TypeError: can only concatenate list (not "X") to list [Fix]

# Table of Contents

1. TypeError: can only concatenate list (not "str") to list
2. TypeError: can only concatenate list (not NoneType) to list

# TypeError: can only concatenate list (not "str") to list

The Python "TypeError: can only concatenate list (not "str") to list" occurs when we try to concatenate a list and a string.

To solve the error, use the append() method to add an item to the list, e.g. my_list.append('my item').

Here is an example of how the error occurs.

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my_list = ['apple', 'banana']

my_str = 'melon'

# ⛔️ TypeError: can only concatenate list (not "str") to list
result = my_list + my_str

If you got the error TypeError: can only concatenate list (not NoneType) to list, click on the following subheading:

• TypeError: can only concatenate list (not NoneType) to list

We tried to use the addition (+) operator to add an item to a list which caused the error.

# Use the append method to add an item to the end of a list

You can use the append() method to add an item to a list.

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my_list = ['apple', 'banana']

my_str = 'melon'

my_list.append(my_str)

print(my_list)  # 👉️ ['apple', 'banana', 'melon']

The list.append() method adds an item to the end of the list.

The method returns None as it mutates the original list.

# Use the extend method to add multiple items to a list

The list.extend() method takes an iterable and extends the list by appending all of the items from the iterable.

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list_1 = ['bobby']
list_2 = ['hadz', '.', 'com']

list_1.extend(list_2)

print(list_1)  # 👉️ ['bobby', 'hadz', '.', 'com']

The list.extend method returns None as it mutates the original list.

# Wrap the string in a list and use the addition operator

Alternatively, you can wrap the string in a list if you'd prefer to use the addition (+) operator.

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my_list = ['apple', 'banana']

my_str = 'melon'

result = my_list + [my_str]

print(result)  # 👉️ ['apple', 'banana', 'melon']

However, using the append() and extend() methods is much more common and intuitive.

Using the addition (+) operator to add items to a list doesn't change the original, it returns a new list.

# Use the remove() method if you need to remove an item from a list

Conversely, if you need to remove an item from a list, use the remove() method.

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my_list = ['apple', 'banana']

my_str = 'banana'

my_list.remove(my_str)

print(my_list)  # 👉️ ['apple']

The list.remove() method removes the first item from the list whose value is equal to the passed-in argument.

The method raises a ValueError if there is no such item.

The remove() method mutates the original list and returns None.

# Use a formatted-string literal to print the contents of a list

If you meant to print the contents of the list, use a formatted string literal.

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my_list = ['apple', 'banana']

my_str = 'are fruits'

result = f'{my_list} {my_str}'

print(result)  # 👉️ ['apple', 'banana'] are fruits

Make sure to wrap expressions in curly braces - {expression}.

# Concatenate an item from the list with a string

If you meant to concatenate an item from the list with a string, access the item at the specific index using square brackets.

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my_list = ['apple', 'banana']

my_str = ' is my fav fruit'

result = my_list[0] + my_str

print(result)  # 👉️ 'apple is my fav fruit'

We accessed the list element at index 0, which is a string, so we were able to concatenate the two strings.

If you aren't sure what type of object a variable stores, use the built-in type() class.

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my_list = ['apple', 'banana']
print(type(my_list))  # 👉️ <class 'list'>
print(isinstance(my_list, list))  # 👉️ True


my_str = 'are fruits'
print(type(my_str))  # 👉️ <class 'str'>
print(isinstance(my_str, str))  # 👉️ True

The type class returns the type of an object.

The isinstance() function returns True if the passed-in object is an instance or a subclass of the passed-in class.

# TypeError: can only concatenate list (not NoneType) to list

The Python "TypeError: can only concatenate list (not "NoneType") to list" occurs when we try to concatenate a list and a None value.

To solve the error, correct the assignment or check if the variable doesn't store a None value before concatenating.

Here is an example of how the error occurs.

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my_list = ['a', 'b']

example = None

# ⛔️ TypeError: can only concatenate list (not "NoneType") to list
result = my_list + example

We tried to use the addition (+) operator to concatenate a list and a None value which caused the error.

# The most common sources of None values

The most common sources of a None value are:

1. Having a function that doesn't return anything (returns None implicitly).
2. Explicitly setting a variable to None.
3. Assigning a variable to the result of calling a built-in function that doesn't return anything.
4. Having a function that only returns a value if a certain condition is met.

# A function that doesn't return anything returns None

Here is an example of getting a None value from a function that doesn't return anything (implicitly returns None).

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# 👇️ this function returns None
def get_list():
    print(['c', 'd'])


my_list = ['a', 'b']

# ⛔️ TypeError: can only concatenate list (not "NoneType") to list
result = my_list + get_list()

The get_list function doesn't return anything, so it implicitly returns None.

You can use a return statement to return a value from the function.

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def get_list():
    return ['c', 'd']


my_list = ['a', 'b']

result = my_list + get_list()

print(result)  # 👉️ ['a', 'b', 'c', 'd']

You can use an if statement if you only want to concatenate the values if the variable doesn't store None.

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my_list = ['a', 'b']

my_other_list = None

if my_other_list is not None:
    result = my_list + my_other_list
    print(result)
else:
    #  👇️ this runs
    print('variable stores a None value')

The if block is only run if the my_other_list variable doesn't store a None value, otherwise the else block is run.

# Use a fallback value if the variable stores None

Alternatively, you can assign a fallback value to the variable if it is None.

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my_list = ['a', 'b']

my_other_list = None

if my_other_list is None:
    my_other_list = []

result = my_list + my_other_list

print(result)  # 👉️ ['a', 'b']

We check if the my_other_list variable stores a None value, and if it does, we set it to an empty list.

# A function that returns a value only if a condition is met

Another common source of None values is having a function that only returns a value if a certain condition is met.

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def get_list(a):
    if len(a) > 2:
        return a


result = get_list(['a', 'b'])

print(result)  # 👉️ None

The if statement in the get_list function is only run if the passed-in argument has a length greater than 2.

To solve the error in this scenario, you either have to check if the function didn't return None, or return a default value if the condition is not met.

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def get_list(a):
    if len(a) > 2:
        return a

    return []


result = get_list(['a', 'b'])

print(result)  # 👉️ []

Now the function is guaranteed to return a value regardless of whether the condition is met.