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How to Resolve "TypeError: str returned non-string" in Python

The TypeError: __str__ returned non-string (type NoneType) error (or a similar error with a different type like int, list, etc.) occurs in Python when the __str__() method of a class does not return a string object. The __str__() method is a special method that's used to define the "informal" or user-friendly string representation of an object. It's what's called when you use str(object), print(object), or format strings with an object.

This guide explains the cause of this error and demonstrates how to fix it.

Understanding the __str__() Method​

The __str__() method is a special ("dunder" or "magic") method in Python classes. Its purpose is to provide a string representation of an object that is:

  • Readable: Intended for end-users.
  • Informal: A clear, concise description, not necessarily a complete representation.

When you call str(object), print(object), or use an object in an f-string, Python automatically calls the object's __str__() method (if it exists) to get a string representation. If __str__() is not defined, Python falls back to __repr__(). If neither method is available, it will return something like <__main__.Employee object at 0x...>.

Common Causes and Solutions​

Forgetting to return a Value**​

The most common mistake is forgetting the return statement in __str__():

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

    def __str__(self):
        print(self.name)  # ⛔️ ERROR:  Prints to console, but RETURNS NONE

emp = Employee('Tom Nolan', 100)
print(emp) # ⛔️ TypeError: __str__ returned non-string (type NoneType)

  • Problem: The __str__() method prints the name, but it doesn't return anything. Functions that don't explicitly return a value implicitly return None.

  • Solution: Always return a string from __str__():

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

        def __str__(self):
            return self.name  # ✅ Correct: Returns a string

Returning a Non-String Value​

The __str__() method must return a string. Returning anything else (an integer, a list, a dictionary, None, etc.) will cause the TypeError.

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

    def __str__(self):
        return self.salary  # ⛔️ ERROR: Returns an integer, not a string

emp = Employee('Tom Nolan', 100)
# print(emp) # TypeError: __str__ returned non-string (type int)

Solution: Convert the value to a string before returning it, typically using str() or an f-string:

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

    def __str__(self):
        return str(self.salary)  # ✅ Correct: Returns a string

Using f-strings and String Concatenation in __str__()​

Within __str__(), you'll often want to create a formatted string representation of your object. F-strings are generally the most readable way to do this:

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

    def __str__(self):
        return f'Name of employee: {self.name}' # Use f-strings for clarity


emp = Employee('Alice', 100)
print(emp) # Output: Name of employee: Alice

You can also use string concatenation, but this is less readable:

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

    def __str__(self):
        return 'Salary: ' + str(self.salary) # Concatenate with string.

Conclusion​

The TypeError: __str__ returned non-string error is always caused by an incorrect return type from the __str__() method.

  • Ensure that your __str__() method always returns a string object.
  • Use str() to convert other types to strings if necessary, and prefer f-strings for creating formatted string representations of your objects.

By following these guidelines, you'll avoid this common error and create classes with clear and helpful string representations.

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