custom sorting and __cmp__

Discussion in 'Python' started by Lee Harr, Nov 30, 2003.

  1. Lee Harr

    Lee Harr Guest

    Let's say I have a class

    class A:
    def __init__(self, level):
    self.level = level

    and I want to put some of these in a list and sort the list
    by level

    a1 = A(1)
    a2 = A(2)
    l = [a1, a2]

    Am I better off creating a __cmp__ method for my class or
    making a cmp function to pass to sort?

    My thought was that __cmp__ would be perfect, but then I
    started thinking about this ...
    .... def __init__(self, level):
    .... self.level = level
    .... def __cmp__(self, other):
    .... if self.level > other.level:
    .... return 1
    .... elif self.level < other.level:
    .... return -1
    .... else:
    .... return 0
    ........ pass
    ....Traceback (most recent call last):
    File "<stdin>", line 1, in ?
    File "<stdin>", line 5, in __cmp__
    AttributeError: C instance has no attribute 'level'

    Should I be catching comparisons to objects that do not have
    my 'level' attribute and falling back to id comparison?
    Or am I worried about nothing (YAGNI :eek:) ?

    Is this related in any way to interfaces?
    Lee Harr, Nov 30, 2003
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  2. Lee Harr
    Instead, how about:
    def __cmp__(self,other):
    try: return cmp(self.level, other.level)
    except return 1 # or -1
    Emile van Sebille, Nov 30, 2003
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  3. I usually solve it like this:

    class A:
    def __init__(self, level):
    self.level = level

    def __cmp__(self, other):
    if isinstance(other, A):
    return cmp(self.level, other.level)
    return -1
    Kristian Ovaska, Nov 30, 2003
  4. Lee Harr

    Aahz Guest

    Python is these days moving more and more toward preventing
    heterogeneous comparisons:
    Traceback (most recent call last):
    File "<stdin>", line 1, in ?
    TypeError: can't compare to int

    However, you should set things up so that you can use ``==``:

    The easy way to handle this is to use the new special methods for rich
    comparisons instead of __cmp__.
    Aahz, Dec 3, 2003
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