Python elegant inverse function of int(string,base)

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

Maybe this shouldn't be an answer, but it could be helpful for some: the built-in format function does convert numbers to string in a few bases:

>>> format(255, 'b') # base 2 '11111111' >>> format(255, 'd') # base 10 '255' >>> format(255, 'o') # base 8 '377' >>> format(255, 'x') # base 16 'ff' 

Answer 2

If you use Numpy, there is numpy.base_repr.

You can read the code under numpy/core/numeric.py. Short and elegant

Answer 3

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. :)

Answer 4

The above answers are really nice. It helped me a lot to prototype an algortithm I had to implement in C

I'd like to come up with a little change (I used) to convert decimal to a base of symbolspace

I also ignored negativ values just for shortness and the fact that's mathematical incorrect --> other rules for modular arithmetics --> other math if you use binary, oct or hex --> diff in unsigned & signed values

def str_base(number, base):
   (d,m) = divmod(number,len(base))
   if d > 0:
      return str_base(d,base)+base[m]
   return base[m]

that lead's to following output

>>> str_base(13,'01')
'1101'
>>> str_base(255,'01')
'11111111'
>>> str_base(255,'01234567')
'377'
>>> str_base(255,'0123456789')
'255'
>>> str_base(255,'0123456789abcdef')
'ff'
>>> str_base(1399871903,'_helowrd')
'hello_world'

if you want to padd with the propper zero symbol you can use

symbol_space = 'abcdest'

>>> str_base(734,symbol_space).rjust(0,symbol_space[0])
'catt'
>>> str_base(734,symbol_space).rjust(6,symbol_space[0])
'aacatt'