)

This amounts to a hidden new API.

“ham” and “spam” are positional-only. The choice of angle brackets was unfortunate, as, first of all, this wasn’t really discussed on python-dev, and second, it’s easy to think that those parameters are optional.

Now, with the AC landing in 3.4, we need to decide how positional-only parameters will look like. Without starting a new discussion similar to what we had prior to PEP 457, I think we have three options: 1. Leave it as is. Obviously, the downside is the potential confusion with “optional” notation.

I think this this is bad, as it has not been discussed and agreed on, and might be changed. It is a plausible alternative to '/', but might possibly even have been rejected. I do not remember.

2. Adopt PEP 457 style: using “/“ to separate positional-only parameters from the rest.

I think this is what Larry proposed, but Nick opposed as a post-beta new feature.

3. Don’t use any notation, just render them as plain arguments: "foo(ham, spam, baz)". The downside here is that the users will be confused, and might try passing those parameters with keywords, or binding them with keywords.

This is the status quo for the docs (both notation and occasional confusion). I think signature should match until we agree on a new convention (and I think one is needed). The first thing to decide is whether to mark each position-only parameter or to put one marker after all of them.

-- Terry Jan Reedy

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