Scope, type and UnboundLocalError

[Paddy]

Bruno Desthuilliers wrote:
> Frank Millman a écrit :
> > Paddy wrote:
> >
> >>Hi,
> >>I am trying to work out why I get UnboundLocalError when accessing an
> >>int from a function where the int is at the global scope, without
> >>explicitly declaring it as global but not when accessing a list in
> >>similar circumstances.
> >>
> >
> >
> > There has just been a long thread about this. I think I understand it
> > now. Here is my explanation.
> >
> > Ignoring nested scopes for this exercise, references to objects (i.e.
> > variable names) can exist in the local namespace or the global
> > namespace. Python looks in the local namespace first, and if not found
> > looks in the global namespace.
> >
> > Any name assigned to within the function is automatically deemed to
> > exist in the local namespace, unless overridden with the global
> > statement.
>
> And this even of the local bindings sequentially comes after another
> access to the name, ie:
>
> g = 0
>
> def fun():
>    x = g # UnboundLocalError here
>    g += 1
>    return x
>
> > With the statement 'm = m + 1', as m is assigned to on the LHS, it is
> > deemed to be local, but as m does not yet have a value on the RHS, you
> > get Unbound Local Error.
>
> Right
>
> > With the statement 'n[0] = n[0] + 1', n is not being assigned to,
> Right
>
> >  as it
> > is mutable.
>
> n is effectively mutable, but this is totally irrelevant. In your
> snippet, n is not 'assigned to', it's "mutated" (ie a state-modifying
> method is called). The snippet:
>
> n[0] = n[0] + 1
>
> is syntactic sugar for
>
> n.__setitem__(0, n.__getitem__(0) + 1)
>
> IOW, it's just method calls on n.

So,
An assignment statement may assign an object to a name, in which case
the name is 'tagged' as being local,
An assignment statement may mutate a mutable object already bound to a
name, in which case the assignment will not 'tag' the name as  being
local.

I guess Bruno, you mean irrelevant as in 'although only mutable objects
can have their state modified; if n has a mutable value but the
assignment statement changed the object referred to by n, then the name
would be tagged as local'?

- Peace,  Paddy.