使用Python建立一个分数类Rational,使之具有如下功能:能防止分母为0，当分数中不是最简形式时，进行约分以及避免分母为负数。用重载运算符完成分数的加、减、乘、除四则运算和大小的比较运算。

class Rational:
    def __init__(self, numerator=0, denominator=1):
        if denominator == 0:
            raise ValueError("Denominator cannot be zero.")
        elif denominator < 0:
            numerator *= -1
            denominator *= -1
        self.numerator = numerator
        self.denominator = denominator
        self.reduce()

    def reduce(self):
        gcd = self.calc_gcd(self.numerator, self.denominator)
        self.numerator //= gcd
        self.denominator //= gcd

    def calc_gcd(self, a, b):
        if b == 0:
            return a
        else:
            return self.calc_gcd(b, a % b)

    def __add__(self, other):
        new_numerator = self.numerator * other.denominator + other.numerator * self.denominator
        new_denominator = self.denominator * other.denominator
        return Rational(new_numerator, new_denominator)

    def __sub__(self, other):
        new_numerator = self.numerator * other.denominator - other.numerator * self.denominator
        new_denominator = self.denominator * other.denominator
        return Rational(new_numerator, new_denominator)

    def __mul__(self, other):
        new_numerator = self.numerator * other.numerator
        new_denominator = self.denominator * other.denominator
        return Rational(new_numerator, new_denominator)

    def __truediv__(self, other):
        if other.numerator == 0:
            raise ValueError("Cannot divide by zero.")
        new_numerator = self.numerator * other.denominator
        new_denominator = self.denominator * other.numerator
        return Rational(new_numerator, new_denominator)

    def __eq__(self, other):
        return self.numerator == other.numerator and self.denominator == other.denominator

    def __lt__(self, other):
        return self.numerator * other.denominator < other.numerator * self.denominator

    def __le__(self, other):
        return self.numerator * other.denominator <= other.numerator * self.denominator

    def __gt__(self, other):
        return self.numerator * other.denominator > other.numerator * self.denominator

    def __ge__(self, other):
        return self.numerator * other.denominator >= other.numerator * self.denominator

    def __str__(self):
        if self.denominator == 1:
            return str(self.numerator)
        else:
            return f"{self.numerator}/{self.denominator}"

使用示例：

a = Rational(3, 4)
b = Rational(1, 2)
print(a + b)  # 输出 5/4
print(a - b)  # 输出 1/4
print(a * b)  # 输出 3/8
print(a / b)  # 输出 3/2
print(a == b) # 输出 False
print(a > b)  # 输出 True
print(a >= b) # 输出 True
print(a < b)  # 输出 False
print(a <= b) # 输出 False