Python multiple intersection

Question

I reimplemented the set in python but i have some problem with multiple intersection.... I followed the book Learning Python but i have problem with my code

class Set:
    def __init__(self,value=[]):
        self.data = []
        self.remDupli(value)

    def remDupli(self,val):
        for i in val:
            if i not in self.data:
                self.data.append(i)

    def intersect(self,other):
        val=[]
        for i in self.data:
            for k in other:
                if i == k:
                    val.append(i)
        return Set(val)

    def union(self,other):
        val=self.data
        for i in other:
            if i not in self.data:
                val.append(i)
        return Set(val)

    def __or__(self,a):       return self.union(a)
    def __and__(self,a):      return self.intersect(a)
    def __len__(self):        return len(self.data)
    def __getitem__(self,key):  return self.data[key]
    def __repr__(self):       return 'Set: ' +repr(self.data)

class Extend(Set):
    def intersect(self, *others):
        val = []
        for i in self:
            for x in others:
                if i in x:
                    val.append(i)
        return Set(val)

but when I run this:

x = Extend([1,2,3,4])
y = Extend([3,4,5])
z = Extend([0,1,2])
print(x & y & z)
print(x.intersect(y, z))

I have two different behavior

Set: []
Set: [1, 2, 3, 4]

I don't understand because the second is different, in my opinion they should have the same behavior, anyone can help me?

x.intersect(y.intersect(z)) will have the same behaviour as x & y & z — Christian Witts
– Christian Witts, Commented May 29, 2012 at 10:35

hamstergene · Accepted Answer · 2012-05-29 10:42:07Z

6

Extend.intersect does not calculate intersection between many sets. It calculates intersection between self and union of others.

The results are different because x & y & z calls Extend.intersect(Extend.intersect(x,y), z), while x.intersect(y,z) calls Extend.intersect(x, *[y,z]) and given what Extend.intersect actually does, those happen to be different operations.

edited May 29, 2012 at 10:42

answered May 29, 2012 at 10:33

hamstergene

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4 Comments

fege Over a year ago

Can I change my code in order to have the same behavior? or do I have only to change way to call in this way x.intersect(y.intersect(z))?

hamstergene Over a year ago

@fege Depends, but I feel that your real problem is that you got the algorithm wrong, so you have to fix it, not the calling code.

John La Rooy Over a year ago

if all(i in x for x in others): val.append(i)

Jeannot Over a year ago

I think the Extend.intersect should use the base implementation defined in the Set class. BTW, I would expect Extend.intersect to return an Extend object.

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