def number_to_string(num):
    '''
      @params
        num: Number
      @return
        string

      If num is a _really_ large number you may expect performance issues
      since there are N iterations to construct the new string.

    '''
    N = str(num)
    if not len(N):
        return ''
    if len(N) <= 3:
        return N

    float_section = N.split('.')
    if len(float_section) > 1:
        N = float_section[0]  # Remove float for now

    negative = num < 0
    if negative:
        N = N[1::]  # Remove negative symbol for now

    number_as_string = ''

    # The first comma will appear in the n-3th position in reverse
    # So we'll find the first comma, then we add an additional index
    # For every 3 characters in N
    commaIndex = range(
        len(N) - 3, 0, -3
    )

    # Then iterate in reverse because we made some assumptions earlier
    for i in reversed(range(len(N))):
        number_as_string = '{}{}'.format(
            '{}{}'.format(
                ',' if i in commaIndex else '', N[i]
            ), number_as_string
        )

    return '{}{}{}'.format(
        '-' if negative else '', number_as_string,
        '.{}'.format(float_section[1]) if len(float_section) > 1 else ''
    )

print (number_to_string(0))
print (number_to_string(1234))
print (number_to_string(12345))
print (number_to_string(123456))
print (number_to_string(1234567))
print (number_to_string(12345678))
print (number_to_string(1234))
print (number_to_string(123))
print (number_to_string(1))
print (number_to_string(123456789))
print (number_to_string(1123456789))

print (number_to_string(-1))
print (number_to_string(-1.2))
print (number_to_string(-1234.2))