Unanswered Questions in python.faqts.com

[Will Ware]

Fiona Czuczman (fiona at sitegnome.com) wrote:
> FAQTS : Computers : Programming : Languages : Python : Language and
> Syntax
> How should a sortable sequence be implemented?
> http://www.faqts.com/knowledge-base/view.phtml/aid/5367/fid/241/lang/

Objects can be sorted as long as they can be compared. Fundamental
types (ints, floats, strings) can be compared with the globally defined
cmp() function. cmp(x,y) returns a negative integer if x<y, zero if x==y,
and a positive integer if x>y. If the 'sort' method is called on a list
of objects, the list will be sorted in increasing order:

x = [3,1,4,1,5,9,2,6]
x.sort()
print x  -->  [1,1,2,3,4,5,6,9]

Classes can be made sortable by defining a __cmp__ method for them. For
example, to sort a list of 3-d points in decreasing order of z, you could
write:

class Point:
    def __init__(self, x,y,z):
        self.x, self.y, self.z = x, y, z
    def __cmp__(self, other):
        return -cmp(self.z, other.z)
        # or: return cmp(other.z, self.z)

Now if you create a list of points (x) and call x.sort(), the points will
be sorted in decreasing-z order.

Another way to define the sorting criterion is to pass a cmp-like function
as the argument to x.sort(). To re-sort your points in increasing-y order,
you can write:

x.sort(lambda self,other: cmp(self.y, other.y))

or if you don't like lambdas:

def y_sorter(u,v):
    return cmp(u.y, v.y)
x.sort(y_sorter)

-- 
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# Resistance is futile. Capacitance is efficacious.
# Will Ware	email:    wware @ world.std.com