[Python] Type Error: Summary of error causes and remedies for'None Type'

[Python] Type Error:'NoneType' object is not iterable Cause and remedy

I was addicted to writing a recursive function, so how to deal with it.

Error that occurred

TypeError: 'NoneType' object is not iterable

In addition to the above, it is also useful for causing and dealing with all errors including "None Tpye".

Other error examples

・ Can only concatenate list(not "NoneType") to list
・ Can't multiply sequence by non-int of type 'NoneType'
・ Unsupported operand type(s) for +: 'NoneType' and 'list'
Such,,,

** ① The execution result of the function without return is "Value None, Type None Type". ** **

** ② When assigning a function to a variable, the function must have a return. ** ** └ Not print. (Because it is not the execution result but one process in the middle of execution) └ return should be the final output of the function. └ It doesn't mean that you only need one return somewhere in the function.

Occurrence Status

Occurs when trying to extract an int from a multidimensional array.

error

arr0 = [1,[2],[[3]]]

def number(arr):
    result=[]
    
    if isinstance(arr, int):
        result.append(arr)
    
    if isinstance(arr, list):
        #Extract elements one by one
        for brr in arr:
            res = number(brr)
            result += res　　#← This is an error
        return result
            
number(arr0)

#output
TypeError: 'NoneType' object is not iterable

** ▼ A little more detailed ** The function number () is assigned to the variable with "res = number (brr)", but since there is no return value in this function itself, None is stored in the variable and the type is NoneType.

"Result + = res" Because I tried to combine this NoneType variable res with the list type variable result.

It is said that res cannot be combined because it is not iterable.

What is NoneTpye?

An error that occurs when no default arguments are passed to the function. The value is None only.

What is (python official) None The only value of type NoneType. None is often used to indicate the absence of a value, such as when no default argument is passed to the function. The assignment to None is invalid and throws a SyntaxError.

** ▼ It looks like this ** When the execution result of the function is empty, the result itself becomes NoneType.

Therefore, if you assign a function whose execution result is empty to a variable, it becomes NoneType.

NoneTpye example

def hello():
    pass

x = hello()
print(type(x))
print(x)

#output
<class 'NoneType'>
None

None Type for print

def hello():
    print("Hello")

x = hello()
print(type(x))

#output
Hello
<class 'NoneType'>

It is output is "Hello", but one that is displayed in the execution process of the function, but does not replace the function itself.

python

def hello():
    return ("Hello")

x = hello()
print(type(x))
print(x)

#output
<class 'str'>
Hello

x is replaced with the string "Hello".

Location of return

It doesn't mean that you only need one or more returns in a function. It is important that a return is returned as the final result of the execution.

▼ An example where return is described but becomes None Type

Example of becoming None Type

def hello(n):
    if n > 2:
        return ("Hello")

x = hello(1)
print(type(x))
print(x)

#output
<class 'NoneType'>
None

Since 1 <2, the output result of the function is empty. (Does not go through the return process)

python

def hello(n):
    if n > 2:
        return ("Hello")

x = hello(3)
print(type(x))
print(x)

#output
<class 'str'>
Hello

function

def hello():
    print("Hello")

print(hello)

#output
<function hello at 0x00000222AE142160>

▼ No parentheses required when assigning the function itself to a variable

Assign function

def hello():
    return ("Hello")

x = hello  #← No parentheses required
print(type(x))
print(x)

#output
<class 'function'>
<function hello at 0x00000222AE1423A0>

▼ Execution example of variable (assigning function)

python

def hello(n):
    if n > 2:
        return ("Hello")
    
x= hello
print(x(3))

#output
Hello

hello () = x ().

** Set return appropriately so that the value assigned to the variable does not become a function. ** **

python

def number(arr):
    result=[]
    
    if isinstance(arr, int):
        result.append(arr)
    
    if isinstance(arr, list):
        #Extract elements one by one
        for brr in arr:
            res = number(brr)
            result += res
    return result  #← Set result as the return value of function number

It worked safely.