A Python app that converts amount on bank cheque to text format

cheque_amount_reader.py

"""Reads the amount on a cheque as a string."""

num_sing = ['', 'one ', 'two ', 'three ', 'four ', 'five ', 'six ', 'seven ',
            'eight ', 'nine ', 'ten ', 'eleven ', 'twelve ', 'thirteen ',
            'fourteen ', 'fifteen ', 'sixteen ', 'seventeen ', 'eighteen ',
            'nineteen ']
num_tens = ['', 'twenty ', 'thirty ', 'forty ', 'fifty ', 'sixty ', 'seventy ',
            'eighty ', 'ninety ']


def get_prefix(amount):
    prefix = None
    appr_amnt = int(amount)
    if amount < 20:
        prefix = num_sing[appr_amnt]
    elif amount < 100:
        sing_conv = int((appr_amnt - appr_amnt % 10) / 10)
        prefix = num_tens[sing_conv - 1] + num_sing[appr_amnt % 10]
    else:
        sing_conv = int((appr_amnt - appr_amnt % 100) / 100)
        sing_conv_alt = int((appr_amnt % 100 - appr_amnt % 10) / 10)
        prefix = num_sing[sing_conv] + 'hundred ' + \
            num_tens[sing_conv_alt - 1] + num_sing[appr_amnt % 10]
    return prefix


def get_suffix(amount):
    suffix = None
    if amount < 1000:
        suffix = ''
    elif amount < 1000000:
        suffix = 'thousand '
    elif amount < 1000000000:
        suffix = 'million '
    elif amount < 1000000000000:
        suffix = 'billion '
    elif amount < 1000000000000000:
        suffix = 'trillion '
    elif amount < 1000000000000000000:
        suffix = 'quadrillion '
    return suffix


def to_text(amnt_on_check):
    small_curr = None
    major_part = None
    max_power_num = 1
    small_plch = None
    big_curr = amnt_on_check[-3:]
    amnt_money = float(amnt_on_check[:-4])
    if amnt_money < 1:
        major_part = ''
        big_plch = ''
    else:
        sub_amnt = amnt_money
        big_plch = big_curr
        while sub_amnt > 1:
            amnt_replacer = sub_amnt
            while sub_amnt / max_power_num >= 1000:
                max_power_num *= 1000
                amnt_replacer /= 1000
            major_part += get_prefix(amnt_replacer) + get_suffix(max_power_num)
            sub_amnt -= int(amnt_replacer) * max_power_num
            max_power_num = 1
    minor_int = int((amnt_money % 1) * 100 + 0.5)
    if minor_int == 0:
        minor_part = ''
    elif minor_int < 20:
        minor_part = num_sing[int(minor_int)]
        small_plch = small_curr
    else:
        minor_part = num_tens[int((minor_int - minor_int %
                                   10) / 10 - 1)] + num_sing[int(minor_int % 10)]
        small_plch = small_curr
    return major_part + big_plch + ' ' + minor_part + small_plch