Scope, type and UnboundLocalError

[Bruno Desthuilliers]

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.