In Python 2.5, generators are further enhanced, with the possibility of receiving a value (or an exception) back from the caller as each yield executes. These advanced features allow generators in 2.5 to implement full-fledged co-routines, as explained at http://www.python.org/peps/pep-0342.html. The main change is that, in 2.5, yield is not a statement, but an expression, so it has a value. When a generator is resumed by calling its method next, the corresponding yield's value is None. To pass a value x  into some generator g  (so that g  receives x as the value of the yield on which it's suspended), instead of calling g.next(), the caller calls g.send(x) (calling g.send(None) is just like calling g.next()). Also, a bare yield without arguments, in Python 2.5, becomes legal, and equivalent to yield None.

Other Python 2.5 enhancements to generators have to do with exceptions, and are covered in "Generator enhancements" on page 126.

Recursion

Python supports recursion (i.e., a Python function can call itself), but there is a limit to how deep the recursion can be. By default, Python interrupts recursion and raises a RecursionLimitExceeded exception (covered in "Standard Exception Classes" on page 130) when it detects that the stack of recursive calls has gone over a depth of 1,000. You can change the recursion limit with function setrecursionlimit of module sys, covered in setrecursionlimit on page 171.

However, changing the recursion limit does not give you unlimited recursion; the absolute maximum limit depends on the platform on which your program is running, particularly on the underlying operating system and C runtime library, but it's typically a few thousand levels. If recursive calls get too deep, your program crashes. Such runaway recursion, after a call to setrecursionlimit that exceeds the platform's capabilities, is one of the very few ways a Python program can crash--really crash, hard, without the usual safety net of Python's exception mechanisms. Therefore, be wary of trying to fix a program that is getting RecursionLimitExceeded exceptions by raising the recursion limit too high with setrecursionlimit. Most often, you'd be better advised to look for ways to remove the recursion or, more specifically, limit the depth of recursion that your program needs.

Readers who are familiar with Lisp, Scheme, or functional-programming languages must in particular be aware that Python does not implement the optimization of "tail-call elimination," which is so important in these languages. In Python, any call, recursive or not, has the same cost in terms of both time and memory space, dependent only on the number of arguments: the cost does not change, whether the call is a "tail-call" (meaning that the call is the last operation that the caller executes) or any other, nontail call.