How to convert an integer to a string in any base?

If you need compatibility with ancient versions of Python, you can either use gmpy (which does include a fast, completely general int-to-string conversion function, and can be built for such ancient versions – you may need to try older releases since the recent ones have not been tested for venerable Python and GMP releases, only somewhat recent ones), or, for less speed but more convenience, use Python code – e.g., for Python 2, most simply:

import string
digs = string.digits + string.ascii_letters


def int2base(x, base):
    if x < 0:
        sign = -1
    elif x == 0:
        return digs[0]
    else:
        sign = 1

    x *= sign
    digits = []

    while x:
        digits.append(digs[int(x % base)])
        x = int(x / base)

    if sign < 0:
        digits.append('-')

    digits.reverse()

    return ''.join(digits)

For Python 3, int(x / base) leads to incorrect results, and must be changed to x // base:

import string
digs = string.digits + string.ascii_letters


def int2base(x, base):
    if x < 0:
        sign = -1
    elif x == 0:
        return digs[0]
    else:
        sign = 1

    x *= sign
    digits = []

    while x:
        digits.append(digs[x % base])
        x = x // base

    if sign < 0:
        digits.append('-')

    digits.reverse()

    return ''.join(digits)

def baseN(num,b,numerals="0123456789abcdefghijklmnopqrstuvwxyz"):
    return ((num == 0) and numerals[0]) or (baseN(num // b, b, numerals).lstrip(numerals[0]) + numerals[num % b])

ref: http://code.activestate.com/recipes/65212/

Please be aware that this may lead to

RuntimeError: maximum recursion depth exceeded in cmp

for very big integers.

>>> numpy.base_repr(10, base=3)
'101'

Note that numpy.base_repr() has a limit of 36 as its base. Otherwise it throws a ValueError

Recursive

I would simplify the most voted answer to:

BS="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
def to_base(n, b): 
    return "0" if not n else to_base(n//b, b).lstrip("0") + BS[n%b]

With the same advice for RuntimeError: maximum recursion depth exceeded in cmp on very large integers and negative numbers. (You could usesys.setrecursionlimit(new_limit))

Iterative

To avoid recursion problems:

BS="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
def to_base(s, b):
    res = ""
    while s:
        res+=BS[s%b]
        s//= b
    return res[::-1] or "0"

Great answers! I guess the answer to my question was "no" I was not missing some obvious solution. Here is the function I will use that condenses the good ideas expressed in the answers.

• allow caller-supplied mapping of characters (allows base64 encode)
• checks for negative and zero
• maps complex numbers into tuples of strings

def int2base(x,b,alphabet='0123456789abcdefghijklmnopqrstuvwxyz'):
    'convert an integer to its string representation in a given base'
    if b<2 or b>len(alphabet):
        if b==64: # assume base64 rather than raise error
            alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
        else:
            raise AssertionError("int2base base out of range")
    if isinstance(x,complex): # return a tuple
        return ( int2base(x.real,b,alphabet) , int2base(x.imag,b,alphabet) )
    if x<=0:
        if x==0:
            return alphabet[0]
        else:
            return  '-' + int2base(-x,b,alphabet)
    # else x is non-negative real
    rets=''
    while x>0:
        x,idx = divmod(x,b)
        rets = alphabet[idx] + rets
    return rets

http://code.activestate.com/recipes/65212/

def base10toN(num,n):
    """Change a  to a base-n number.
    Up to base-36 is supported without special notation."""
    num_rep={10:'a',
         11:'b',
         12:'c',
         13:'d',
         14:'e',
         15:'f',
         16:'g',
         17:'h',
         18:'i',
         19:'j',
         20:'k',
         21:'l',
         22:'m',
         23:'n',
         24:'o',
         25:'p',
         26:'q',
         27:'r',
         28:'s',
         29:'t',
         30:'u',
         31:'v',
         32:'w',
         33:'x',
         34:'y',
         35:'z'}
    new_num_string=''
    current=num
    while current!=0:
        remainder=current%n
        if 36>remainder>9:
            remainder_string=num_rep[remainder]
        elif remainder>=36:
            remainder_string='('+str(remainder)+')'
        else:
            remainder_string=str(remainder)
        new_num_string=remainder_string+new_num_string
        current=current/n
    return new_num_string

Here's another one from the same link

def baseconvert(n, base):
    """convert positive decimal integer n to equivalent in another base (2-36)"""

    digits = "0123456789abcdefghijklmnopqrstuvwxyz"

    try:
        n = int(n)
        base = int(base)
    except:
        return ""

    if n < 0 or base < 2 or base > 36:
        return ""

    s = ""
    while 1:
        r = n % base
        s = digits[r] + s
        n = n / base
        if n == 0:
            break

    return s

def base_conversion(num, base):
    digits = []
    while num > 0:
        num, remainder = divmod(num, base)
        digits.append(remainder)
    return digits[::-1]

>>> import string
>>> def int2base(integer, base):
        if not integer: return '0'
        sign = 1 if integer > 0 else -1
        alphanum = string.digits + string.ascii_lowercase
        nums = alphanum[:base]
        res = ''
        integer *= sign
        while integer:
                integer, mod = divmod(integer, base)
                res += nums[mod]
        return ('' if sign == 1 else '-') + res[::-1]


>>> int2base(-15645, 23)
'-16d5'
>>> int2base(213, 21)
'a3'

def base_changer(number,base):
    buff=97+abs(base-10)
    dic={};buff2='';buff3=10
    for i in range(97,buff+1):
        dic[buff3]=chr(i)
        buff3+=1   
    while(number>=base):
        mod=int(number%base)
        number=int(number//base)
        if (mod) in dic.keys():
            buff2+=dic[mod]
            continue
        buff2+=str(mod)
    if (number) in dic.keys():
        buff2+=dic[number]
    else:
        buff2+=str(number)

    return buff2[::-1]   

Here is an example of how to convert a number of any base to another base.

from collections import namedtuple

Test = namedtuple("Test", ["n", "from_base", "to_base", "expected"])


def convert(n: int, from_base: int, to_base: int) -> int:
    digits = []
    while n:
        (n, r) = divmod(n, to_base)
        digits.append(r)    
    return sum(from_base ** i * v for i, v in enumerate(digits))


if __name__ == "__main__":
    tests = [
        Test(32, 16, 10, 50),
        Test(32, 20, 10, 62),
        Test(1010, 2, 10, 10),
        Test(8, 10, 8, 10),
        Test(150, 100, 1000, 150),
        Test(1500, 100, 10, 1050000),
    ]

    for test in tests:
        result = convert(*test[:-1])
        assert result == test.expected, f"{test=}, {result=}"
    print("PASSED!!!")

Say we want to convert 14 to base 2. We repeatedly apply the division algorithm until the quotient is 0:

14 = 2 x 7

7 = 2 x 3 + 1

3 = 2 x 1 + 1

1 = 2 x 0 + 1

The binary representation is just the remainder read from bottom to top. This can be proved by expanding

14 = 2 x 7 = 2 x (2 x 3 + 1) = 2 x (2 x (2 x 1 + 1) + 1) = 2 x (2 x (2 x (2 x 0 + 1) + 1) + 1) = 2^3 + 2^2 + 2

The code is the implementation of the above algorithm.

def toBaseX(n, X):
strbin = ""
while n != 0:
    strbin += str(n % X)
    n = n // X
return strbin[::-1]

This is my approach. At first converting the number then casting it to string.

    def to_base(n, base):
        if base == 10:
            return n
        
        result = 0
        counter = 0
        
        while n:
            r = n % base
            n //= base
            result += r * 10**counter
            counter+=1
        return str(result)

I have written this function which I use to encode in different bases. I also provided the way to shift the result by a value 'offset'. This is useful if you'd like to encode to bases above 64, but keeping displayable chars (like a base 95).

I also tried to avoid reversing the output 'list' and tried to minimize computing operations. The array of pow(base) is computed on demand and kept for additional calls to the function.

The output is a binary string

pows = {}

######################################################
def encode_base(value,
                base = 10,
                offset = 0) :

    """
    Encode value into a binary string, according to the desired base.

    Input :
        value : Any positive integer value
        offset : Shift the encoding (eg : Starting at chr(32))
        base : The base in which we'd like to encode the value

    Return : Binary string

    Example : with : offset = 32, base = 64

              100 -> !D
              200 -> #(
    """

    # Determine the number of loops
    try :
        pb = pows[base]

    except KeyError :
        pb = pows[base] = {n : base ** n for n in range(0, 8) if n < 2 ** 48 -1}

    for n in pb :
        if value < pb[n] :
            n -= 1
            break

    out = []
    while n + 1 :
        b = pb[n]
        out.append(chr(offset + value // b))
        n -= 1
        value %= b

    return ''.join(out).encode()

This function converts any integer from any base to any base

def baseconvert(number, srcbase, destbase):
    if srcbase != 10:
        sum = 0
        for _ in range(len(str(number))):
            sum += int(str(number)[_]) * pow(srcbase, len(str(number)) - _ - 1)
        b10 = sum
        return baseconvert(b10, 10, destbase)
    end = ''
    q = number
    while(True):
        r = q % destbase
        q = q // destbase
        end = str(r) + end
        if(q<destbase):
            end = str(q) + end
            return int(end)

The below provided Python code converts a Python integer to a string in arbitrary base ( from 2 up to infinity ) and works in both directions. So all the created strings can be converted back to Python integers by providing a string for N instead of an integer. The code works only on positive numbers by intention (there is in my eyes some hassle about negative values and their bit representations I don't want to dig into). Just pick from this code what you need, want or like, or just have fun learning about available options. Much is there only for the purpose of documenting all the various available approaches ( e.g. the Oneliner seems not to be fast, even if promised to be ).

I like the by Salvador Dali proposed format for infinite large bases. A nice proposal which works optically well even for simple binary bit representations. Notice that the width=x padding parameter in case of infiniteBase=True formatted string applies to the digits and not to the whole number. It seems, that code handling infiniteBase digits format runs even a bit faster than the other options - another reason for using it?

I don't like the idea of using Unicode for extending the number of symbols available for digits, so don't look in the code below for it, because it's not there. Use the proposed infiniteBase format instead or store integers as bytes for compression purposes.

    def inumToStr( N, base=2, width=1, infiniteBase=False,\
    useNumpy=False, useRecursion=False, useOneliner=False, \
    useGmpy=False, verbose=True):
    ''' Positive numbers only, but works in BOTH directions.
    For strings in infiniteBase notation set for bases <= 62 
    infiniteBase=True . Examples of use:
    inumToStr( 17,  2, 1, 1)             # [1,0,0,0,1]
    inumToStr( 17,  3, 5)                #       00122
    inumToStr(245, 16, 4)                #        00F5
    inumToStr(245, 36, 4,0,1)            #        006T
    inumToStr(245245245245,36,10,0,1)    #  0034NWOQBH
    inumToStr(245245245245,62)           #     4JhA3Th 
        245245245245 == int(gmpy2.mpz('4JhA3Th',62))
    inumToStr(245245245245,99,2) # [25,78, 5,23,70,44]
    ----------------------------------------------------
    inumToStr( '[1,0,0,0,1]',2, infiniteBase=True ) # 17 
    inumToStr( '[25,78, 5,23,70,44]', 99) # 245245245245
    inumToStr( '0034NWOQBH', 36 )         # 245245245245 
    inumToStr( '4JhA3Th'   , 62 )         # 245245245245
    ----------------------------------------------------
    --- Timings for N = 2**4096, base=36: 
                                      standard: 0.0023
                                      infinite: 0.0017
                                      numpy   : 0.1277
                                      recursio; 0.0022
                                      oneliner: 0.0146
                For N = 2**8192: 
                                      standard: 0.0075
                                      infinite: 0.0053
                                      numpy   : 0.1369
    max. recursion depth exceeded:    recursio/oneliner
    '''
    show = print
    if type(N) is str and ( infiniteBase is True or base > 62 ):
        lstN = eval(N)
        if verbose: show(' converting a non-standard infiniteBase bits string to Python integer')
        return sum( [ item*base**pow for pow, item in enumerate(lstN[::-1]) ] )
    if type(N) is str and base <= 36:
        if verbose: show('base <= 36. Returning Python int(N, base)')
        return int(N, base)
    if type(N) is str and base <= 62:
        if useGmpy: 
            if verbose: show(' base <= 62, useGmpy=True, returning int(gmpy2.mpz(N,base))')
            return int(gmpy2.mpz(N,base))
        else:
            if verbose: show(' base <= 62, useGmpy=False, self-calculating return value)')
            lstStrOfDigits="0123456789"+ \
                "abcdefghijklmnopqrstuvwxyz".upper() + \
                "abcdefghijklmnopqrstuvwxyz"
            dictCharToPow = {}
            for index, char in enumerate(lstStrOfDigits):
                dictCharToPow.update({char : index}) 
            return sum( dictCharToPow[item]*base**pow for pow, item in enumerate(N[::-1]) )
        #:if
    #:if        
        
    if useOneliner and base <= 36:  
        if verbose: show(' base <= 36, useOneliner=True, running the Oneliner code')
        d="0123456789abcdefghijklmnopqrstuvwxyz"
        baseit = lambda a=N, b=base: (not a) and d[0]  or \
        baseit(a-a%b,b*base)+d[a%b%(base-1) or (a%b) and (base-1)]
        return baseit().rjust(width, d[0])[1:]

    if useRecursion and base <= 36: 
        if verbose: show(' base <= 36, useRecursion=True, running recursion algorythm')
        BS="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        def to_base(n, b): 
            return "0" if not n else to_base(n//b, b).lstrip("0") + BS[n%b]
        return to_base(N, base).rjust(width,BS[0])
        
    if base > 62 or infiniteBase:
        if verbose: show(' base > 62 or infiniteBase=True, returning a non-standard digits string')
        # Allows arbitrary large base with 'width=...' 
        # applied to each digit (useful also for bits )
        N, digit = divmod(N, base)
        strN = str(digit).rjust(width, ' ')+']'
        while N:
            N, digit = divmod(N, base)
            strN = str(digit).rjust(width, ' ') + ',' + strN
        return '[' + strN
    #:if        
    
    if base == 2:
        if verbose: show(" base = 2, returning Python str(f'{N:0{width}b}')")
        return str(f'{N:0{width}b}')
    if base == 8:
        if verbose: show(" base = 8, returning Python str(f'{N:0{width}o}')")
        return str(f'{N:0{width}o}')
    if base == 16:
        if verbose: show(" base = 16, returning Python str(f'{N:0{width}X}')")
        return str(f'{N:0{width}X}')

    if base <= 36:
        if useNumpy: 
            if verbose: show(" base <= 36, useNumpy=True, returning np.base_repr(N, base)")
            import numpy as np
            strN = np.base_repr(N, base)
            return strN.rjust(width, '0') 
        else:
            if verbose: show(' base <= 36, useNumpy=False, self-calculating return value)')
            lstStrOfDigits="0123456789"+"abcdefghijklmnopqrstuvwxyz".upper()
            strN = lstStrOfDigits[N % base] # rightmost digit
            while N >= base:
                N //= base # consume already converted digit
                strN = lstStrOfDigits[N % base] + strN # add digits to the left
            #:while
            return strN.rjust(width, lstStrOfDigits[0])
        #:if
    #:if
    
    if base <= 62:
        if useGmpy: 
            if verbose: show(" base <= 62, useGmpy=True, returning gmpy2.digits(N, base)")
            import gmpy2
            strN = gmpy2.digits(N, base)
            return strN.rjust(width, '0') 
            # back to Python int from gmpy2.mpz with 
            #     int(gmpy2.mpz('4JhA3Th',62))
        else:
            if verbose: show(' base <= 62, useGmpy=False, self-calculating return value)')
            lstStrOfDigits= "0123456789" + \
                "abcdefghijklmnopqrstuvwxyz".upper() + \
                "abcdefghijklmnopqrstuvwxyz"
            strN = lstStrOfDigits[N % base] # rightmost digit
            while N >= base:
                N //= base # consume already converted digit
                strN = lstStrOfDigits[N % base] + strN # add digits to the left
            #:while
            return strN.rjust(width, lstStrOfDigits[0])
        #:if
    #:if    
#:def

I'm presenting a "unoptimized" solution for bases between 2 and 9:

  def to_base(N, base=2):
    N_in_base = ''
    while True:
        N_in_base = str(N % base) + N_in_base
        N //= base
        if N == 0:
            break
    return N_in_base

This solution does not require reversing the final result, but it's actually not optimized. Refer to this answer to see why: https://stackoverflow.com/a/37133870/7896998

Simple base transformation

def int_to_str(x, b):
    s = ""
    while x:
        s = str(x % b) + s
        x //= b
    return s

Example of output with no 0 to base 9

s = ""
x = int(input())
while x:
    if x % 9 == 0:
        s = "9" + s
        x -= x % 10
        x = x // 9
    else:
        s = str(x % 9) + s
        x = x // 9

print(s)

num = input("number")
power = 0
num = int(num)
while num > 10:
    num = num / 10
    power += 1

print(str(round(num, 2)) + "^" + str(power))

Well I personally use this function, written by me

import string

def to_base(value, base, digits=string.digits+string.ascii_letters):    # converts decimal to base n

    digits_slice = digits[0:base]

    temporary_var = value
    data = [temporary_var]

    while True:
        temporary_var = temporary_var // base
        data.append(temporary_var)
        if temporary_var < base:
            break

    result = ''
    for each_data in data:
        result += digits_slice[each_data % base]
    result = result[::-1]

    return result

This is how you can use it

print(to_base(7, base=2))

Output: "111"

print(to_base(23, base=3))

Output: "212"

Please feel free to suggest improvements in my code.

This is an old question but I thought i'd share my take on it as I feel it is somewhat simpler that other answers (good for bases from 2 to 36):

def intStr(n,base=10):
    if n < 0   : return "-" + intStr(-n,base)         # handle negatives
    if n < base: return chr([48,55][n>9] + n)         # 48 => "0"..., 65 => "A"...
    return intStr(n//base,base) + intStr(n%base,base) # recurse for multiple digits

I know this is an old post, but I'm just leaving my solution here just in case.

def decimal_to_given_base(integer_to_convert, base):
     remainder = integer_to_convert // base
     digit = integer_to_convert % base
     if integer_to_convert == 0:
         return '0'
     elif remainder == 0:
         return str(digit)
     else:
         return decimal_to_given_base(remainder, base) + str(digit)

I made my function to do this. Run well on windows 10, python 3.7.3.

def number_to_base(number, base, precision = 10):
    if number == 0:
        return [0]
    
    positive = number >= 0
    number = abs(number)
    
    ints = []  # store the integer bases
    floats = []  # store the floating bases

    float_point = number % 1
    number = int(number)
    while number:
        ints.append(int(number%base))
        number //= base
    ints.reverse()
    
    while float_point and precision:
        precision -= 1
        float_point *= base
        floats.append(int(float_point))
        float_point = float_point - int(float_point)

    return ints, floats, positive


def base_to_str(bases, string="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"):
    """bases is a two dimension list, where bases[0] contains a list of the integers,
    and bases[1] contains a list of the floating numbers, bases[2] is a boolean, that's
    true when it's a positive number
    """
    ints = []
    floats = []

    for i in bases[0]:
        ints.append(string[i])

    for i in bases[1]:
        floats.append(string[i])

    if len(bases[1]) > 0:
        return (["-", ""][bases[2]] + "".join(ints)) + "." + ("".join(floats))
    else:
        return (["-", ""][bases[2]] + "".join(ints))
    

    

Example:

>>> base_to_str(number_to_base(-6.252, 2))
'-110.0100000010'