formatting float value

# question: http://pastebin.com/QwrjQn9n

import numpy as np

def exponential_approx(x, acc, digits=15):
    approx = 0
    real = np.math.exp(x)
    diff = 1
    ind = 0

    while diff > acc:
        term = (x**ind)/(np.math.factorial(ind))
        approx += term
        diff = (real - approx)
        ind += 1

    #return '%.15f' % approx
    return '%.*f' % (digits, approx)

print(exponential_approx(0.1, 1e-3))      # default 15 digits
print(exponential_approx(0.1, 1e-3, 3))
print(exponential_approx(0.1, 1e-3, 10))
print(exponential_approx(0.1, 1e-3, 15))
print(exponential_approx(0.1, 1e-3, 20))

'''
1.105000000000000
1.105
1.1050000000
1.105000000000000
1.10499999999999998224
'''