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To: python-list@python.org
From: "Frank Millman" <frank@chagford.com>
Subject: Re: Is this unpythonic?
Date: Fri, 8 May 2015 15:10:52 +0200
References: <mihqhb$r1k$1@ger.gmane.org> <85y4kzb90f.fsf@benfinney.id.au>
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"Ben Finney" <ben+python@benfinney.id.au> wrote in message 
news:85y4kzb90f.fsf@benfinney.id.au...
> "Frank Millman" <frank@chagford.com> writes:
>
>> If I use a list as an argument, I only use it to pass values *into*
>> the function, but I never modify the list.
>
> You've had good reasons why a mutable default argument is a good idea.
>
> I'll address another aspect of the question: passing a structured
> collection of values via a single parameter.
>
> If those values *always* go together, it sounds like you have a class
> which should be defined to make it explicit that they go together.
>
[...]
>
> Too much overhead? If you want to collect a meaningful collection of
> values together in the same way each time, but don't need any special
> behaviour, then a class might be overkill.
>
> Python has the 'namedtuple' type::
>
[...]
>
> <URL:https://docs.python.org/3/library/collections.html#collections.namedtuple>
> documents the surprisingly useful 'namedtuple' factory.
>

Useful thoughts - thanks, Ben.

Here is a typical use case.

I have a function which constructs a sql SELECT statement from various input 
parameters. One of the parameters is used to build up an ORDER BY clause.

If ordering is required, the caller passes a list of tuples, each tuple 
consisting of a column name and a boolean indicating ascending/descending.

If no ordering is required, it is convenient for the caller to omit the 
argument altogether, but then the callee needs a default argument. 
Previously I had 'order=[]' as a keyword argument, but that gave rise to my 
query. I have now changed it to 'order=None'.

Frank