Python - one variable equals another variable when it shouldn't

i=1
while (i>0):
    x_past = x_present

    j=0
    while(j<3):
        value=0
        k=0
        while(k<3):
            if(k!=j):
                if(i==1):
                    if(k>j):
                        value=value+0
                    else:
                        value=value+x_present[k]*eqn[j][k]
                else:
                    value=value+x_present[k]*eqn[j][k]
            else:
                value=value+eqn[j][k]
            k=k+1
        x_present[j:j+1]=[value]
        j=j+1
    print "X_PAST"
    print x_past
    print "X_PRESENT"
    print x_present
    if(error(x_past, x_present)<10**-2):
        break;
    i=i+1

x_present[j:j+1]=[value]

x_past=x_present

>>> x_present = [4,5,6]
>>>
>>> x_past = x_present
>>>
>>> x_past
[4, 5, 6]
>>>
>>> x_present.append(7)
>>>
>>> x_past
[4, 5, 6, 7]
>>>

>>> x_past = x_present[:]
>>> x_past
[4, 5, 6, 7]
>>>
>>> x_present.append(8)
>>>
>>> x_past
[4, 5, 6, 7]

>>> import copy
>>> a = range(10)
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> b = a
>>> a += 10, 11
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
>>> b
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
>>> c = copy.copy(a) # shallow copy
>>> c
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
>>> del a[3:]
>>> a
[0, 1, 2]
>>> b
[0, 1, 2]
>>> c
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

import copy
# assert(len(x_present) >= len(eqn))

first = True
while True:
    x_past = copy.copy(x_present) # copy

    for j, eqj in enumerate(eqn):
        x_present[j] = sum(x_present[k] * eqj[k]
                           for k in range(j if first else len(eqj))
                           if k != j)
        x_present[j] += eqj[j]

    print "X_PASTn%snX_PRESENTn%s" % (x_past, x_present)
    if allequal(x_past, x_present, tolerance=10**-2):
        break
    first = False

def allequal(x, y, tolerance):
    return (len(x) == len(y) and
            all(-tolerance < (xx - yy) < tolerance
                for xx, yy in zip(x, y)))