Question

Apologies if I’ve got the terminology wrong here—I can’t think what this particular idiom would be called.

I’ve been trying to create a Python 3 class that statically declares instances of itself inside itself—sort of like an enum would work. Here’s a simplified version of the code I wrote:

class Test:
    A = Test("A")
    B = Test("B")

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

    def __str__(self):
       return "Test: " + self.value

print(str(Test.A))
print(str(Test.B))

Writing this, I got an exception on line 2 (A = Test("A")). I assume line 3 would also error if it had made it that far. Using __class__ instead of Test gives the same error.

  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in Test
NameError: name 'Test' is not defined

Is there any way to refer to the current class in a static context in Python? I could declare these particular variables outside the class or in a separate class, but for clarity’s sake, I’d rather not if I can help it.

To better demonstrate what I’m trying to do, here’s the same example in Java:

public class Test {
    private static final Test A = new Test("A");
    private static final Test B = new Test("B");

    private final String value;

    public Test(String value) {
        this.value = value;
    }

    public String toString() {
        return "Test: " + value;
    }

    public static void main(String[] args) {
        System.out.println(A);
        System.out.println(B);
    }
}

This works as you would expect: it prints:

Test: A
Test: B

How can I do the same thing in Python?

Answer 1

After you defined the class, just add these two lines:

Test.A = Test("A")
Test.B = Test("B")

A class in Python is an object like any other and you can add new variables at any time. You just can’t do it inside the class since it’s not defined at that time (it will be added to the symbol table only after the whole code for the class has been parsed correctly).