On Fri, Jul 23, 2010 at 11:30 AM, SeanMon <smono...@gmail.com> wrote: > > I was playing around with Python functions returning functions and the > scope rules for variables, and encountered this weird behavior that I > can't figure out. > > Why does f1() leave x unbound, but f2() does not? > > def f1(): > x = 0 > def g(): > x += 1 > return x > return g1 > > def f2(): > x = [] > def g(): > x.append(0) > return x > return g > > a = f1() > b = f2() > > a() #UnboundLocalError: local variable 'x' referenced before > assignment > b() #No error, [0] returned > b() #No error, [0, 0] returned > --
It's not closure related at all. Same thing happens at the module level. x = 0 def f1() : x += 1 #gives UnboundLocalError x = [] def f2() : x.append(1) #succeeds. The reason for it is that if you have any assignments to the variable in the function, Python creates a new local variable for it. x += 1 is an assignment, not a modification. Python 2.x allows you to assign to the global scope (using the global keyword) but support for assigning to the outer function's scope wasn't added until Python 3 (with the nonlocal keyword) def f1(): x = 0 def g(): nonlocal x x += 1 return x return g1 > > http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
