“local variable referenced before assignment” — on

Question

Take the following code:

import something

def Foo():
    something = something.SomeClass()
    return something

…this is apparently not valid code:

UnboundLocalError: local variable 'something' referenced before assignment

…as the local variable something is created, but not assigned, before the RHS of the = is evaluated. (See, for example, this related answer's comment.) This seems a bit odd to me, but sure, I'll go with it. Now, why is the following valid code?

class Foo(object):
    something = something.SomeClass()

My understanding was that the inside of a class definition was essentially a scope:

The class’s suite is then executed in a new execution frame (see section Naming and binding), using a newly created local namespace and the original global namespace.

So, then, why does that code act differently than that of a function?

Answer 1

From the python class documentation:

Class definitions place yet another namespace in the local scope.

A special quirk of Python is that – if no global statement is in effect – assignments to names always go into the innermost scope. Assignments do not copy data — they just bind names to objects. The same is true for deletions: the statement del x removes the binding of x from the namespace referenced by the local scope. In fact, all operations that introduce new names use the local scope: in particular, import statements and function definitions bind the module or function name in the local scope. (The global statement can be used to indicate that particular variables live in the global scope.)

So within a function (or a scope) the assignment creates a local unbound variable that is accessed before it is bound, whereas in a class definition it creates an entry in the "namespace" dictionary of that class on assignment, allowing the resolution of something to the outer namespace (the module namespace).

Answer 2

Consider the following example which may help to clarify this:

import datetime

class Foo(object):
    datetime = datetime.datetime

>>> datetime
<module 'datetime' from '/usr/lib/python2.6/lib-dynload/datetime.so'>
>>> Foo.datetime
<type 'datetime.datetime'>

Note that the line datetime = datetime.datetime is actually assigning to the name Foo.datetime, which is not ambiguous with the global datetime (like it would be if the same code were in the function).

In summary, because class definitions create a new namespace as well as a new scope, you are allowed to directly access a name in an enclosing scope and assign to the same name in the local scope.