On Sat, 2007-09-22 at 18:21 +0000, John J. Lee wrote: > Carsten Haese <[EMAIL PROTECTED]> writes: > > The second half of my post illustrates a difference of opinion about > > what constitutes a generator function. You state that frange() is not a > > generator function because it doesn't use yield, but it behaves like > > one. My point is that it *is* a generator function because the generator > > expression is merely syntactic sugar for an equivalent for/yield loop. > > Seems to me that's a definitional thing, with no conceptual content, > so "difference of opinion" seems an odd choice of words. It would be > nice to nail the definitions down. Do the Python docs do that? > > > > Of course, the distinction of whether frange() *is* a generator function > > or merely *behaves* as one is immaterial in practice, and we can both be > > right in the absence of a formal definition of what a generator function > > is. PEP 255 says "A function that contains a yield statement is called a > > generator function," but that was written before generator expressions > > were introduced. > > Ah, they do -- thanks. Though I'm now left puzzled why you express > your "difference of opionion" above...
It's a matter of opinion whether the excerpt from the PEP constitutes the formal definition of what a generator function is or isn't. A formal definition needs conditions that are both sufficient and necessary. The PEP only says that having a yield statement is sufficient for making a generator function. It doesn't say that a yield statement is necessary for making a generator function. In other words, it doesn't say that a function that doesn't contain a yield statement isn't a generator function. The language reference is equally wishy-washy: "Using a yield statement in a function definition is *sufficient* to cause that definition to create a generator function instead of a normal function." [emphasis mine] Again, no indication that a yield statement is necessary for making a generator function. It then proceeds to saying "When a generator function is called, it returns an iterator known as a generator iterator, or more commonly, a generator." That condition seems to be true for the OP's frange() function, even though it doesn't have a yield statement. Until somebody can point out a definition that says unequivocally "an object is a generator function if and only if ...", it's up in the air whether frange() is a generator function or merely impersonates a generator function. Ultimately, though, this is a purely academic question without a useful answer. The useful conclusion is that a function can behave like a generator function without a yield statement, and we have reached that conclusion a long time ago. -- Carsten Haese http://informixdb.sourceforge.net -- http://mail.python.org/mailman/listinfo/python-list
