What is a 'NoneType' object?

Question

I'm getting this error when I run my python script:

TypeError: cannot concatenate 'str' and 'NoneType' objects

I'm pretty sure the 'str' means string, but I dont know what a 'NoneType' object is. My script craps out on the second line, I know the first one works because the commands from that line are in my asa as I would expect. At first I thought it may be because I'm using variables and user input inside send_command.

Everything in 'CAPS' are variables, everything in 'lower case' is input from 'parser.add_option' options.

I'm using pexpect, and optparse

send_command(child, SNMPGROUPCMD + group + V3PRIVCMD)
send_command(child, SNMPSRVUSRCMD + snmpuser + group + V3AUTHCMD + snmphmac + snmpauth + PRIVCMD + snmpencrypt + snmppriv)

Answer 1

NoneType is the type for the None object, which is an object that indicates no value. None is the return value of functions that "don't return anything". It is also a common default return value for functions that search for something and may or may not find it; for example, it's returned by re.search when the regex doesn't match, or dict.get when the key has no entry in the dict. You cannot add None to strings or other objects.

One of your variables is None, not a string. Maybe you forgot to return in one of your functions, or maybe the user didn't provide a command-line option and optparse gave you None for that option's value. When you try to add None to a string, you get that exception:

send_command(child, SNMPGROUPCMD + group + V3PRIVCMD)

One of group or SNMPGROUPCMD or V3PRIVCMD has None as its value.

Answer 2

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

if obj is None:

or

if obj is not None:

Answer 3

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.

Answer 4

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

Answer 5

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

Answer 6

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

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

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

Answer 11

NoneType is the type of None.

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

Answer 12

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