list of arbitrary-base digits to int

Question

Given a list of integers representing the digits of a number in base b, how do I convert this list to an int for any b most efficiently?

numlist = [1, 2, 3, 4, 5]

def list2int(numList, b):
    if b == 10: return int(''.join(map(str, numList)))
    else: ?

print list2int(numList, 7)
>>> 3267

I can only think of the naive approach to do this but this scales really horribly.

def list2int(numList, b):
    num = 0
    for i, ii in enumerate(numList): num += ii * b**(len(numList) - i - 1)
    return num

Are there any better ways?

Answer 1

You could use reduce:

In [3]: reduce(lambda x,y:7*x+y, numlist)
Out[3]: 3267

where 7 is the base.

Answer 2

You can pass the base to int(), but this will work only up to base 36:

>>> int(''.join(map(str, numlist)), 7)
3267
>>> int(''.join(map(str, numlist)), 15)
58115
>>> int(''.join(map(str, numlist)), 36)
1776965