[Python-Dev] more timely detection of unbound locals

Steven D'Aprano steve at pearwood.info
Mon May 9 18:39:14 CEST 2011

Eli Bendersky wrote:
>> I think you are making an unwarranted assumption about what is "more
>> expected". I presume you are thinking that the expected behaviour is that
>> foo() should:
>> print global x (5)
>> assign 1 to local x
>> print local x (1)
>> If we implemented this change, there would be no more questions about
>> UnboundLocalError, but instead there would be lots of questions like "why is
>> it that globals revert to their old value after I change them in a
>> function?".
> True, but this is less confusing and follows the rules in a more
> straightforward way. x = 1 without a 'global x' assigns a local x, this make
> sense and is similar to what happens in C where an inner declaration
> temporarily shadows a global one.

I disagree that it is less confusing. Instead of a nice, straightforward 
error that you can google, the function will silently do the wrong 
thing, giving no clue that weirdness is happening.

def spam():
     if x < 0:  # refers to global x
         x = 1  # now local
     if x > 0:  # could be either global or local
         x = x - 1  # local on the LHS of the equal
         # sometimes global on the RHS
         x += 1  # local x, but what value does it have?

Just thinking about debugging the mess that this could make gives me a 


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