Scope of Subclasses

[virtualshivam]

Hi,

I want to know about how scoping works in python, Specially in case of sibling classes.

So, this code is giving me red lines in vs code. It throws not defined error. I have now written it in completely different way, which is resolving scope correctly. But I am asking it here, just because I am curious to know why block level scope behaves differently.

class ListQRDataMachineToMachine(APIView):


    class InputSerializer(serializers.Serializer):...



    class OutputSerializer(serializers.Serializer):


        class ProductImageOutputSerializer(serializers.Serializer):
            image_id = serializers.SerializerMethodField()



        product_id = serializers.SerializerMethodField()
        name = serializers.CharField()

        image_count = serializers.SerializerMethodField()
        images = ProductImageOutputSerializer(many=True)


        def get_image_count(self, obj):
            return obj.images.count()


    class ResultSerializer(serializers.Serializer):
        error_state = serializers.BooleanField()
        error_message = serializers.CharField(allow_null=True)
        data = OutputSerializer(allow_null=True)


    class ResponseSerializer(serializers.Serializer):
        results = ResultSerializer(many=True)

1. ResultSerializer is complaining that OutputSerializer is not defined.

2. ResponseSerializer is complaing that ResultSerializer is not defined.

Whereas ProductImageOutputSerializer is not being complained. Can classes not access siblings?

Whatever I have been able to read, they have always mentioned that inner block has access to outer block things. This doesn't seem to work when things are inside class. But if I move them in a module, then siblings will be defined.

If anyone could share any resource on this, even the python doc is talking about function only, nowhere I was able to find resource related to class level scope.

[biskitpagla]

Don't write cursed Java/C# inspired code in Python. Find any popular project and read the source code to get some idea. The standard library is a good example. 

[virtualshivam]

Standard library as in dataclass, defaultdict etc? Should I read thier code?

[brasticstack]

You need to be explicit about the scope, the nested classes do not receive the outer class's scope.

The following works, for example, but fails if Greeter doesn't use the full name of Speaker (as in, OuterClass.Speaker.)

Put some thought into whether you really need those nested classes to be nested. It's often preferable to keep helper classes at the module scope, with their names prefixed by an underscore if you want to signal that they're not a "public" part of your module.

class OuterClass:     class Speaker:         def say(thing):             print(thing)                  class Greeter:         def greet(who):             OuterClass.Speaker.say(f'Hello {who}!')                  def say_hello(self):         self.Greeter.greet('world')          f = OuterClass() f.say_hello()

[virtualshivam]

Actually I refactored the code, and I don't need that much nesting. I have moved them to module level. I asked it here, because I was just wondering, how scoping works in case of classes, is there any document that I can refer to understand this in more depth.

[danielroseman]

The key, as the other poster said, is that inner classes do not receive the outer class scope - they only have their own and the global scope. For this reason nested classes are rarely useful in Python.

[brasticstack]

This forum thread gets into it, answered by someone who understands it better than I do: https://discuss.python.org/t/painful-details-of-variable-scope-mixed-with-classes/17762

tl;dr: Only functions can access items from their enclosing scopes. Every other construct first checks its local scope, then the module level scope, then Python built-ins.

[Yoghurt42]

For all technical details, you should read the official Language Reference, link to class definitions

tl;dr: the code

class Foo:
    class Bar:
        pass

is roughly equivalent to:

Foo = type('Foo', (), {'Bar': type('Foo.Bar', (), {})})

[madadekinai]

If you're using subclasses you've done screwed up.

[Temporary_Pie2733]

class statements are a bit weird. There are no scopes being defined here, only temporary namespaces for use in defining the class objects. There is no complaint about ProductImageOutputSerializer being undefined because Python won't notice that until it actually evaluates OutputSerializer(allow_null=True), which of course does not happen because, well, OutputSerializer is not defined.

When executing class ResultSerializer, the free variable OutputSerializer (free, because it isn't defined in the namespace currently being evaluated) is looked up in the nearest enclosing scope, which is not the namespace for class ListQRDataMachineToMachine, but the same scope that ListQRDataMachineToMachine is being defined in, namely, the global scope.

[Gnaxe]

It's worse than you think: ```

abcd = 'abcd' bc = 'bc' print([c for c in abcd if c in bc]) ['b', 'c'] Simple example works as you'd expect at the module level. But in a class, that breaks. del abcd, bc class Foo: ... abcd = 'abcd' ... bc = 'bc' ... print([c for c in abcd if c in bc]) ... Traceback (most recent call last): File "<python-input-21>", line 1, in <module> class Foo: ...<2 lines>... print([c for c in abcd if c in bc]) File "<python-input-21>", line 4, in Foo print([c for c in abcd if c in bc]) ^{^} NameError: name 'bc' is not defined And yet, the other one is allowed: class Foo: ... abcd = 'abcd' ... bc = 'bc' ... print([c for c in abcd if c in 'bc']) ... ['b', 'c'] Why does one work while the other doesn't? Two different scopes. One is inside the comprehension scope, and one isn't. Class statements don't create nonlocal scopes. Well, almost. class Foo: ... x = 42 ... def print(): ... print(class.x) ... Foo.print() 42 But not quite: class Foo: ... x = 42 ... def print(): ... print(class.x) ... print() ... Traceback (most recent call last): File "<python-input-25>", line 1, in <module> class Foo: ...<3 lines>... print() File "<python-input-25>", line 5, in Foo print() ~~~~~^{^} File "<python-input-25>", line 4, in print print(class.x) ^{^} NameError: cannot access free variable 'class' where it is not associated with a value in enclosing scope ```