invalid-str-return-type (PLE0307)

Derived from the Pylint linter.

What it does

Checks for __str__ implementations that return a type other than str.

The __str__ method should return a str object. Returning a different type may cause unexpected behavior.

class Foo:
    def __str__(self):
        return True

Use instead:

class Foo:
    def __str__(self):
        return "Foo"