Recursion and Generators
eppstein at ics.uci.edu
Sat Jul 10 20:36:54 CEST 2004
In article <b4a8ffb6.0407101006.567e82eb at posting.google.com>,
tkpmep at hotmail.com (Thomas Philips) wrote:
> I'm totally comfortable with it backing up levels, yielding a new
> result each time. It is intuitive to me that at each level it should
> yield a list followed by a StopIteration. What is not clear to me is
> why it feels an uncontrollable urge to descend through the list of
> nested calls to testRecursion one last time after it has come up to
> the highest level and yielded [1,2,3,4] StopIteration.
After it has yielded at the top level, all of your recursive for-loops
are still looping, and won't stop looping until each generator has been
activated again and has raised a StopIteration.
E.g. consider the following greatly-simplified example
for x in gen():
What actually happens here:
gen() is called and creates a generator g. g is not called yet.
First iteration of loop:
g is called for the first time and yields 17. Execution of g is
suspended within the yield statement. 17 is printed.
Second iteration of loop:
g is called for the second time; execution of g is continued after the
yield statement. Since there is nothing left of g, it raises
StopIteration. The exception causes the for loop to exit.
In your recursive example, the state of the generators "after it has
come to the highest level and yielded [1,2,3,4]" is the same as in this
smaller example between the first and second iterations: the generators
are all suspended within their yield statements. You need to call into
them again to get them to raise StopIteration.
David Eppstein http://www.ics.uci.edu/~eppstein/
Univ. of California, Irvine, School of Information & Computer Science
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