def a(b=[]): b.append(1) print b Now if you do, a() a() you might expect to see [1] [1] but you will actually see [1] [1, 1] The

def a(b=[]): b.append(1) print b Now if you do, a() a() you might expect to see [1] [1] but you will actually see [1] [1, 1] The list is not created at function call time, but at function definition time.