Trouble with variable "leakage"?

[Jeremy N]

I am working with Python in Maya, and have run into a problem with a
variable changing its contents without being scripted to do so. The
various print() statements below were from my efforts to track down
where it was occurring. I left them in so that anyone running this
will more easily see what's happening.

On the line that reads  'dx = d1 / dx ; print("dx = %f" % dx)'  there
is something happening to the variable that is being printed
repeatedly between the lines. The print statements prior to this
particular line print  '...xlist[0][1] = 0.5'  . However, on this
line, that variable is being updated to reflect a new value, when no
assignment to that variable has been made at that time.

This leads me to believe that the variables  'dx'  and  'xlist[0][1]'
are inexplicably linked. I have no idea why. Please help me.

a=[5,0,3,4]
b=[8,3,0,10]
c=[2,4,10,0]

nlist = [a,b,c]
xlist = [[],[],[]]

for i in range(len(nlist)) :
	relist = list(nlist)
	relist.pop(i)
	dlist = list(nlist[i])
	dlist.pop(0) ; dlist.pop(i)
	for j in range(len(relist)) :
		d1 = float(nlist[i][0])
		d2 = float(relist[j][0])
		dx = float(dlist[j])
		r1 = 1 - ( abs(d1-dx) / float(d2) )
		if r1 == 0.0 :
			r1 += (d1 < d2)
		xlist[i].append(float(r1))

del d1, d2, dx, relist, dlist

ylist = list(xlist)
print(xlist)
print(ylist)

for i in range(len(xlist)) :
	relist = list(xlist)
	relist.pop(i)
	for j in range(len(relist)) :
		print( "!!!!!!!!!!!!!!! NEW LOOP AT ( %d:%d ) !!!!!!!!!!!!!!!" %
( i, j ) )
		print("%s / (%s + %s)" % ( str(xlist[i][j]), str(xlist[i][j]),
str(relist[j][( (i!=0) * ((j>=i)+(i-1)) )]) ) )
		d1 = float(xlist[i][j]) ; print("d1 = %f" % d1)
		print( "...xlist[0][1] = %s" % str(xlist[0][1]) )
		d2 = relist[j][( (i!=0) * ((j>=i)+(i-1)) )] ; print("d2 = %f" % d2)
		print( "...xlist[0][1] = %s" % str(xlist[0][1]) )
		dx = d1 + d2 ; print("dx = %f" % dx)
		print( "...xlist[0][1] = %s" % str(xlist[0][1]) )
		dx = d1 / dx ; print("dx = %f" % dx)
		ylist[i][j] = float(dx) ; #print(ylist[i][j])
		print( "...xlist[0][1] = %s" % str(xlist[0][1]) )
		print( "||| xlist[2][0] = %s" % str(xlist[2][0]) )
		print( "...\nxlist = %s\n..." % str(xlist) )

print(xlist)
print(ylist)