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. -- http://mail.python.org/mailman/listinfo/python-list
