How to create distinct instances of a class in Python? [duplicate]

Question

Could somebody please explain the following code to me.

class InnerTest:

    def __init__(self, value = 0):
        self.value = value

class OuterTest:

    def __init__(self, inner_test = InnerTest()):
        self.inner_test = inner_test

a = OuterTest()
b = OuterTest()

a.inner_test.value = 42
print b.inner_test.value

It prints 42, I expected 0.

I meant to create two instances of OuterTest, which would contain a distinct instance of InnerTest each. Instead I got two instances of OuterTest which reference the same instance of InnerTest.

Also what would be a correct way to implement what I wanted please?

Answer 1

The default parameter in functions are evaluated only once, at the time of function definition. So there is only one instance of InnerTest being used for both the object.

That means, when you create the two objects:

a = OuterTest()
b = OuterTest()

Both, a.inner_test and b.inner_test, are referring to the same instance, and hence the result.

To solve this, change the default value to None, and create instance conditionally:

class OuterTest:
    def __init__(self, inner_test=None):
        if not inner_test:
            inner_test = InnerTest()
        self.inner_test = inner_test

Answer 2

You could move the InnerTest() into the init or call it in the init, passing a reference to the class n the second case.

Using def __init__(self, inner_test = InnerTest()): is evaluated once so the object is shared among all instances of the OuterTest class making it a class attribute as opposed to an instance attribute by instantiating it in the init.

class OuterTest:
    def __init__(self):
        self.inner_test = InnerTest()

Or:

class OuterTest:
    def __init__(self, inner_test = InnerTest):
        self.inner_test = inner_test()

Both methods will work as desired, passing a reference to a class means you have the option of passing in whatever class you want:

In [11]: class OuterTest:
   ....:     def __init__(self, inner_test=InnerTest):
   ....:             self.inner_test = inner_test()
   ....:         

In [12]: a = OuterTest()

In [13]: b = OuterTest()

In [14]: a.inner_test.value = 42

In [15]: print(a.inner_test.value)
42

In [16]: print(b.inner_test.value)
0

In [17]: class OuterTest:
   ....:     def __init__(self):
   ....:             self.inner_test = InnerTest()
   ....:         

In [18]: a = OuterTest()

In [19]: b = OuterTest()

In [20]: a.inner_test.value = 42

In [21]: print(a.inner_test.value)
42

In [22]: print(b.inner_test.value)