getrecursiondepth

Manlio Perillo NOmanlio_perilloSPAM at libero.it
Tue Sep 28 11:59:40 CEST 2004


On Sun, 26 Sep 2004 17:45:38 GMT, Andrew Dalke <adalke at mindspring.com>
wrote:

>Manlio Perillo wrote:
>> 1) To write code that execute once in a function (as C static
>> variables)
>> 2) To guard against too many recursion
>
>Here's a way to get 1).  (And I don't know how
>checking the stack depth can be used to emulate
>C's static variable.)

def spam(x):
	if getrecursiondepth() == 1:
		# initialization code

This is equivalent to C++ code:

struct Init
{
Init() { /* initialization code */ }
};

void spam(int x)
{
	static Init init;
	...
}

> [...]

>
>In any case, what you're saying is that you
>want to take over control of how to limit
>Python's stack use.  

Not really...

>It also means you can't call someone else's
>code because you don't know how that might
>affect the stack.  

I don't think that calling someone else's code can affect the current
recursion depth.


>If you don't call someone
>else's code then you can always track your
>stack use yourself by passing a stack depth
>value into your recursion
>

This is simply what I want to do!

>def my_function(x, y, z, max_depth = 20):
>   if max_depth == 0: raise RuntimeError( ....)
>   ...
>     my_function(x-1,y+3,z/2, max_depth-1)
>   ...
>
>That is a more common way to solve your problem.
>

I have already used this 'pattern'.
But sometimes I don't want to expose the 'limit' on the argument list.
Actually I have resolved this by doing:

def my_function(x, y, z, __max_depth = 20)

But this means I can't use keyword argument in my function!

>>> Unlike getrecursionlimit it is something that can be
>>>calculated pretty easily.
>>>
>> 
>> I think getrecursiondepth is easy to calculate!
>
>True, and I feel somewhat silly in retrospect for
>having said that.  For some reason I was thinking
>of a non-recusive way to get that information.
>
>> Wait!
>> I have said that the real getrecursiondepth 'redefines' the origin, so
>> that outside any function it always returns 0.
>
>Ahh, I see your point.  But then you don't know
>how much stack space you have available.  

This is not a problem!.
getrecursiondepth is not intended for such things.

> [...]

>BTW,  I can fool your code
>
>def fool_recusion_base(n=950):
>   if n == 0:
>     import pystate
>     return
>   fool_recursion_base(n-1)
>
>fool_recursion_base()
>import pystate
>print pystate.getrecursiondepth()
>
>

Ok, but remember the Python paradigm: we are adult programmers...

>This will probably print a number near -950.
>
>To summarize, I don't see how your solution helps
>you with #1 and think there are better ways to
>handle #2.  Could you post code to show how you
>would use your new function?
>

Anyway I have asked why getrecursiondepth is not included in sys
module because many members of the PyThreadState struct are accessible
from Python.



Regards  Manlio Perillo



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