Recursion in Python RuntimeError maximum recursion depth exceeded while calling a Python object

Question

Possible Duplicate    Maximum recursion depth        I have another problem with my code   I m wrtiting my first program in Vpython and I have to make a simulation of mixing two gases  First i had a problem with borders  but now when the balls that represents the gas particles  stay within the borders there is sth different wrong  After a few seconds i get an error  which is shown below the source code of my function   Code   def MovingTheBall listOfBalls position numCell flagOfExecution       flag   0     if flagOfExecution  0          positionTmp   position     else          positionTmp    position 0  choice  -1 0 1   position 1  choice  -1 0 1   0      for i in range  0  len listOfBalls             if positionTmp  listOfBalls i  pos              flag 1       if flag  1          return MovingTheBall lista  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1      else          if positionTmp 0   0 or positionTmp 0  gt  numCell or positionTmp 0  lt  -numCell or positionTmp 1  gt  numCell or positionTmp 1  lt  -numCell              return MovingTheBall lista  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1           return positionTmp   the error is       return MovingTheBall listOfBalls  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1    File  gaz txt   line 138  in MovingTheBall     return MovingTheBall listOfBalls  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1    File  gaz txt   line 138  in MovingTheBall     return MovingTheBall listOfBalls  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1    File  gaz txt   line 138  in MovingTheBall     return MovingTheBall listOfBalls  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1    File  gaz txt   line 138  in MovingTheBall     return MovingTheBall listOfBalls  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1    File  gaz txt   line 138  in MovingTheBall     return MovingTheBall listOfBalls  position 0  choice  -1 0 1   position 1  choice  -1 0 1   0  numCell 1    File  gaz txt   line 130  in MovingTheBall     if positionTmp  listOfBalls i  pos  RuntimeError  maximum recursion depth exceeded while calling a Python object   Can anybody think of a way to simplify my function   I run the function it while loop   while 1          rate 20          for i in range 0 len self listOfBalls                self listOfBalls i  pos poruszanie self listOfBalls self listOfBalls i  pos self numCell 0

User · Answer

That s the error you get when a function makes too many recursive calls to itself  It might be doing this because the base case is never met  and therefore it gets stuck in an infinite loop  or just by making an large number of calls to itself  You could replace the recursive calls with while loops

User · Answer

I ve changed the recursion to iteration    def MovingTheBall listOfBalls position numCell   while 1      stop 1     positionTmp    position 0  choice  -1 0 1   position 1  choice  -1 0 1   0      for i in range 0 len listOfBalls            if positionTmp  listOfBalls i  pos              stop 0     if stop  1          if  positionTmp 0   0 or positionTmp 0  gt  numCell or positionTmp 0  lt  -numCell or positionTmp 1  gt  numCell or positionTmp 1  lt  -numCell               stop 0         else              return positionTmp   Works good  D

User · Answer

The error is a stack overflow  That should ring a bell on this site  right  It occurs because a call to poruszanie results in another call to poruszanie  incrementing the recursion depth by 1  The second call results in another call to the same function  That happens over and over again  each time incrementing the recursion depth   Now  the usable resources of a program are limited  Each function call takes a certain amount of space on top of what is called the stack   If the maximum stack height is reached  you get a stack overflow error

User · Answer

Python lacks the tail recursion optimizations common in functional languages like lisp  In Python  recursion is limited to 999 calls  see sys getrecursionlimit    If 999 depth is more than you are expecting  check if the implementation lacks a condition that stops recursion  or if this test may be wrong for some cases   I dare to say that in Python  pure recursive algorithm implementations are not correct safe  A fib   implementation limited to 999 is not really correct  It is always possible to convert recursive into iterative  and doing so is trivial    It is not reached often because in many recursive algorithms the depth tend to be logarithmic  If it is not the case with your algorithm and you expect recursion deeper than 999 calls you have two options   1  You can change the recursion limit with sys setrecursionlimit n  until the maximum allowed for your platform      sys setrecursionlimit limit         Set the maximum depth of the Python interpreter stack to limit  This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python       The highest possible limit is platform-dependent  A user may need to set the limit higher when she has a program that requires deep recursion and a platform that supports a higher limit  This should be done with care  because a too-high limit can lead to a crash    2  You can try to convert the algorithm from recursive to iterative  If recursion depth is bigger than allowed by your platform  it is the only way to fix the problem  There are step by step instructions on the Internet and it should be a straightforward operation for someone with some CS education  If you are having trouble with that  post a new question so we can help

[python] Recursion in Python? RuntimeError: maximum recursion depth exceeded while calling a Python object

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