[Tutor] Sometimes unreferenced!?

[John Abbe]

At 9:23 AM -0800 on 2003-02-06, Jeff Shannon typed:
>John Abbe wrote:
>
>>def f():
>>    records = [4, 5]
>>    groupData += records
>
>>UnboundLocalError: local variable 'groupData' referenced before assignment
>
>The trick is that the parser examines every variable in a function 
>as it's reading in the def, and decides at that time whether a 
>variable is local or global.
>
>In your first function e(), you modify the object that groupData 
>points to, by calling its reverse() method.  But it's still the same 
>list object, so there's no re-binding, so groupData is considered a 
>global name.  This means that Python looks it up properly at runtime.
>
>In your f() function, though, you use += to create a *new* list 
>(which is the union of groupData and records), and then re-bind 
>groupData to point to that new list.  Here's where things get tricky 
>-- Python sees the re-binding before running the code, so it 
>presumes that groupData is a local variable here.
>So a new, local name is created that shadows the global groupData. 
>Now, 'groupData += records' is (loosely) equivalent to 'groupData = 
>groupData + records', so Python tries to look up the contents of the 
>(local) groupData variable to add them to the contents of the 
>'records' variable -- but at this point, the local version of 
>groupData doesn't point to anything yet.

Okay, i understand *that* this is happening (and thanks for the fix), 
but i don't get why. If the parser can know that groupData is a 
global variable and thus not choke on groupData.reverse(), then why 
can't it know that groupData is a global variable when the function 
tries to look up its contents?

Life,
John
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