recursion function recurring too often-CodePudding

Relatively early in my python journey and coding in general. I've come across an example question that asks that should flatten a list of lists. Now how many lists of lists exist I wouldn't know. Which leads me to doing my first recursion attempt:

array = [[1,2,3,['a','b','c'],4],'d', [5,6],[7],8,9]
flattened = []

def flattenme (iteritem):
    
    for item in iteritem:
        if isinstance(item, Iterable) or isinstance(item, (str,bytes)):
            flattenme(item)
        else:
            flattened.append(item)

When I run this with

print(f"flatten method: {list(flattenme(array))}")

I get the error:

RecursionError: maximum recursion depth exceeded in comparison

Shouldn't

flattenme(item)

at the last time it's called just return the non-list item? I'm not sure what is going wrong and where my understanding of recursion is failing.

CodePudding user response：

Strings and bytes are iterable. Specifically, strings yield strings of length 1 when iterated over.

This means your code recurses infinitely.

Furthermore, your function doesn't return anything.

Here is my suggestion, which makes flattenme a generator that doesn't split strings:

array = [[1,2,3,['a','b','c'],4],'d', [5,6],[7],8,9]

def flattenme (iteritem):
    for item in iteritem:
        if isinstance(item, Iterable) and not isinstance(item, (str,bytes)):
            yield from flattenme(item)
        else:
            yield item