lists: += vs. .append() & oddness with scope of variables
tjreedy at udel.edu
Sun Mar 5 20:02:25 CET 2006
"Sandro Dentella" <sandro at e-den.it> wrote in message
news:slrne0lk30.ek2.sandro at bluff.diade.it...
> I'd like to understand why += operator raises an error while .append()
Your mistake is thinking of '+=' as an operator. In Python terms it is
not, any more than '=' is. In Python, neither 'a=b' nor 'a+=b' is an
expression. Both symbols are statement symbols that define assigment
statements (augmented in the former case). "a.append(b)" is an expression
with side effects used as a statement.
> Traceback (most recent call last):
> File "c1.py", line 26, in ?
> x = foo()
> File "c1.py", line 7, in __init__
> print "a: ", a
> UnboundLocalError: local variable 'a' referenced before assignment
This is the clue that you did not get. It tells you that the parser thinks
'a' is local, which means you rebound the name 'a' *somewhere* in the
function, even if not as obviously as a simple assignment "a = whatever".
It turns out that augmented assignment statements are assignments
Terry Jan Reedy
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