Python TypeError: issubclass() arg 1 must be a class - Solution and Explanation

The error message 'TypeError: issubclass() arg 1 must be a class' occurs when you pass something other than a class as the first argument to the issubclass() function in Python. The issubclass() function is used to check if one class is a subclass of another class.

To resolve this error, ensure that the argument passed to issubclass() is a valid class. Let's see an example:

class MyClass:
    pass

class MySubClass(MyClass):
    pass

print(issubclass(MySubClass, MyClass))  # Output: True

In this example, MySubClass is a subclass of MyClass, and therefore issubclass(MySubClass, MyClass) returns True.

Common Causes of the Error:

• Passing a variable: You might be mistakenly passing a variable or an instance of a class instead of the class itself.
• Typo: Double-check that you've spelled the class name correctly.
• Incorrect Import: If you're using a class from another module, make sure you've imported it correctly.

Remember: The issubclass() function requires a class as its first argument to determine if it's a subclass of the second argument. Always verify you're passing valid classes to avoid this error.