I have been using Python typing's 'NoReturn' wrong

I am a huge fan of python’s new typing module which was introduced in version 3.5 see the version overview here. The addition of this module gives us a range of tools to help people read and understand your code in the future.

Since I have started using typing I have been adding argument types, and return types to almost all of my public functions. One that keeps popping up is NoReturn, and as it turns out I have been using it wrong.

My Mistake

I had completely forgotten about Python’s implicit returns. Almost no function in python actually returns nothing… In my mind for what ever reason I thought that if I don’t explicitly return something then the NoReturn typing makes sense, however this is not the case.

Below is an example of a function that I thought should be typed as NoReturn but was very wrong:

def foobar():
    print("BAZBAR")

The above function does not explicitly return anything, but python will add an implicit return of None when it ends. You can see this by assigning the return value of that function to a variable eg: rtn = foobar() then see the type by running type(rtn), you should get <class 'NoneType'> back! This is an example of python adding return None at the end of your function.

So what is NoReturn for?

The goal of the NoReturn type is to signify that calling this function will end all further processing, it is the point of NO RETURN! This type should be used in functions that will raise an exception, or a function that ends the program, below are a few examples.

Examples of NoReturn used correctly

def raise_error() -> NoReturn:
    """
    Always raise an error.
    """
    raise RuntimeError("An Error occurred")

def quit_program() -> NoReturn:
   """
   Quit the process running this code.
   """
   import sys
   sys.exit()

I now need to go over 100s of lines of code, and fix this mistake! To fix this mistake, I will need to go though all the instances where I have put NoReturn and if in reality the function ends processing, then I will leave it as it is. In most cases though I will need to replace -> NoReturn with -> None .

So what did I learn?

I could have avoided this if I just RTFM a little better! The python docs are a great resource, and I should have read the NoReturn section more carefully. You can find the docs on NoReturn here.

---

Tags: Python, Typing, Mistakes