visit all elements in nested lists and dicts without recursion

Question

I have a structure consisting of nested lists and dicts. I want to apply a function to every element. How to do it without recursion.

def visit(data, func):
    if isinstance(data, dict):
        for k, v in data.items():
            data[k] = visit(v, func)
        return data
    elif isinstance(data, list):
        for i, v in enumerate(data):
            data[i] = visit(v, func)
        return data
    else:
        return func(data)

The recursive version works for small data, but I hit the RecursionError exception when the data is big.

I looked for general ways to eliminate recursion, the ones I found rely on first transforming the recursive call to a tail call, my problem with this is the recursive call in my example is inside a loop.

Answer 1

This approach will work. For the record, though, I agree with Sven Marnach that there is something definitely fishy going on with your data structures if you have nesting that is breaking the recursion limit. If as Sven conjectures,you have cycles in your data, this approach will also break.

data = [1,2, {'a':1, 'b':{'a':[1,2,3]}},3]

def apply(data, f):
    stack = []
    stack.append(data)
    while stack:
        data = stack.pop()
        if isinstance(data, dict):
            for k,v in data.items():
                if isinstance(v, (dict,list)):
                    stack.append(v)
                else:
                    data[k] = f(v)
        if isinstance(data, list):
            for i,e in enumerate(data):
                if isinstance(e, (dict,list)):
                    stack.append(e)
                else:
                    data[i] = f(e)

In the interpreter shell:

$ python -i apply.py
>>> apply(data, lambda x: x + 1)
>>> data
[2, 3, {'a': 2, 'b': {'a': [2, 3, 4]}}, 4]
>>>