This is supposed to be a program in Python which returns the position of a target element in a list recursively:

def linear_search_arrayList(arr, target, arr2 = [], num = 0):
    if(len(arr) == num):
        return arr2
    elif(arr[num]==target):
        return linear_search_arrayList(arr,target, arr2.append(num) , num 1)
    else:
        return linear_search_arrayList(arr,target, arr2, num  1)
    
    
array_list = [1,2,3,4,6,7,4,9]
print(linear_search_arrayList(array_list, 4))

This program returns an error

AttributeError: 'NoneType' object has no attribute 'append'

CodePudding user response：

this is because you are passing to linear_search_arrayList the arr2.append(num) argument, witch returns None, so you are passing the None argument to the function, since you cannot append a value to a NoneType object, you cannot pass that.

if your purpose was to add to the list and then pass it through the function, you can do:

def linear_search_arrayList(arr, target, arr2 = [], num = 0):

    if(len(arr) == num):
        return arr2
    elif(arr[num]==target):
        return linear_search_arrayList(arr,target, arr2 [num] , num 1)
    else:
        return linear_search_arrayList(arr,target, arr2, num  1)
    
    
array_list = [1,2,3,4,6,7,4,9]
print(linear_search_arrayList(array_list, 4))

CodePudding user response：

This code had several issues.

1. The name linear_search_arrayList does not fit Python's style guidelines; functions have lowercase names (in the snake_case format)
2. You had unnecessary parentheses in your if an elif statements.
3. Having arr2 = None in your function arguments means that arr2 will always point to the same list, which is generated once the moment the program is run. If you want it to be a new, empty list every time, you have to explicitly create a new list. A good practice for this is to set the default value to None, then check for that with is None. You should not do == None because that is not good practice in Python (explanation).
4. The append error was happening because append returns nothing; it affects the original list. If you want to append something to a list, you can either do it beforehand like arr2.append(num), or do arr2 [num] if you don't want to affect arr2.

With all that in mind, here is my attempt at fixing your code; you'll have to determine if it functions how you want, though.

def linear_search_array_list(arr, target, arr2=None, num=0):
    if arr2 is None:
        arr2 = []
    if len(arr) == num:
        return arr2
    elif arr[num] == target:
        return linear_search_array_list(arr, target, arr2   [num], num   1)
    else:
        return linear_search_array_list(arr, target, arr2, num   1)


array_list = [1, 2, 3, 4, 6, 7, 4, 9]
print(linear_search_array_list(array_list, 4))