[Solved] TypeError: str returned non-string (type NoneType)

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The Python error “TypeError: str returned non-string (type NoneType)” means you aren’t returning a string from your class’ __str__ method. To fix this, make sure you’re actually returning a string.

Python has methods called “dunder” methods, which is shorthand for “double underscore”. They are special methods you can add to your classes to do things like, in this case, automatically convert your instantiated class to a string when treated as such (for example, when placed in an f-string like this: f"My Class: {my_class}".

These “dunder” methods only work if you hold up your end of the bargain; i.e., return the type they’re supposed to be converting to.

Problem: you’re not returning a str from __str__

Let’s say you have the following class defined, and are trying to make use of the __str__ dunder method:

class MyClass:
    def __init__(self, some_string):
        self.my_string = some_string

    def __str__(self):
        print(self.my_string)

my_class = MyClass("Hello world!")
print(str(my_class))
Python

If you’re new to Python, this might seem correct to you. After all, you want to print the value of my_string when you treat the class instance as a string. However, if you try to run this, you’ll get the following error:

Hello world!
Traceback (most recent call last):
  File "/home/user/main.py", line 9, in <module>
    print(str(my_class))
TypeError: __str__ returned non-string (type NoneType)

As you can see, it first printed out “Hello world!” like we were expecting, then it threw an error complaining that you didn’t return a str from the method. Well, it’s not lying, we didn’t! We called print which doesn’t return anything, not to mention we never put a return statement in our method to begin with. And when a method doesn’t “return” anything, it really returns the value None, hence the error message.

Solution: make sure you’re returning a string

This is an easy enough problem to fix. Just return self.my_string instead of printing it:

class MyClass:
    def __init__(self, some_string):
        self.my_string = some_string

    def __str__(self):
        return self.my_string

my_class = MyClass("Hello world!")
print(str(my_class))
# -> Hello world!
Python

Conclusion

Python’s special “dunder” method __str__ must return a string, or you’ll get this error. Take a look at your stack trace, see where it’s originating, and make sure that the __str__ method it’s complaining about is actually returning a str.