On Thu, Dec 11, 2014 at 3:10 AM, Jean-Michel Pichavant <jeanmic...@sequans.com> wrote: > ----- Original Message ----- >> From: "ast" <nom...@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 -- https://mail.python.org/mailman/listinfo/python-list
