Conflicting needs for __init__ method

[Ben Finney]

"BJörn Lindqvist" <bjourne at gmail.com> writes:

> import rational
> rational.rational(45)
> rational.rational(45.0)
> rational.rational([45, 45.5])
>
> def rational(obj):
>     initers = [(int, from_int), (basestring, from_str), (list, from_list)]
>     for obj_type, initer in initers:
>         if isinstance(obj, obj_type):
>             return initer(obj)
>     raise ValueError("Can not create a rational from a %r" % type(obj).__name__)

You've just broken polymorphism. I can't use your factory function
with an instance of my custom type that behaves like a list, but is
not derived from list (or a non-'int' int, or a non-'basestring'
string).

Use the supplied value as you expect to be able to use it, and catch
the exception (somewhere) if it doesn't work. That will allow *any*
type that exhibits the correct behaviour, without needlessly
restricting it to a particular inheritance.

-- 
 \      "This sentence contradicts itself -- no actually it doesn't."  |
  `\                                             -- Douglas Hofstadter |
_o__)                                                                  |
Ben Finney