Controlling a generator the pythonic way

Mike Meyer mwm at
Sat Jun 11 16:44:26 EDT 2005

Thomas Lotze <thomas at> writes:
> A related problem is skipping whitespace. Sometimes you don't care about
> whitespace tokens, sometimes you do. Using generators, you can either set
> a state variable, say on the object the generator is an attribute of,
> before each call that requires a deviation from the default, or you can
> have a second generator for filtering the output of the first. Again, both
> solutions are ugly (the second more so than the first). One uses
> side-effects instead of passing parameters, which is what one really
> wants, while the other is dumb and slow (filtering can be done without
> taking a second look at things).

I wouldn't call the first method ugly; I'd say it's *very* OO.

Think of an object instance as a machine. It has various knobs,
switches and dials you can use to control it's behavior, and displays
you can use to read data from it, or parts of its state . A switch
labelled "ignore whitespace" is a perfectly reasonable thing for a
tokenizing machine to have.

Yes, such a switch gets the desired behavior as a side effect. Then
again, a generator that returns tokens has a desired behavior
(advancing to the next token) as a side effect(*). If you think about
these things as the state of the object, rather than "side effects",
it won't seem nearly as ugly. In fact, part of the point of using a
class is to encapsulate the state required for some activity in one

Wanting to do everything via parameters to methods is a very top-down
way of looking at the problem. It's not necessarily correct in an OO


*) It's noticable that some OO languages/libraries avoid this side
effect: the read method updates an attribute, so you do the read then
get the object read from the attribute. That's very OO, but not very
Mike Meyer <mwm at>
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.

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