TypeError: __str__ returned non-string (type NoneType)

# TypeError: str returned non-string (type NoneType)

The Python "TypeError: __str__ returned non-string (type NoneType)" occurs when we forget to return a value from the __str__() method.

To solve the error, make sure to return a string from the method.

typeerror str returned non string type nonetype

Here is an example of how the error occurs.

main.py

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class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def __str__(self):
        print(self.name) # 👈️ Forgot to return a value


emp = Employee('Bobby Hadz', 100)

# ⛔️ TypeError: __str__ returned non-string (type NoneType)
print(emp)

We forgot to return a value from the __str__() method in the Employee class.

# Use a `return` statement to return a value from `str()`

To solve the error, use a return statement to return a string from the method.

main.py

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class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def __str__(self):
        return self.name # 👈️ return a string


emp = Employee('Bobby Hadz', 100)

print(emp) # 👉️ Bobby Hadz

use return statement to return value from str method

The return value of the __str__() method must be a string object.

# The `str()` method must return a value of type string

If the value you are returning is not a string, use the str() class to convert it.

main.py

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class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def __str__(self):
        return str(self.salary)  # 👈️ Return a string


emp = Employee('Bobby Hadz', 100)

print(emp)  # 👉️ 100

the str method must return value of type string

We used the str class to convert the integer to a string.

This is necessary because returning a value of a different type from the __str__ method is not allowed.

Here is an example that tries to return an integer from the method.

main.py

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class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def __str__(self):
        # ⛔️ TypeError: __str__ returned non-string (type int)
        return self.salary


emp = Employee('Bobby Hadz', 100)

print(emp)  # 👉️ 100

dont return an integer from str method

The code sample causes the following error.

shell

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Traceback (most recent call last):
  File "/home/borislav/Desktop/bobbyhadz_python/main.py", line 12, in <module>
    print(emp)  # 👉️ 100
    ^^^^^^^^^^
TypeError: __str__ returned non-string (type int)

We tried to return an integer from the __str__ method, which caused the TypeError.

You can use a formatted string literal if you need to include expressions in a string.

main.py

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class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def __str__(self):
        return f'Name of employee: {self.name}'


emp = Employee('Alice', 100)

print(emp)  # 👉️ Name of employee: Alice

Formatted string literals (f-strings) let us include expressions inside of a string by prefixing the string with f.

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

You can also use the addition (+) operator to concatenate strings.

main.py

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class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def __str__(self):
        return 'Salary: ' + str(self.salary)


emp = Employee('Bobby Hadz', 100)

print(emp)  # 👉️ Salary: 100

using addition operator to concatenate strings

Note that we had to use the str() class to convert the salary attribute to a string.

This is necessary because the values on the left and right-hand sides of the addition (+) operator need to be of compatible types (e.g. both strings or both numbers).

The __str__() method is called by str(object) and the built-in functions format() and print() and returns the informal string representation of the object.

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TypeError: __str__ returned non-string (type NoneType)