在另一个类构造函数中实例化一个类的唯一实例

Question

I have two classes. One which instantiates the other in its constructor.

class Details(object):
    pass

class Overview(object):
    def __init__(self, details = Details()):
        self.details = details

When I create two instances of overview, they change the same instance of details. Shouldn't Overview create a new instance of Details() upon every instantiation?

ov1 = Overview()
ov2 = Overview()
print(id(ov1.details))
print(id(ov2.details))

# 2940786890344
# 2940786890344

Answer 1

The default argument to details is evaluated at class creation time! All instances of Overview will have the identical Details instance if not provided. Change along the lines of:

class Overview(object):
    def __init__(self, details=None):
        self.details = Details() if details is None else details

This is a common source of surprise for beginners, especially with mutable default arguments .