[Python-ideas] from __future__ import runtime_default_kwargs

Steven D'Aprano steve at pearwood.info
Mon Mar 11 07:58:17 EDT 2019

Hi James,

Some weeks ago, you started a discussion here about "Clearer 
Communication". Here's another suggestion to help: don't expect your 
readers to either guess, or infer from the code, what your proposal 
means. As the Zen of Python says:

Explicit is better than implicit.

Looking at your sample code, I'm guessing that you want support for late 
binding of function parameter defaults. Python uses early binding, see 
the FAQs:


Am I correct? If not, can you please explain what it is that you are 
actually suggesting.

Note that early binding is not a bug to be fixed, it is a design choice 
which is sometimes useful and sometimes not useful. Mutable defaults 
being shared is sometimes a good feature to have, and for immutable 
defaults, early binding is more efficient.

I don't think that changing the behaviour will be acceptible, I know I 
would argue strongly in favour of keeping early binding.

(If the language defaults to early binding, it is easy to implement late 
binding in the body of the function; but if the language uses late 
binding, it is difficult and annoying to get early binding when you want 
it. We should stick to the status quo.)

But having a nice, clean way to get late binding semantics as an 
alternative might be acceptible, if we can agree on acceptible syntax 
and a mechanism.


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