Functions Returning True on Success

Functions Returning True on Success

Many beginning programmers latch on to the idea of having functions return True if they succeed and False if they fail so that you have code like this:

if do_something():
    # It worked!
    logger.error("Something didn't work!")

For me, I think it was when I saw some code a teaching assistant wrote in my university operating systems class this way, and it seemed so elegant. But:

This boolean success/fail is actually not such a great idea. Instead, follow this pattern:

A function succeeds or raises an exception. If it raises an exception, make that exception information-rich.

Some of the reasons are in Why Exceptions Are Better Than Returned Error Codes, but there are others for this particular boolean success/fail variation.

What if I have a function that can't fail? Should I do this?

def do_something():
    # do the thing
    return True

Consistency is a good thing, right? So shouldn't all my functions work the same way?

Except that there's now a pointless line of code in this function, and the contract for it implies that at any time I could start returning False and the caller ought to have realized this and added a previously pointless check that it returned successfully. The code bloats, and it is too easy to accidentally miss the return value.

More problematic is the False return value when a function can and does fail. As the caller I'm probably interested in knowing why it failed, but False doesn't give me that. "Oh, that's easy," one might say. "Just do the logging inside the function instead of at the call level."

def do_something():
    if do_some_internal_thing():
        return True
        logger.error("Something failed because of the internal thing!")
        return False

This is also not a great idea because it blurs the distinction between:

  1. Indicating that something went wrong, which is the function's responsibility, and
  2. What to do about it, which is the caller's responsibility

Consider this situation:

def some_use_case():
   if not do_something():
        # No worries, fall back...

The do_something function assumes that because it failed, that must be an error that the application should report. But from the caller's perspective, it isn't an error at all, and now the application log is littered with misleading errors:

ERROR Something failed because of the internal thing!

Note that even when using exceptions, the same problem of confused responsibilities still exists if do_something logs itself:

def do_something():
    except Exception:
        logger.error("Something failed because of the internal thing!")

If do_something can add valuable information to the exception raised by do_some_internal_thing, then it can catch it and put the valuable information into a new exception so that it is available to the caller should it choose to log it. Otherwise, just let the underlying exception through.

Notice that the Python Logging HOWTO page says that the error(), exception() and critical() logging functions are the best tool to "Report suppression of an error without raising an exception" (emphasis added).