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Date: Thu, 11 Dec 2014 03:16:42 +1100
Subject: Re: How to detect that a function argument is the default one
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Thu, Dec 11, 2014 at 3:10 AM, Jean-Michel Pichavant
<jeanmichel@sequans.com> wrote:
> ----- Original Message -----
>> From: "ast" <nomail@invalid.com>
>> I have the idea to write:
>>
>> def __init__(center=(0,0), radius=10, mass=None)):
>>
>>     if  mass == None:
>>         self.mass = radius**2
>>     else:
>>         self.mass = mass
>>
>> but maybe Python provides something clever.
>>
>> Thx
>
> If you like one-liners,
>
> def __init__(self, center=(0,0), radius=10, mass=None):
>     self.center = center
>     self.radius = radius
>     self.mass = (mass is None and radius**2) or mass

Leaving aside the obvious point that not everything needs to be a
one-liner, there is a MUCH better way to spell this: the if/else
ternary expression.

self.mass = radius**2 if mass is None else mass

> But there's an issue with that solution: self.mass being computed during the instance initialization, what if radius is changing ?
>
> c1 = Circle((0,0), 10, None)
> print c1.mass
> 20
> c1.radius = 20
> print c1.mass
> 20 # that is unexpected
>
> Everytime an attribute is computed from another attribute, it should ring a python bell : property
>
> def __init__(self, center, radius, mass):
>     self.center = center
>     self.radius = radius
>     self._mass = mass
>
>     @property
>     def mass(self):
>          return self._mass if self._mass is not None else self.radius*2
>
>
> c1 = Circle((0,0), 10, None)
> print c1.mass
> 20
> c1.radius = 20
> print c1.mass
> 40

I think that juuuuuust miiiiight count as scope creep :) Though it
does highlight an interesting point, namely that sometimes the state
of "argument wasn't specified" needs to carry through into an object's
"attribute wasn't specified".

> Note : what is the mass of a circle ?

Fortunately for us, this is a programming class, not a philosophical
one. Maybe the circles in question are being cast in steel? :)

ChrisA