Tracking recursion depth with Python decorators

Question

I am trying to write a decorator that tracks the recursion depth of a recursive function in Python.

Take, for example, a recursive function such as


def fib(n):
    if n == 0:
        return 0
    if n == 1:
        return 1
    else:
        return fib(n-1) + fib(n-2)

Usually, to track recursion depth you could write something like so


    def fib(n, level=0):
        print(level)
        if n == 0:
            return 0
        if n == 1:
            return 1
        else:
            return fib(n-1, level=level+1) + fib(n-2, level=level+1)

However, after fiddling around with decorators for a while and a lot of googling, I'm not sure this is even possible.

I have tried things like


def visualise(func):
    def modify(*args, **kwargs):
        kwargs["count"] += 1
        print(kwargs["count"])
        return func(*args, **kwargs)

    return modify

@visualise
def fib(n):
    ...

fib(4, count=0)

But it complains that count is an unexpected keyword argument, which I don't really understand because I was under the impression that the wrapper modify replaces all occurrences of fib but I guess not?

Pointers would be greatly appreciated.

The problem is you pass `kwargs` to the wrapped function – juanpa.arrivillaga Aug 04 '18 at 23:39 — juanpa.arrivillaga, Aug 04 '18 at 23:39

score 6 · Accepted Answer · answered Aug 04 '18 at 23:11

6

You can specify level variable for decorator and then use it in function calls like this:

def visualise(func):
     visualise.level = 0
     def wrapper(*args, **kwargs):
         print("In:", visualise.level)
         visualise.level += 1
         result = func(*args, **kwargs)
         visualise.level -= 1
         print("Out:", visualise.level)
         return result
     return wrapper

answered Aug 04 '18 at 23:11

Lev Zakharov

2,409
1
10
24

1

Don't use a function attribute, just use the closure, or else your decorator is only good for one function – juanpa.arrivillaga Aug 04 '18 at 23:40
1

For those who want to know what _closures_ are: https://stackoverflow.com/a/2796916/4594973 – code_dredd Jan 13 '19 at 10:45

score 3 · Answer 2 · answered Aug 09 '21 at 19:19

Here's a class based approach of the previous answer from Lev Zakharov above

from functools import wraps
class Visualize(object):
    def __init__(self):
        self.level = 0

    def __call__(self, func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            print("In:", self.level)
            self.level += 1
            result = func(*args, **kwargs)
            self.level -= 1
            print("Out:", self.level)
            return result
        return wrapper

@Visualize()
def fib(n):
...

About @wraps see here

score 0 · Answer 3 · answered Aug 04 '18 at 23:14

0

Could you not define the function as the following?

def fib(n, count=0):
    # ....

answered Aug 04 '18 at 23:14

N Chauhan

3,407
2
7
21

But now you need to modify the original function and then you might as well not even have a decorator. – Quinten C Jun 03 '20 at 23:09

Tracking recursion depth with Python decorators

3 Answers3

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