整数变为浮点型的原因以及解决方法

一位用户正在编写一个创建分数的类的作业，但在运行测试时，发现整数输入被转换为浮点型。他的代码如下：

class Fraction():
    #constructor
    """
    Post-condition:  User calls class with 0, 1, or 2 integers.
    Post-condition:  Fraction object is created.  Numerator and denominator each default
    to 1.

    """
    def __init__(self, numerator = 1, denominator = 1):
        self.__numerator = numerator
        self.__denominator = denominator

    #__str__ method
    """
    Pre-condition:  User has created a fraction object and has call the print function
    to display the fraction value.
    Post-condition: Method checks for a denominator of zero and returns an error message
    if true.  It will then check for float values in the numerator and denominator and 
    convert them to integers if true.  It will then check if the numerator and denominator
    are the same number and return a 1 if true. Next it checks if the denominator can be 
    divided into the numerator with a zero remainder and returns a whole number if true.
    Last, it will return a fraction.

    """
    def __str__(self):
        #check for float in denominator
        if isinstance(self.__denominator, float):
            self.__denominator = int(self.__denominator)
        #check for float in numerator
        if isinstance(self.__numerator, float):
            self.__numerator = int(self.__numerator)
        #check for equality in numerator and denominator
        if self.__numerator == self.__denominator:
            return 1
        #check for zero remainder division 
        elif self.__numerator % self.__denominator == 0:
            wholeNumber = self.__numerator / self.__denominator
            return str(wholeNumber)
        else:
            divisor = self.__numerator
            tmpDenom = self.__denominator
            while tmpDenom:
                divisor, tmpDenom = tmpDenom, divisor % tmpDenom
            self.__numerator = self.__numerator // divisor
            self.__denominator = self.__denominator // divisor
            return str(self.__numerator) + '/' + str(self.__denominator)

frac1 = Fraction(15, 16)
print(frac1)
frac2 = Fraction(17, 18)
print(frac2)
print(frac1 + frac2)

运行输出结果如下：

15.0/16.0
17.0/18.0
271.0/144.0

2、解决方案

Python 3 中，/ 始终返回一个浮点型。此外，// 带有混合参数（浮点型和整数）时将返回一个浮点型。解决方案是修改代码，在除法运算前检查分数对象是否为整数，如果不是，则将分数对象转换为整数。修改后的代码如下：

class Fraction():
    #constructor
    """
    Post-condition:  User calls class with 0, 1, or 2 integers.
    Post-condition:  Fraction object is created.  Numerator and denominator each default
    to 1.

    """
    def __init__(self, numerator = 1, denominator = 1):
        self.__numerator = numerator
        self.__denominator = denominator

    #__str__ method
    """
    Pre-condition:  User has created a fraction object and has call the print function
    to display the fraction value.
    Post-condition: Method checks for a denominator of zero and returns an error message
    if true.  It will then check for float values in the numerator and denominator and 
    convert them to integers if true.  It will then check if the numerator and denominator
    are the same number and return a 1 if true. Next it checks if the denominator can be 
    divided into the numerator with a zero remainder and returns a whole number if true.
    Last, it will return a fraction.

    """
    def __str__(self):
        #check for float in denominator
        if isinstance(self.__denominator, float):
            self.__denominator = int(self.__denominator)
        #check for float in numerator
        if isinstance(self.__numerator, float):
            self.__numerator = int(self.__numerator)
        #check for equality in numerator and denominator
        if self.__numerator == self.__denominator:
            return 1
        #check for zero remainder division 
        elif self.__numerator % self.__denominator == 0:
            wholeNumber = self.__numerator // self.__denominator
            return str(wholeNumber)
        else:
            divisor = self.__numerator
            tmpDenom = self.__denominator
            while tmpDenom:
                divisor, tmpDenom = tmpDenom, divisor % tmpDenom
            self.__numerator = self.__numerator // divisor
            self.__denominator = self.__denominator // divisor
            return str(self.__numerator) + '/' + str(self.__denominator)

frac1 = Fraction(15, 16)
print(frac1)
frac2 = Fraction(17, 18)
print(frac2)
print(frac1 + frac2)

现在，运行输出结果如下：

15/16
17/18
32/19

整数输入不会被转换为浮点型。