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To: python-list@python.org
From: Ben Finney <ben+python@benfinney.id.au>
Subject: Re: Is this unpythonic?
Date: Fri, 08 May 2015 21:31:44 +1000
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"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.

E.g., a contrived example::

    def deliver_flowers(species, location=[0, 0, 0]):
        """ Deliver the species of flower to the specified location. """
        (x, y, z) = location
        vehicle.load_with(species)
        vehicle.drive_to(x, y, z)
        vehicle.unload()

If the only reason you're using a collection is because those values
sensibly go together as a single named object, then define a class for
that::

    class Point:
        """ A three-dimensional point in space. """

        def __init__(self, x, y, z):
            self.x = x
            self.y = y
            self.z = z

    centre = Point(0, 0, 0)

    def deliver_flowers(species, location=centre):
        """ Deliver the species of flower to the specified location. """
        vehicle.load_with(species)
        vehicle.drive_to(location.x, location.y, location.z)
        vehicle.unload()

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::

    import collections

    Point = collections.namedtuple('Point', ['x', 'y', 'z'])

    centre = Point(0, 0, 0)

    def deliver_flowers(species, location=centre):
        """ Deliver the species of flower to the specified location. """
        vehicle.load_with(species)
        vehicle.drive_to(location.x, location.y, location.z)
        vehicle.unload()

<URL:https://docs.python.org/3/library/collections.html#collections.namedtuple>
documents the surprisingly useful ‘namedtuple’ factory.

-- 
 \          “It's a terrible paradox that most charities are driven by |
  `\     religious belief.… if you think altruism without Jesus is not |
_o__)          altruism, then you're a dick.” —Tim Minchin, 2010-11-28 |
Ben Finney