ln (Natural Log) in Python

math.log is the natural logarithm:

math.log(x[, base]) With one argument, return the natural logarithm of
x (to base e).

Your equation is therefore:

n = math.log((1 + (FV * r) / p) / math.log(1 + r)))

Note that in your code you convert n to a str twice which is unnecessary

Here is the correct implementation using numpy (np.log() is the natural logarithm)

import numpy as np
p = 100
r = 0.06 / 12
FV = 4000

n = np.log(1 + FV * r/ p) / np.log(1 + r)

print (Number of periods =  + str(n))

Output:

Number of periods = 36.55539635919235