Skip Montanaro <skip.montan...@gmail.com>:
> On Wed, Dec 10, 2014 at 9:14 AM, ast <nom...@invalid.com> wrote:
>> 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.
>
> This is almost the correct idiom. In general, pick defaults from
> values outside your valid domain. None is often perfect for
> this. Since you're comparing to a singleton value, "is" is the usual
> comparison, so your code becomes:
>
> def __init__(center=(0,0), radius=10, mass=None)):
> if mass is None:
> self.mass = radius**2
> else:
> self.mass = mass
In this case, None works well. However, sometimes None is a valid input
value. Then, a sentinel object is the way out:
_OMITTED = object()
class SomeClass:
def __init__(self, center=(0,0), radius=10, mass=_OMITTED)):
if mass is _OMITTED:
self.mass = radius**2
else:
self.mass = mass
Marko
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