Fix Python TypeError: can only concatenate str (not "NoneType") to str

Python shows TypeError: can only concatenate str (not "NoneType") to str when you try to concatenate a string with a None value with the + operator.

To fix this error, you need to avoid concatenating a None value with a string.

Let’s see an example. Suppose you have the Python code below:

x = None

# ❌ TypeError: can only concatenate str (not "NoneType") to str
print("The value of x is " + x)

Because the x variable is None, concatenating it with a string in the print() function produces the error:

Traceback (most recent call last):
  File ...
    print("The value of x is " + x)
TypeError: can only concatenate str (not "NoneType") to str

To avoid concatenating a None with a string, you can first check the value of your variable using an if statement.

When the variable is not None, then concatenate the variable to a string as shown below:

x = None

if x is not None:
    print("The value of x is " + x)
else:
    print("The value of x is None")

Alternatively, you can also provide a default value for the variable when the value is None as follows:

x = None

if x is None:
    x = "Y"

print("The value of x is " + x)  # ✅

By adding a default value to replace None, you ensure that the TypeError: can only concatenate str (not "NoneType") to str is avoided.

The None value is a special value in Python that represents the absence of a value or a null value. It is often used to indicate that a variable or a function has no return value.

To avoid the TypeError, you need to be aware of the circumstances under which the None value is produced by Python and handle it appropriately in your code.

Several common sources of None value when running Python code are:

1. Assigned None to a variable explicitly
2. Calling a function that returns nothing
3. Calling a function that returns only under a certain condition

You’ve seen an example of the first case, so let’s look at the second and third cases.

When you call a function that has no return statement, the function will return a None value implicitly.

Consider the code below:

def greet():
    pass


result = greet()
print(result)  # None

When a function doesn’t have a return statement defined, it returns None.

This also applies to Python built-in functions, such as the sort() method of the list object.

listings = [5, 3, 1]

# call sort() on list
output = listings.sort()

print(output)  # None
print(1 in listings)  # True ✅

The sort() method sorts the original listings variable without returning anything. When you assign the result to a variable, that variable contains None.

Finally, you can also get None when you have a function that has a conditional return statement.

Consider the greet() function below:

def greet(name):
    if name:
        return f"Hello {name}!"


# Call greet with empty string
output = greet("")
print(output)  # None

output = greet("Nathan")
print("n" in output)  # True ✅

The greet function only returns a string value when the name variable evaluates to True.

Since an empty string evaluates to False, the first call to the greet function returns None.

To avoid having None returned by your function, you need to add another return statement as follows:

def greet(name):
    if name:
        return f"Hello {name}!"
    return "Hello Unknown!"

In the code above, the second return statement will be executed when the name variable evaluates to False.

Now you’ve learned several common cases that may cause a None value to enter your code.

Conclusion

The TypeError: can only concatenate str (not "NoneType") to str error in Python occurs when you concatenate a None value with a string value.

To fix this error, you need to avoid concatenating a None with a string.

You can do this by using an if .. is not None statement to check if the variable is None before concatenating it.

You can also use the if .. is None expression and provide a default value to use when the variable contains None.