在python中不满足条件时不创建对象？

Question

Is it possible to not create an object if certain conditions are not met in the constructor of a class?

Eg:

class ABC:
    def __init__(self, a):
        if a > 5:
            self.a = a
        else:
            return None

a = ABC(3)
print(a)

This should print None (since it should not create an Object but return None in this case) but currently prints the Object...

Answer 1

you can use a classmethod as an alternate constructor and return what you want:

class ABC:
    def __init__(self, a):
        self.a = a

    @classmethod
    def with_validation(cls, a):
        if a > 5:
            return cls(a)
        return None


a = ABC.with_validation(10)

a
<__main__.ABC at 0x10ceec288>

a = ABC.with_validation(4)

a

type(a)
NoneType

Answer 2

This code seems to show that an exception raised in an __init__() gives you the effect you want:

class Obj:
    def __init__(self):
        raise Exception("invalid condition")

class E:
    def __call__(self):
        raise Exception("raise")

def create(aType):
    return aType()

def catchEx():
    e = E()
    funcs=[Obj, int, e]

    for func in funcs:
        try:
            func()
            print('No exception:', func)
        except Exception as e:
            print(e)

catchEx()

Output:

invalid condition
No exception: <class 'int'>
raise

Answer 3

I think this shows the principle. Note that returning None is not returning a new object because None is a singleton in Python, but of course it is still an object. Note also that __init__ will not be called as None is not an A class object.

class A():
    def __new__(cls, condition):
        if condition:
            obj = super().__new__(cls)
            return obj

a = A(True)
print(a)
a1 = A(False)
print(a1)

This outputs:

<__main__.A object at 0x7f64e65c62e8>
None

Answer 4

With the help from @progmatico and a little try and error I managed to come to this solution:

class ABC:
    def __new__(cls, *args, **kwargs):
        if len(args) > 0:
            arg = args[0]
        else:
            arg = kwargs['a']

        if arg <= 5:
            return None
        return object.__new__(cls)

    def __init__(self, a):
        self.a = a

    def __str__(self):
        return str(self.a)

a = ABC(a=3)
print(a)
b = ABC(a=7)
print(b)