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Mike Brooks3,389 Points
Setting argument = None causes no accumulation
From the Python tutorial I excerpted the following code:
def f(a, L=): L.append(a) return L def f2(a, L=None): if L is None: L =  L.append(a) return L print(f(1)) print(f(2)) print(f(3)) print(f2(1)) print(f2(2)) print(f2(3))
Which yields the following results:
 [1, 2] [1, 2, 3]   
It was showing how in the case of function f(), the contents of the L list are accumulating with each call and how if you alternatively in f2() set L = None it does not.
Maybe this is just something I should take as True but what about setting L = None overrides the notion that the argument is only instantiated at function definition time.