Inner class usage python. Global name is not defined

Question

I have a question about inner classes usage in python. I know that this is a bad practice but anyway. I am wondering about scope where inner class is defined. I got an error 'global name 'ClassName' is not defined'. My code snippet looks like this:

I discovered that to avoid getting this error I can use:

ClassWithEnum.EnumClass

instead of:

EnumClass

inside of doSomethingWithEnum() function. So I am wondering if there is any other way to make EnumClass defined inside of doSomethingWithEnum() function? And why there is no such error when I declare EnumClass as a default parameter in doSomethingWithEnum() function?

Please post your code as code so it's easily copiable.

deceze
– deceze ♦

2017-06-16 11:13:42 +00:00
Commented Jun 16, 2017 at 11:13 — deceze
– deceze ♦, Commented Jun 16, 2017 at 11:13
@deceze, Ok, will do so in future.

Bill
– Bill

2017-06-16 12:04:25 +00:00
Commented Jun 16, 2017 at 12:04 — Bill
– Bill, Commented Jun 16, 2017 at 12:04

deceze · Accepted Answer · 2017-06-16 11:49:05Z

5

class ClassWithEnum(object):
    class EnumClass(object):
        ...

    def doSomethingWithEnum(self, m = EnumClass....):
        ...

Python class construction executes as code. The def statement is really a line of code being executed that creates a function. The class keyword introduces a namespace. Putting these two mechanisms together, it means that class EnumClass really creates an object by that name in the current namespace, not much different from what foo = 'bar' does, so within the same namespace you can refer to it by that name, which is what happens in the def statement.

Also compare:

class Foo:
    bar = 'baz'
    print bar
    baz = bar

Every line of code inside a class block is a regular executable line of code.

Once your class definition is done, you're out of the ClassWithEnum namespace and cannot access EnumClass anymore simply by that name; it's now only available as ClassWithEnum.EnumClass; whether from "outside" the class or from within a function (any function, including class methods).

To get access to the class without typing its name from within the method you could do:

type(self).EnumClass

Or simply self.EnumClass, since properties are looked up up the chain.

answered Jun 16, 2017 at 11:49

deceze♦

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1 Comment

SRC Over a year ago

+1 for the detailed explanation and of course for this line Python class construction executes as code

Michael Gendin · Accepted Answer · 2017-06-16 11:15:59Z

2

When you are inside doSomethingWithEnum function, you are in the different namespace. To access anything that is defined in your class, like EnumClass, you should call self.EnumClass. If it were a class method, it would be cls.EnumClass.

answered Jun 16, 2017 at 11:15

Michael Gendin

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