为什么 function 返回无？

Question

Why does the cat1.set_size() function return None instead of "small" and the cat2.color function returns "<__ main __. Tiger object at 0x000002711C6D4DF0>" instead of "white" ?

class Cat:
    def __init__(self, color, cat_type):
        self.size = "undefined"
        self.color = color
        self.cat_type = cat_type

    def set_size(self, cat_type):
        if self.cat_type == "indoor":
            self.size = "small"
        else:
            pass


class Tiger(Cat):
    def __init__(self, color, cat_type):
        super().__init__(color, cat_type)

    def set_size(self, cat_type):
        super().__init__(self, cat_type)
        if self.cat_type == "wild":
            self.size = "big"
        else:
            self.size = "undefined"


cat1 = Cat(color="black", cat_type="indoor")
cat1_size = cat1.set_size("indoor")
print(cat1.color, cat1.cat_type, cat1_size)

cat2 = Tiger(color="white", cat_type="wild")
cat2.set_size("wild")
print(cat2.color, cat2.cat_type, cat2.size)

Conclusion:

black indoor None
<__main__.Tiger object at 0x000002711C6D4DF0> wild big

Answer 1

It never prints something because you never return anything from the method. You need to return whatever you need at the end of the method.

return self.size

At the end of set_size method.

Answer 2

Here's a better organization.

class Cat:
    def __init__(self, color, cat_type):
        self.size = "undefined"
        self.color = color
        self.cat_type = cat_type

    def set_size(self):
        if self.cat_type == "indoor":
            self.size = "small"

class Tiger(Cat):
    def set_size(self):
        if self.cat_type == "wild":
            self.size = "big"

cat1 = Cat(color="black", cat_type="indoor")
cat1.set_size()
print(cat1.color, cat1.cat_type, cat1.size)

cat2 = Tiger(color="white", cat_type="wild")
cat2.set_size()
print(cat2.color, cat2.cat_type, cat2.size)

This still has problems, though. The size of the cat should not be determined by "wild" or "indoor". A Tiger , as a specific kind of Cat , should always have its size set to "big". You might have another subclass called Domestic that has its size set to "small".

Answer 3

There are two problems here:

1. You are not returning anything from set_size

2. You are rerunning the __init__ function in Tiger 's set_size

Here is the corrected code:

class Cat:
    def __init__(self, color, cat_type):
        self.size = "undefined"
        self.color = color
        self.cat_type = cat_type

    def set_size(self):
        if self.cat_type == "indoor":
            self.size = "small"
        return self.size


class Tiger(Cat):
    def __init__(self, color, cat_type):
        super().__init__(color, cat_type)

    def set_size(self):
        super().set_size()
        if self.cat_type == "wild":
            self.size = "big"
        return self.size


cat1 = Cat(color="black", cat_type="indoor")
cat1_size = cat1.set_size("indoor")
print(cat1.color, cat1.cat_type, cat1_size)

cat2 = Tiger(color="white", cat_type="wild")
cat2.set_size("wild")
print(cat2.color, cat2.cat_type, cat2.size)

Answer 4

If there is no return in the method it always returns None.

Answer 5

This code has been slightly modified.

class Cat:
    def __init__(self, color, cat_type):
        self.size = "undefined"
        self.color = color
        self.cat_type = cat_type

    def set_size(self):
        if self.cat_type == 'indoor':
            self.size = 'small'
        return self.size

class Tiger(Cat):
    def __init__(self, color, cat_type):
        super().__init__(color, cat_type)

    def set_size(self):
        super().set_size()
        if (self.cat_type == 'wild'):
            self.size = 'big'
        return self.size


cator = Cat(color='black', cat_type='indoor')

cator_size = cator.set_size()

print("Cat is {}, {}, {}".format(cator.color, cator.cat_type, cator_size))