Christoph Groth a écrit :
Dear all,
sometimes it is handy to have a function which can take as argument
anything which can be converted into something, e.g.
def foo(arg):
arg = float(arg)
# ...
I would like to mimic this behavior of float for a user-defined type,
e.g.
def bar(arg):
arg = My_type(arg)
# ...
Now I wonder what is the most pythonic way to write the __init__ method
of My_type? The following comes to my mind:
class My_type:
def __init__(self, other):
if isinstance(other, type(self)):
self.a = other.a
self.b = other.b
return
# initialize self in some other way
It seems to me that in this way I might get problems when I pass an
instance of Derived_from_my_type to bar, as it will become an instance
of My_type.
The instance you pass to bar won't "become" anything else. You create a
new My_type instance from the Derived_from_my_type one's values, and
rebinding the _local_ name 'arg' only affects the local namespace.
<OT>
BTW, the convention in Python is to use TitleCase for class names
(except - for historical reasons - for most builtin types which are
lowercase).
</OT>
What is a good way to express this?
Depends on what are the possible initializers / arguments for My_type.
There are a few possible solutions but which one is best depends on the
concrete case and personal tastes.
In C++
Forget about C++ - Python is a different beast !-)
(which I know better than
python) I would make bar accept a const reference to My_type. Then I
could use it directly with instances of My_type, Derived_from_my_type
and other types which can be converted into My_type.
If you only worry about being able to use any "My_type like" object -
that is, any object implementing a given subset of My_type's interface,
then just document your expectations in the function's docstring and use
whatever object is passed in as if it was a My_type instance. Period. As
long as you document what your function expects, it's the caller's
responsaibility to make sure it provides something compatible. If he
don't, well he'll get a nice traceback.
I know this might look very freestyle and a bit frightening when coming
from one of these B&D languages, but that's really the Python way, and
from experience, it JustWork(tm).
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