[Python-ideas] Why does += trigger UnboundLocalError?

Thu Jun 2 00:56:52 CEST 2011

On 6/1/2011 1:41 PM, Terry Reedy wrote:
> On 6/1/2011 12:52 AM, Carl M. Johnson wrote:
>
>> So, my proposal is that += by itself should not cause x to be considered
>> a local variable.

> While I do understand your point, I also value uniformity.
> -1

There is another problem I had not thought of before. Right now, Python 
has (always had) a simple rule: code in a function CANNOT rebind names 
in outer scopes unless the function has a global or nonlocal 
declaration. This simple, uniform rule benefits not only the interpreter 
but human readers. It should not be broken.

def f():
   'doc for f'
   def g():
     'docstring of g'
     <body of g>
   <body of f>

If g is the only nested function and the body of g does not have a 
nonlocal declaration (which OUGHT to be at the top if present), then a 
reader or maintainer of f knows (without reading g in detail) that 
nothing other that <body of f> can rebind f's locals.

-- 
Terry Jan Reedy