A Number class

number.py

#!/usr/bin/env python3
# _*_ coding:utf-8 _*_

import re


def str_to_int(s: str) -> int:
    if not s:
        return 0
    i = ord(s[0]) - ord("0")
    if len(s) > 1:
        for c in s[1:]:
            i = i * 10 + (ord(c) - ord("0"))
    return i


class Number:
    REGEX = re.compile(
        r"(?P<sign>[\+-])?"
        r"(?P<integer>\d+)"
        r"\.?"
        r"(?P<decimal>\d+)?"
        r"[eE]?"
        r"(?P<expsign>[\+-])?"
        r"(?P<exponent>\d+)?"
    )

    def __init__(self, s: str):
        self._str_repr = s
        matched = self.REGEX.match(s)
        if not matched:
            raise ValueError(f"expected a number string, got {s}")
        self.sign = matched.group("sign") == "-" and -1 or 1
        self.integer_part = matched.group("integer")
        self.decimal_part = matched.group("decimal") or "0"
        self.exp_sign = matched.group("expsign") == "-" and -1 or 1
        self.exponent = str_to_int(matched.group("exponent") or "0")

    def __str__(self) -> str:
        return self._str_repr

    def __repr__(self) -> str:
        return self._str_repr

    def as_int(self) -> int:
        if self.exponent == 0:
            i = str_to_int(self.integer_part) * self.sign
            if str_to_int(self.decimal_part[0]) >= 5:
                return i + 1
            return i
        if self.exp_sign == 1:
            tail_zeroes = self.exponent - len(self.decimal_part)
            if tail_zeroes < 0:
                abs_val = str_to_int(
                    self.integer_part + self.decimal_part[: self.exponent]
                )
            else:
                abs_val = str_to_int(
                    self.integer_part + self.decimal_part.ljust(self.exponent, "0")
                )
            return abs_val * self.sign
        if self.exponent - len(self.integer_part) < 0:
            i = str_to_int(self.integer_part[: self.exponent])
            if str_to_int(self.integer_part[self.exponent]) >= 5:
                i += 1
            return i * self.sign
        return 0


if __name__ == "__main__":
    while True:
        user_input = input(
            "Please enter a number, supports these formats"
            ": 123|1.234|1.2e34\n"
        )
        if user_input.lower() in ["q", "quit", "exit"]:
            break
        print(Number(user_input).as_int())