Why does min(A, B) not raise an error if A, B actually can't be compared?

[Claudio Grondi]

Thanks to Raymond for his reply.

If I understand him right, there is no problem with
min() as such, but with the definition of the class, which
when used in min() should define at least __cmp__().

I have attached code with another
  class  PythonObject_classWithDefined__cmp__:
where the behaviour of min() is like I would
expect it.

Using classes without defined __cmp__ in comparison
operations violates the directive:
  "Explicit is better than implicit"
and is just to be considered bad programming
style or to name it more directly, just a way of
obfuscating code, right?

The still open question for me then is:
Why does min() not raise an error in case
there is no comparison function definition
for the feeded objects available?

Claudio

ATTACHMENT:

class PythonObject_classWithDefined__cmp__:
  def __init__(self, value = 1):
    self.value = value
  #:def
  def __cmp__(self, otherInstance):
    if ( self.value < otherInstance.value ):
      return -1
    elif(self.value == otherInstance.value ):
      return 0
    else:
      return 1
    #:if/else
  #:def
#:class

PythonObject_classWithDefined__cmp__instanceA =
PythonObject_classWithDefined__cmp__()
PythonObject_classWithDefined__cmp__instanceB =
PythonObject_classWithDefined__cmp__()
print "min(A,B) is A: "
print "in case of classes with defined __cmp__() as parameter: " +
str(min(PythonObject_classWithDefined__cmp__instanceA,
PythonObject_classWithDefined__cmp__instanceB) is
PythonObject_classWithDefined__cmp__instanceA)
print "min(B,A) is A: "
print "in case of classes with defined __cmp__() as parameter: " +
str(min(PythonObject_classWithDefined__cmp__instanceB,
PythonObject_classWithDefined__cmp__instanceA) is
PythonObject_classWithDefined__cmp__instanceA)

 outputs:

min(A,B) is A:
in case of classes with defined __cmp__() as parameter: True
min(B,A) is A:
in case of classes with defined __cmp__() as parameter: False


"Raymond Hettinger" <python at rcn.com> schrieb im Newsbeitrag
news:1125084040.571865.154920 at o13g2000cwo.googlegroups.com...
> Claudio Grondi wrote:
> > Is there any deeper reason I don't understand
> > explaining why does min(A,B) behave different
> > for classes than for lists?
>
> Yes, the sort order for lists is determined by their contents.  With
> your example, the lists have identical contents, so min() returns the
> first minimum value encountered which is A for min(A,B) and B for
> min(B,A).
>
> For instances, the sort order is determined by custom __cmp__ or rich
> comparision methods.  In the absence of those, the default ordering is
> determined by the object's id.  In your example, the default is used
> and either object may be returned as the minimum depending on which
> object id is a higher number (that is an implementation and state
> dependent).  Since the two objects have unique ids, min() will
> consistently find one to be lower than the other irrespective of
> argument order, if min(A,B) is A, then min(B,A) will also be A.
>
> The best way to develop your understanding here is view the object ids
> for the instances and experiment with the results of A<B, A<=B, A==B,
> etc.
>
> Then write a simple, pure python version of min() that returns the
> first occurence of the lowest valued element.  Trace through its
> execution and all will become clear.
>
>
> Raymond
>