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<p class=MsoNormal>Hello there.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Consider this code:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>def factorial(n):<o:p></o:p></p>
<p class=MsoNormal> def helper(n):<o:p></o:p></p>
<p class=MsoNormal> if n:<o:p></o:p></p>
<p class=MsoNormal> return n*helper(n-1)<o:p></o:p></p>
<p class=MsoNormal> else:<o:p></o:p></p>
<p class=MsoNormal> return 1<o:p></o:p></p>
<p class=MsoNormal> return helper(n)<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Ok, this is a bit contrived, but there are other valid
reasons for calling a closure recursively.<o:p></o:p></p>
<p class=MsoNormal>The problem with this is that once you have called
factorial() once, you end up with a recursive cycle. „factorial“
has become a cell object, referencing the „helper“ function, which
again refers to the outer cell object. This requires „gc“ to clean
up. Also, it is entirely non-obvious. the problem becomes worse if the inner
function also refers to some large, temporary variable, since it will get
caught up in the reference loop.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>A workaround is this. Replace the final line with:<o:p></o:p></p>
<p class=MsoNormal> try:<o:p></o:p></p>
<p class=MsoNormal> return helper(n)<o:p></o:p></p>
<p class=MsoNormal> finally:<o:p></o:p></p>
<p class=MsoNormal> helper = None<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>This clears the cell before function exit. Note that it is
not possible to „del“ it, you get a syntax error for that.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Now, the reason I‘m posting this here, and not to
python-lang, is this: Is there a way to implement this differently so that
this pattern doesn‘t result in a cycle?<o:p></o:p></p>
<p class=MsoNormal>Possible ideas would be:<o:p></o:p></p>
<p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span
style='mso-list:Ignore'>1)<span style='font:7.0pt "Times New Roman"'>
</span></span><![endif]>clear „helper“ on function exit. Not good
if the internal function is exported, though.<o:p></o:p></p>
<p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span
style='mso-list:Ignore'>2)<span style='font:7.0pt "Times New Roman"'>
</span></span><![endif]>Use have ‚helper‘ store a weakref to the
cell object<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Cheers,<o:p></o:p></p>
<p class=MsoNormal>Kristján <o:p></o:p></p>
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