How to resolve TypeError: can only concatenate list (not NoneType) to list in Python

If you are looking for a solution to the error TypeError: can only concatenate list (not NoneType) to list. To fix this, the variable or function value must be a value other than None and check the variable data type before passing it to the function. Details are below.

What causes the TypeError: can only concatenate list (not NoneType) to list error?

The TypeError: can only concatenate list (not NoneType) to list error happens because you’re concatenating a list with a None value.

Example: 

listInfor = ['learnshareit', 'website']
invalidVar = None

# Concatenating a list with a None value will cause an error.
print(listInfor + invalidVar )

Output:

Traceback (most recent call last):
 File "./prog.py", line 5, in <module>
TypeError: can only concatenate list (not "NoneType") to list

The error TypeError: can only concatenate list (not “NoneType”) to list also occurs when you declare a function whose return value is not explicitly.

Example:

listInfor = ['learnshareit', 'website']

def noValueFunc():
    print (['visit'])   

noValue = noValueFunc()
print(listInfor + noValue)

Output:

Traceback (most recent call last):
 File "./prog.py", line 7, in <module>
TypeError: can only concatenate list (not "NoneType") to list

How to solve this error?

Check the NoneType object.

You can use the Authentication operator to check if a variable can validly perform a concatenation.

Example:

• Declare a variable whose value is None.
• Use the Authentication operator. If the variable contains the value None, execute the if statement; otherwise, you can join two objects together.
• You can replace the ‘is’ operator with the ‘is not’ operator (substitute statements accordingly).

invalidVar = None
listInfor = ['learnshareit', 'website']

# Use the 'is' operator
if invalidVar is None:
    print('Object is None')
else:
    print('Object is not None')
    print(invalidVar + listInfor)     

Output:

Object is None

You can use the relational operator ‘!=’ for error handling.

The ‘!=’ operator compares the values ​​of the arguments: if they are different, it returns True. If equal, returns False.

Example:

invalidVar = None
listInfor = ['learnshareit', 'website']

# Use the relational operator '!='
if invalidVar != None:
    print('Object is not None')
    print(invalidVar + listInfor)
    
else:
     print('Object is None')

Output:

Object is None

Use the isinstance() function to check NoneType. 

Example:

invalidVar = None
listInfor = ['learnshareit', 'website']

if isinstance(invalidVar, type(None)) is True:
   print('Object is None')

else:
    result = invalidVar + listInfor
    print(result)

Output:

Object is None

Avoid initializing functions that do not return results or return None.

Create can initialize a variable that is a list or a string as long as it is not None, then concatenating two objects will not cause an error.

Example:

invalidVar = ['visit']
listInfor = ['learnshareit', 'website']

# Use the 'is' operator
if invalidVar is None:
    print('Object is None')

else:
    print('Object is not None')
    result = invalidVar + listInfor     
    print(result)

Output:

Object is not None
['visit', 'learnshareit', 'website']

When you instantiate a function that needs to return an explicit value, which can be a list containing elements, the concatenation will not cause an error.

Example:

listInfor = ['learnshareit', 'website']

def ValueFunc():
    return (['visit'])   

Value = ValueFunc()
print(Value + listInfor)

Output:

['visit', 'learnshareit', 'website']

Summary

The problem TypeError: can only concatenate list (not NoneType) to list in Python is probably solved. If you have any questions or have a more creative way, I hope you will share it with everyone by commenting below. Thank you for reading my article.

Jason Wilson

My name is Jason Wilson, you can call me Jason. My major is information technology, and I am proficient in C++, Python, and Java. I hope my writings are useful to you while you study programming languages.

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Name of the university: HHAU
Major: IT
Programming Languages: C++, Python, Java