Namespaces within a Python Class

Question

I have this:

class MyClass:
    """A simple example class"""
    i = 12345
    def f(self):
        print i                     # self.i will work just fine
        return 'hello world'

When I do:

>>> x = MyClass()
>>> 
>>> x.f()

I get an error, as expected.

My question is:

Why do I get the error?
Why is there no namespace between the namespace of the function(or method) definition and the global namespace of the module containing the class?
Is there any other way to reference i inside f in this case other than using self?

possible duplicate of [Closures in a class scope](http://stackoverflow.com/questions/13381861/closures-in-a-class-scope) — Martijn Pieters, Jan 12 '13 at 23:25
As for 3: Yes, by referencing it via the class: `MyClass.i`. — Martijn Pieters, Jan 12 '13 at 23:25
Also related: [Accessing class variables from a list comprehension in the class definition](http://stackoverflow.com/questions/13905741/13913933#13913933) — Martijn Pieters, Jan 12 '13 at 23:26
Actually, there is between the global namespace and the function, but there isn't between class variables and the function (read only unless you use `global`). If you want to declare instance variables, do it in the constructor (`__init__()`). — Gareth Latty, Jan 12 '13 at 23:28
This is from PEP227: Names in class scope are not accessible. Names are resolved in the innermost enclosing function scope. If a class definition occurs in a chain of nested scopes, the resolution process skips class definitions. And Martin's comment below on Alex's response totally makes sense. — Ankur Agarwal, Jan 12 '13 at 23:38

score 3 · Accepted Answer · edited Jan 13 '13 at 00:12

3

You've got an error because print i is trying to print a global (for the module) variable i. If you want to use the member of the class you should write self.i.
Because Guido Van Rossum decided not to use namespaces. Actually, I don't know how to answer anymore.
Yes, but no. Yes, because you can use "reflection" (I can't remember how it is called in python) and access any member of the class. No, because using self is the only usable way.

edited Jan 13 '13 at 00:12

ford

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answered Jan 12 '13 at 23:26

Alex G.P.

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For 2: It doesn't make *sense* to make classes a namespace. What would it mean if you altered the contents of a mutable `i`? Is that changed for the class object, the instance, for subclasses? Explicit is better than implicit, so for classes you have to be explicit about what you want to reference. A class attribute, an inherited class attribute, or the instance attribute itself. – Martijn Pieters Jan 12 '13 at 23:29

score 0 · Answer 2 · edited Feb 02 '20 at 03:40

Why do I get the error?

Answer: This is because the variable i & the symbol/level f which points to a function/method code are in MyClass namespace where as the value/callable code/body of the function f lies in global namespace.

Why is there no namespace between the namespace of the function(or method) definition and the global namespace of the module containing the class?

Answer: Not sure why the python designer decided to put function name/symbol in the class but its body in global namespace.

Is there any other way to reference i inside f in this case other than using self?

Answer: I don't think so, but even if it is then it might not be easier logic/code

about the last point: `i` is a class variable. Yes, you can reach it via `self.i` (i.e. python looks in the instance variables and if it cannot find it, it look into the variables of the class), and you can also reach it via `MyClass.i` — Pynchia, Apr 13 '23 at 14:50

Namespaces within a Python Class

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