How to debug recursive functions in Python?

Recursive function is the natural way to solve problems when you encounter functional programming. However it is sometimes hard to realize the run-time execution flow even the function is simple. I used to running python debugger and use "commands" to investigate the behavior:

1. (Pdb) l
2.   1  -> def iseven(n):
3.   2         return True if n == 0 else isodd(n - 1)
4.   3
5.   4     def isodd(n):
6.   5         return False if n == 0 else iseven(n - 1)
7.   6
8.   7     print(iseven(4))
9. [EOF]
10. (Pdb) b 2
11. Breakpoint 1 at /home/mars/try/python/decorator/use_pdb.py:2
12. (Pdb) b 5
13. Breakpoint 2 at /home/mars/try/python/decorator/use_pdb.py:5
14. (Pdb) commands 1
15. (com) p n
16. (com) c
17. (Pdb) commands 2
18. (com) p n
19. (com) c
20. (Pdb) c
21. 4
22. > /home/mars/try/python/decorator/use_pdb.py(2)iseven()
23. -> return True if n == 0 else isodd(n - 1)
24. 3
25. > /home/mars/try/python/decorator/use_pdb.py(5)isodd()
26. -> return False if n == 0 else iseven(n - 1)
27. 2
28. > /home/mars/try/python/decorator/use_pdb.py(2)iseven()
29. -> return True if n == 0 else isodd(n - 1)
30. 1
31. > /home/mars/try/python/decorator/use_pdb.py(5)isodd()
32. -> return False if n == 0 else iseven(n - 1)
33. 0
34. > /home/mars/try/python/decorator/use_pdb.py(2)iseven()
35. -> return True if n == 0 else isodd(n - 1)
36. True
37. The program finished and will be restarted
38. > /home/mars/try/python/decorator/use_pdb.py(1)<module>()
39. -> def iseven(n):
40. (Pdb)

Since I rarely use heavy recursion in Python, I did not feel anything wrong with this approach until seeing an article yesterday. Eli Bendersky uses a decorator to investigate this more effectively. His trick can shows cleaner flow.

1. [~/try/python/decorator] (master) 1410h11m $ python use_trace.py
2. iseven(4)
3.   isodd(3)
4.     iseven(2)
5.       isodd(1)
6.         iseven(0)
7. True
8. [~/try/python/decorator] (master) 1410h12m $ cat use_trace.py
9. from Trace import TraceCalls
10.  
11. @TraceCalls()
12. def iseven(n):
13.     return True if n == 0 else isodd(n - 1)
14.  
15. @TraceCalls()
16. def isodd(n):
17.     return False if n == 0 else iseven(n - 1)
18.  
19. print(iseven(4))

This lead to me thinking more possible applications of decorator. We might use decorator to draw callgraph, logging, profiling, unit testing...etc. On the fly!! Simple and beautiful, isn't it? :-)