Robert Johansson wrote: > This is my attempt to generate all pairs of relatively prime pairs of > nonnegative integers (n, m) such that n + m <= N, using the Stern-Brocot > tree. > > def rp_bounded_sum(n, i = 0, p = (0, 1), q = (1, 1), S = set([(0, 1), (1, > 1)])): > # Returns a set S with all relatively prime pairs (a, b) > # such that 0 <= a <= b and a + b <= n. > r = p[0] + q[0], p[1] + q[1] > if r[0] + r[1] <= n: > S.add(r) > rp_bounded_sum(n, 1, p, r, S) > rp_bounded_sum(n, 1, r, q, S) > if not i: > return S
Default values for function arguments are shared across function invocations. If you have mutable defaults, e. g. a list or set, and modify that default value during the function call the modifications will be visible the next time you invoke the function. A simple example: >>> def f(x, items=[]): ... items.append(x) ... return items ... >>> f(1) [1] >>> f(2) [1, 2] >>> f(3) [1, 2, 3] >>> f(1, [42]) [42, 1] >>> f(4) [1, 2, 3, 4] The standard idiom to avoid this issue is to use a special default value to signal that the argument was not provided. In most cases that special value is None: >>> def f(x, items=None): ... if items is None: ... items = [] # if no list was provided make a new one ... items.append(x) ... return items ... >>> f(1) [1] >>> f(2) [2] >>> f(3) [3] >>> f(1, [42]) [42, 1] >>> f(4) [4] _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
