On Sun, 05 Aug 2007 23:50:24 -0700, Lee Fleming wrote: > Hello, > I have a simple question. Say you have the following function: > > def f(x, y = []): > y.append(x) > return y > > print f(23) # prints [23] > print f(42) # prints [23, 42] > > As far as I understand, the default value y, an empty list, is created > when the def statement evaluates. With this thought in mind, the above > calls > to f make sense. > > But this, the code that "fixes" the list accumulation confounds me: def > f(x, y=None): > if y is None: y = [] > y.append(x) > return y > > print f(23) # prints [23] > print f(42) # prints [42] > > Why didn't the second call to f, f(42) return [23, 42]? As I understand > it, y is only None at the beginning of f(23). Then y changes from None > to 23. When f ends, doesn't y still have 23 in it, > just as it did in the first function I discussed? And if y has 23 in it, > won't the second call to f not execute what's in the if statement? > > In other words, what's going on here? How is it that y accumulates > argument values between function calls in the first function, but > doesn't in the second one?
You're just unluckily shadowing the name `y` in the local scope of your function. Your code snippet could be rewritten as:: def f(x, y=None): if y is None: my_y = [] else: my_y = y my_y.append(x) return my_y HTH, Stargaming -- http://mail.python.org/mailman/listinfo/python-list