[Twisted-Python] Maximum recursion depth reached

Steve Freitas sflist at ihonk.com
Fri May 9 05:23:48 EDT 2008

Hi all,

I'm hitting the recursion limit in my code (well, somewhere inside
Twisted, that is), and I'd appreciate any insight as to why. I wouldn't
think what I'm doing -- returning a callback from method A which calls
method B, which then in turn calls method A again -- would be
controversial, since I'd assume Python would be letting go of the stack
for method A, but I guess there's more to it than that.

Code demonstrating the issue follows. Running it results in:
    File "/usr/lib/python2.5/site-packages/twisted/python/reflect.py",
line 550, in accumulateBases
        accumulateBases(base, l, baseClass)
    exceptions.RuntimeError: maximum recursion depth exceeded

(This doesn't actually perfectly reflect my problem in the real code I'm
working with. I'm seeing "'maximum recursion depth exceeded' in <bound
method DebugInfo.__del__ of <twisted.internet.defer.DebugInfo instance
at 0xb1d99b6c>>", but I assume this is coming from the same basic
problem in my code.)


from twisted.internet import defer, reactor

def yo_print(s, d):
    print s
    return s

def some_deferred(item):
    d = defer.Deferred()
    reactor.callLater(0, lambda: yo_print(item, d))
    return d

def start(items):
    dl = []
    for item in items:
        if len(dl) > 4:
    items = items[len(dl):]
    if dl:
        return defer.DeferredList(dl).addCallback(next_batch, items)

def next_batch(_, items):
    return start(items)

if __name__ == '__main__':
    items = []
    for i in range(1651): # 1650 doesn't do it
    print start(items)


So I guess the rule is to never, within a callback chain, call a
function which was invoked earlier in the callback chain?

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