Why is SageMath generating errors during compilation?

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Whenever compile the following codes, it generates errors. But I don't know why is such errors occurred.

 F.<x> = GF(3^15)
def NP(a):
    return F(a.digits(3))

import random
b=[0,0]
for r in srange(0,2):
    b[r]=random.randint(0,3^15-1)

for i in srange(1,4):
    q=NP(b[0])+NP(b[1])
    print q

Here are the errors:

 File "file.sage.py", line 16, in <module>
    q=NP(b[_sage_const_0 ])+NP(b[_sage_const_1 ])
  File "file.sage.py", line 8, in NP
    return F(a.digits(_sage_const_3 )) #integer 2 polynomial
AttributeError: 'int' object has no attribute 'digits'