Classes nested in functions and attribute lookup

Question

Hi all!

The following works Ok, i.e. it doesn't give any errors:

def foo(arg):
    class Nested(object):
        x = arg

foo('hello')

But the following throws an exception:

def foo(arg):
    class Nested(object):
        arg = arg # note that names are the same

foo('hello')

Traceback:

Traceback (most recent call last):
  File "test.py", line 6, in <module>
    foo('hello')
  File "test.py", line 3, in foo
    class Nested(object):
  File "test.py", line 4, in Nested
    arg = arg
NameError: name 'arg' is not defined

I can't understand the reason of such behavior. Could anybody explain?

Answer 1

The arg property shadows the arg function argument (inner scoping)

def foo(arg):
    class Nested(object):
        arg = arg # you try to read the `arg` property which isn't initialized

You get the same error if you type i = i in the interpreter window without having initialized the i variable.

Answer 2

If you try and assign to a variable within a function, Python assumes that variable is local to that function. So by trying to assign arg to the value of itself, you are implicitly declaring it as a local variable.

Answer 3

It is due to Python's scoping rules:

def foo(arg): # (1)
    class Nested(object):
        arg = arg # (2)

(2) defines a new 'arg' name in the class' namespace, which opaques the value of the other 'arg' in the outer namespace (1).

However, (2) is unnecessary and the following is completely valid:

def foo(arg):
    class Nested(object):
        def message(self):
            print arg
    return Nested()

nested = foo('hello')
nested.message()

(prints hello)