What is a 'NoneType' object?

It means you’re trying to concatenate a string with something that is None.

None is the “null” of Python, and NoneType is its type.

This code will raise the same kind of error:

>>> bar = "something"
>>> foo = None
>>> print foo + bar
TypeError: cannot concatenate 'str' and 'NoneType' objects

NoneType is simply the type of the None singleton:

>>> type(None)
<type 'NoneType'>

From the latter link above:

None

The sole value of the type NoneType. None is frequently used to represent the absence of a value, as when default arguments are not passed to a function. Assignments to None are illegal and raise a SyntaxError.

In your case, it looks like one of the items you are trying to concatenate is None, hence your error.

In Python, to represent the absence of a value, you can use the None value types.NoneType.None

Your error’s occurring due to something like this:
>>> None + "hello world"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
>>>

Python’s None object is roughly equivalent to null, nil, etc. in other languages.

In the error message, instead of telling you that you can’t concatenate two objects by showing their values (a string and None in this example), the Python interpreter tells you this by showing the types of the objects that you tried to concatenate. The type of every string is str while the type of the single None instance is called NoneType.

You normally do not need to concern yourself with NoneType, but in this example it is necessary to know that type(None) == NoneType.

For the sake of defensive programming, objects should be checked against nullity before using.

if obj is None:

or

if obj is not None:

One of the variables has not been given any value, thus it is a NoneType. You’ll have to look into why this is, it’s probably a simple logic error on your part.

NoneType is the type of None.

See the Python 2 docs here:
https://docs.python.org/2/library/types.html#types.NoneType

It’s returned when you have for instance print as your last statement in a function instead of return:

def add(a, b):
    print(a+ b)

x = add(5,5)
print(x)
print(type(x))

y = x + 545
print(y)

TypeError: unsupported operand type(s) for +: ‘NoneType’ and ‘int’
<class ‘NoneType’>

def add(a, b):
    return (a+ b)

x = add(5,5)
print(x)
print(type(x))

10
<class 'int'>
555

In Python

• NoneType is the type of the None object.
• There is only one such object.
  Therefore, "a None object" and "the None object" and
  "None" are three equivalent ways of saying the same thing.
• Since all Nones are identical and not only equal,
  you should prefer x is None over x == None in your code.
• You will get None in many places in regular Python
  code as pointed out by the accepted answer.
• You will also get None in your own code when you
  use the function result of a function that does not end with
  return myvalue or the like.

Representation:

• There is a type NoneType in some but not all versions of Python,
  see below.
• When you execute print(type(None)), you will get
  <type 'NoneType'>.
  This is produced by the __repr__ method of NoneType.
  See the documentation of repr
  and that of
  magic functions
  (or "dunder functions" for the double underscores in their names) in general.

In Python 2.7

• NoneType is a type defined in the
  standard library module types

In Python 3.0 to 3.9

• NoneType has been
  removed
  from
  module types,
  presumably because there is only a single value of this type.
• It effectively exists nevertheless, it only has no built-in name:
  You can access NoneType by writing type(None).
• If you want NoneType back, just define
  NoneType = type(None).

In Python 3.10+

• NoneType is again a type defined in the
  standard library module types,
  introduced in order to
  help type checkers do their work

If you’re getting type None for an object, make sure you’re returning in the method. For example:

class Node:
    # node definition

then,

def some_funct():
    # some code
    node = Node(self, self.head)
    self.head = node

if you do not return anything from some_func(), the return type will be NoneType because it did not return anything.

Instead, if you return the node itself, which is a Node object, it will return the Node-object type.

def some_func(self):
    node = Node(self, self.head)
    self.head = node
    return node

NoneType is type of None. Basically, The NoneType occurs for multiple reasons,

• Firstly when you have a function and a condition inside (for instance), it will return None if that condition is not met.
  Ex:-
  def dummy(x, y): if x > y: return x res = dummy(10, 20) print(res) # Will give None as the condition doesn't meet.

To solve this return the function with 0, I.e return 0, the function will end with 0 instead of None if the condition is not satisfied.

• Secondly, When you explicitly assign a variable to a built-in method, which doesn’t return any value but None.
  my_list = [1,2,3]
  my_list = my_list.sort()
  print(my_list) #None sort() mutate the DS but returns nothing if you print it.
  Or
  lis = None
  re = lis.something())
  print(re) # returns attribute error NonType object has no attribute something