python 中的切换器给出的 Object 问题

Question

I seem to have a problem with a switch-fashioned code when I try to create an object where its class depends on an input.

Let us say there are two classes, Class1 and Class2. Both of them, when I create an object, give me a message "Object of Class<1 or 2> created" Then there is this new class which have a method that take a string when I create the object.this string is "Class1" or "Class2".

Here the tricky part: based on this string I create an object Class1 or an object Class2 as follows:

def type_to_object(self, type):
        switcher = {
            "Class1": Class1(),
            "Class2": Class2(),
        }
        return switcher.get(type, "Invalid Class")

Now, this worked well in similar cases. The problem here is that it seems to create both the objects, even if I only got one in return when I call the method (obviously). I say this because refering to what I said above, I get both the messages "Object of Class1 created" and "Object of Class2 created"

Thank you in advance

PS: what it seems super strange, is that it should not enter all the cases, only the one equal to type

Answer 1

The problem is that you create instances of both your classes in the switcher dictionary.

Try this instead:

class Class1:
    def __init__(self):
        self.who_am_i = "Class1"


class Class2:
    def __init__(self):
        self.who_am_i = "Class2"


def type_to_object(class_type):
    switcher = {
        "Class1": Class1,  # No parentheses here, we don't want an instance. Just a type
        "Class2": Class2,  #
    }
    selected_class = switcher.get(class_type, None)
    if selected_class is None:
        raise RuntimeError("Invalid Class")
    return selected_class()  # Now, we use parentheses: we want to create an instance


c = type_to_object("Class1")
print(c.who_am_i)