lists: += vs. .append() & oddness with scope of variables

Sandro Dentella sandro at e-den.it
Sun Mar 5 12:49:53 CET 2006


I'd like to understand why += operator raises an error while .append() does
not. My wild guess is the parses treats them differently but I cannot
understand why this depends on scope of the variables (global or class
variables):


a = [0]

class foo(object): 

    def __init__(self):
        print "a: ", a
        # += does not work if 'a' is global
        #a += [1]
        a.append(2)
        print "a= ", a

class bar(object): 

    b = [0]

    def __init__(self):
        print "b: ", self.b
        # += *does* work if 'a' is class var
        self.b += [1]
        self.b.append(2)
        print "b= ", self.b


if __name__ == '__main__':
    x = foo()
    y = bar()

a:  [0]
a=  [0, 2]
b:  [0]
b=  [0, 1, 2]

uncommenting 'a += [1]' would raise:

a:
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

TIA
sandro
*:-)


-- 
Sandro Dentella  *:-)
e-mail: sandro at e-den.it 
http://www.tksql.org                    TkSQL Home page - My GPL work



More information about the Python-list mailing list