[Python-Dev] AC Derby and accepting None for optional positional arguments

Steven D'Aprano steve at pearwood.info
Fri Jan 17 09:21:06 CET 2014

On Thu, Jan 16, 2014 at 01:08:47PM -0800, Ryan Smith-Roberts wrote:

> socket.getservbyname(servicename[, protocolname])
> This is not an inspectable signature, since pure Python does not support
> bracketed arguments. To make it inspectable, we must give protocolname a
> (valid Python) default value:
> socket.getservbyname(servicename, protocolname=None)
> Unfortunately, while useful and inspectable, this signature is not correct.
> For a pure Python function, passing None for protocolname is the same as
> omitting it. However, if you pass None to getservbyname(), it raises a
> TypeError. So, we have these three options:
> 1) Don't give getservbyname() an inspectable signature.
> 2) Lie to the user about the acceptability of None.
> 3) Alter the semantics of getservbyname() to treat None as equivalent to
> omitting protocolname.
> Obviously #2 is out. My question: is #3 ever acceptable? It's a real
> change, as it breaks any code that relies on the TypeError exception.

The answer seems straightforward to me: it should be treated as any 
other change of behaviour, and judged on a case-by-case basis. I 
think the bug tracker is the right place to ask. Since it's not a 
bug fix, it may be able to be changed, but not lightly, and not in a 
bug-fix release. 
The fact that the motivation for the behaviour change is Argument Clinic 
should not change the decision, as far as I can see. Would a feature 
request "Allow None as default protocolname" be accepted?


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