Question

Python allows conversions from string to integer using any base in the range [2,36] using:

int(string,base)

I am looking for an elegant inverse function that takes an integer and a base and returns a string.

For example:

>>> str_base(224,15)
'ee'

I came up with the following solution:

def digit_to_char(digit):
    if digit < 10: return chr(ord('0') + digit)
    else: return chr(ord('a') + digit - 10)

def str_base(number,base):
    if number < 0:
        return '-' + str_base(-number,base)
    else:
        (d,m) = divmod(number,base)
        if d:
            return str_base(d,base) + digit_to_char(m)
        else:
            return digit_to_char(m)

Note: digit_to_char() works for bases <= 169 arbitrarily using ASCII characters after z as digits for bases above 36.

Is there a Python built‑in, library function, or a more elegant inverse function of int(string,base)?

Answer 1

This thread has some example implementations.

Actually I think your solution looks rather nice, it’s even recursive which is somehow pleasing here.

I’d still simplify it to remove the else, but that’s probably a personal style thing. I think if foo: return is very clear, and doesn’t need an else after it to make it clear it’s a separate branch.

def digit_to_char(digit):
    if digit < 10:
        return str(digit)
    return chr(ord('a') + digit - 10)

def str_base(number,base):
    if number < 0:
        return '-' + str_base(-number, base)
    (d, m) = divmod(number, base)
    if d > 0:
        return str_base(d, base) + digit_to_char(m)
    return digit_to_char(m)

I simplified the 0-9 case in digit_to_char(), I think str() is clearer than the chr(ord()) construct. To maximize the symmetry with the >= 10 case an ord() could be factored out, but I didn’t bother since it would add a line and brevity felt better. 🙂