python – What is a NoneType object?

python – What is a NoneType object?

NoneType is the type for the None object, which is an object that indicates no value. None is the return value of functions that dont 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, its returned by when the regex doesnt 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 didnt provide a command-line option and optparse gave you None for that options 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.

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

if obj is None:


if obj is not None:

python – What is a NoneType object?

NoneType is simply the type of the None singleton:

>>> type(None)
<type NoneType>

From the latter link above:


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.

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