python - Error while calculating math.sqrt -

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i faced strange error, , wasted 2 hours on it, couldn't come explanation

      import math      ipd={1:[2,3],2:[1],3:[1],4:[5,6,7],5:[4,6],6:[4,5],7:[4]}     gkeys = [k k in ipd.iterkeys()]     key_map = {}      x in range(len(gkeys)):         key_map[x] = gkeys[x]      val = len(gkeys)     #adj matrix...     w = [[0.0]*val]*val     print w     in range(len(gkeys)):         j in range(len(gkeys)):             #if , j has edge...             if key_map[j] in ipd[key_map[i]]:                 deg1 = len(ipd[key_map[j]])                 deg2 = len(ipd[key_map[i]])                      v = 1.0/math.sqrt(deg1*deg2)                 w[i][j] = v                 print i,j,deg1,deg2,w[i][j], w[j][i]  

here whole problem

for i=3 , j=5 first w[i][j] 0.4248.. i=5, , j=3 w[j][i] giving 0.5, how possible.

thanks!

w = [[0.0]*val]*val 

the above gives val copies of the exact same list. try this:

a = [[0] * 4] * 4 print a[0][0] = 99 print 

output:

[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]] [[99, 0, 0, 0], [99, 0, 0, 0], [99, 0, 0, 0], [99, 0, 0, 0]]