CODEWITHSUNDEEP
python
I I
I I

Reputation: 41

copy function in class definition in python

Im doing a class for polynomial and i have a problem with a copy function. It suppose to create a copy of the Poly object and return a reference to the new Poly object. Im really stuck on this copy idea. Thanks for any help

class Poly:

    def __init__ (self, p):
        self.a = p
        self.deg= len(p) -1
        if len(p) == 1 and p[0] == 0:
            self.deg = -1

    def evalPoly(self,x0):
        ''' evaluates the polynomial at value x'''
        b=0
        for coefficients in reversed(self.a):
            b=b*x0+int(coefficients)
        return b

    def polyPrime(self):
        '''replaces the coeffiecients of self with the coefficients           
        of the derivative polynomial '''
        if self.deg == 0:
            return np.zeroes(1,float), 0
        else:
            newdeg=self.deg-1
            p=[i*self.a[i] for i in range(1,self.deg+1)]
            p=str(p)[1: -1]
            p=eval(p)
        return p

    def copy(self):
        return Poly(self.a)

I'm stuck on how to create a copy of the Poly object and return a reference to the new Poly object

Upvotes: 0

Views: 5334

Answers (4)

BenH
BenH

Reputation: 894

Can you please elaborate why it doesn't work? This works perfectly for me:

class Poly(object):

    def __init__ (self, p):
        self.a = p
        self.deg= len(p) -1
        if len(p) == 1 and p[0] == 0:
            self.deg = -1

    def evalPoly(self,x0):
        ''' evaluates the polynomial at value x'''
        b=0
        for coefficients in reversed(self.a):
            b=b*x0+int(coefficients)
        return b

    def polyPrime(self):
        '''replaces the coeffiecients of self with the coefficients           
        of the derivative polynomial '''
        if self.deg == 0:
            return np.zeroes(1,float), 0
        else:
            newdeg=self.deg-1
            p=[i*self.a[i] for i in range(1,self.deg+1)]
            p=str(p)[1: -1]
            p=eval(p)
        return p

    def __str__(self):
        return "%s %s" % (self.a, self.deg)

    def copy(self):
        return Poly(self.a)

if __name__ == "__main__":
    p = Poly((1,3))
    print p
    x = p.copy()
    print x

Edit: Okay I see now he was passing in a list which was mutable is the general consensus.

Upvotes: 1

Matus
Matus

Reputation: 613

Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other.

Check out copy module:

http://docs.python.org/library/copy.html

Upvotes: 2

Santiago Alessandri
Santiago Alessandri

Reputation: 6855

I think the problem you are having is that as self.a is a list then you are passing a reference to that list in the instantiation of the new Poly object.

You should copy the list and give that copy to instantiate the object:

import copy

class Poly:
    ...
    def copy(self):
        return Poly(copy.copy(self.a))

Upvotes: 5

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799082

The problem is actually hiding up in __init__().

    self.a = p[:]

Upvotes: 2

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