Re: richcmpfunc semantics
harold fellermann <[EMAIL PROTECTED]> writes:
> Thank you Greg,
>
> I figured most of it out in the meantime, myself. I only differ
> from you in one point.
>
>>> What has to be done, if the function is invoked for an operator
>>> I don't want to define?
>>
>> Return Py_NotImplemented. (Note that's return, *not* raise.)
>
> I used
>
> PyErr_BadArgument();
> return NULL;
>
> instead. What is the difference between the two and which one
> is to prefer.
If you do it your way you're a bad neighbour: If your object is the
first one (left-hand side) of the operator, it will prevent the other
object from handling the case if it can. This is the same advice as
for all of the other operators (__add__, etc.)
Consider the pure-python version:
class A:
def __init__(self, what_to_do='return'):
self.what_to_do = what_to_do
def __eq__(self, other):
print 'A.__eq__'
if self.what_to_do == 'return':
return NotImplemented
else:
raise Exception
class B:
def __eq__(self, other):
print 'B.__eq__'
return True
>>> a = A('return')
>>> b = B()
>>> a == b
A.__eq__
B.__eq__
True
>>> b == a
B.__eq__
True
>>> a == a
A.__eq__
A.__eq__
A.__eq__
A.__eq__
True
So the B class handles the case where A doesn't know what to do. Also
note the last case, where Python falls back on id() comparisions to
determine equality.
Now, compare with this:
>>> a = A('raise')
>>> b = B()
>>> a == b
A.__eq__
Traceback (most recent call last):
File "", line 1, in ?
File "x.py", line 9, in __eq__
raise Exception
Exception
>>> b == a
B.__eq__
True
>>> a == a
A.__eq__
Traceback (most recent call last):
File "", line 1, in ?
File "x.py", line 9, in __eq__
raise Exception
Exception
So now comparing A and B objects can fail. If you *know* A and B
objects can't be compared for equality, it'd be ok to raise a
TypeError, but that should be after a type test.
> Also, do you need to increment the reference count
> of Py_NotImeplemented before returning it?
Yes; it's a singleton like Py_None.
--
|>|\/|<
/--\
|David M. Cooke
|cookedm(at)physics(dot)mcmaster(dot)ca
--
http://mail.python.org/mailman/listinfo/python-list
Re: richcmpfunc semantics
Thank you Greg, I figured most of it out in the meantime, myself. I only differ from you in one point. What has to be done, if the function is invoked for an operator I don't want to define? Return Py_NotImplemented. (Note that's return, *not* raise.) I used PyErr_BadArgument(); return NULL; instead. What is the difference between the two and which one is to prefer. Also, do you need to increment the reference count of Py_NotImeplemented before returning it? Thanks, - harold - -- I like pigs. Dogs look up to us. Cats look down to us. Pigs treat us as equal. -- Winston Churchill -- http://mail.python.org/mailman/listinfo/python-list
Re: richcmpfunc semantics
harold fellermann wrote: richcmpfunc compare(PyObject *,PyObject, int); I supposed the two objects passed are the ones to be compared. Yes. What is the meaning of the integer argument? Does it specify the kind of comparision operator (e.g. __eq__ or __le__), and if so, how? < 0 <= 1 == 2 != 3 > 4 >= 5 What is my richcmpfunc supposed to return? 0 or 1 indicating False or True? In the usual case, yes, although it can return any Python object. Rich comparison doesn't impose any semantics on the operations -- this is one of the reasons for its existence. What has to be done, if the function is invoked for an operator I don't want to define? Return Py_NotImplemented. (Note that's return, *not* raise.) Maybe there is some good documentation available, but I cannot find it. I found most of this out by reading the source, I think. -- Greg Ewing, Computer Science Dept, University of Canterbury, Christchurch, New Zealand http://www.cosc.canterbury.ac.nz/~greg -- http://mail.python.org/mailman/listinfo/python-list
richcmpfunc semantics
Hi all, I want to implement rich comparision in an extension class. Problem is I cannot find good documentation of the richcmpfunc semantics. Given the signature richcmpfunc compare(PyObject *,PyObject, int); I supposed the two objects passed are the ones to be compared. What is the meaning of the integer argument? Does it specify the kind of comparision operator (e.g. __eq__ or __le__), and if so, how? What is my richcmpfunc supposed to return? 0 or 1 indicating False or True? What has to be done, if the function is invoked for an operator I don't want to define? Maybe there is some good documentation available, but I cannot find it. So, any references or help is appreciated. Cheers, - harold - -- "Scientist are very good in answering the questions they choose to answer." -- Richard Alley -- http://mail.python.org/mailman/listinfo/python-list
