Duncan Booth writes: > Neil Cerutti wrote: > > On 2013-10-03, Duncan Booth <duncan.booth@invalid.invalid> wrote: > >> It isn't hard to imagine adding a TAIL_CALL opcode to the > >> interpreter that checks whether the function to be called is > >> the same as the current function and if it is just updates the > >> arguments and jumps to the start of the code block. > > > > Tail call optimization doesn't involve verification that the > > function is calling itself; you just have to verfify that the > > call is in tail position. > > You misunderstood me. As usually implemented tail call recursion > doesn't require verifying that the function is calling itself, but > in Python the function could be rebound so a check would be a > necessary protection against this unlikely situation. Consider this > Python: > > @some_decorator > def fact(n, acc=1): > if n <= 1: > return acc > return fact(n-1, n*acc) > > Is that tail recursion or not? > > You cannot tell whether or not that is tail recursion unless you > know the definition of 'some_decorator' and even then the answer may > vary from run to run.
Ignoring the decorator, fact(n-1, n*acc) is in a tail position because it follows the keyword return. It doesn't matter what function fact is at the time: the remaining action in the caller is to return. Tail positions, with respect to enclosing code, are defined syntactically. -- https://mail.python.org/mailman/listinfo/python-list
