How to sort by a particular attribute of a class?

I have a model with an attribute which keeps track of the price. Right now, I have a list of that certain model. Is there anyway to rearrange the list to sort by that particular attribute? Is python smart enough to know that the attribute is a value which can be sorted? I am not keeping keeping track of the instances of a particular model using a database (it is not needed for what I am doing, so I cannot just retrieve the instances from th开发者_如何学JAVAe database in sorted order) Thanks!

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You can use the inbuilt sorted function, together with a custom-made function that returns the price of an object:

class Dummy(object) :
    pass

def getPrice(obj) :
    return obj.price

d0 = Dummy()
d0.price = 56.
d1 = Dummy()
d1.price=16.

d2 = Dummy()
d2.price=786.

d3 = Dummy()
d3.price=5.5

elements = [d0, d1, d2, d3]

print 'Pre-sorting:'
for elem in elements :
    print elem.price

sortedElements = sorted(elements, key=getPrice)

print 'Post-sorting:'
for elem in sortedElements :
    print elem.price

This would also work via any method of your class that returns the price, e.g.

class Dummy(object) :
    def __init__(self, price) :
        self._price = price
    def getPrice(self) :
        return self._price

...

sortedElements = sorted(elements, key = Dummy.getPrice)

See http://wiki.python.org/moin/HowTo/Sorting/ for more.

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Alternatively you can use operator.attrgetter():

list_of_objects.sort(key=operator.attrgetter('name_of_attribute_to_sort_by'))

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To sort in-place you can use the .sort method of list using a function that defines the key to sort on.

>>> class Data(object):
...     def __init__(self,x,y):
...             self.x=x
...             self.y=y
... 
>>> l=[Data(i,i+1) for i in xrange(10,-1,-1)]
>>> print ", ".join("%s %s"%(x.x,x.y) for x in l)
10 11, 9 10, 8 9, 7 8, 6 7, 5 6, 4 5, 3 4, 2 3, 1 2, 0 1
>>> l.sort(key=lambda obj:obj.y)
>>> print ", ".join("%s %s"%(x.x,x.y) for x in l)
0 1, 1 2, 2 3, 3 4, 4 5, 5 6, 6 7, 7 8, 8 9, 9 10, 10 11

To get another list while keeping the original one intact, use the sorted function with the optional key parameter defined similarly.

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The best place to look is http://wiki.python.org/moin/HowTo/Sorting

Personally, the class __cmp__ function is much handier when dealing with classes, since typically you always want to sort them the same way.

Here's some quick examples:

class Foo :
    def __init__(self, x, y) :
        self.x = x
        self.y = y

    def __cmp__(self, x) :
        return cmp(self.x, x)

    def __repr__(self) :
        return "Foo(%d)" % self.x

# Simple list of objects
data = [
    Foo(1, 99),
    Foo(5, 94),
    Foo(6, 93),
    Foo(2, 97),
    Foo(4, 95),
    Foo(3, 96),
]

# sort using the __cmp__ class method - in numeric order
print sorted(data)

# sort using the key lambda, which reverse sorts... 
print sorted(data, key=lambda a : a.y)