Python – When to use a dictionary vs tuple in Python

Using a dict let's you translate the key into a callable. The key doesn't need to be hardcoded though, as in your example.

Usually, this is a form of caller dispatch, where you use the value of a variable to connect to a function. Say a network process sends you command codes, a dispatch mapping lets you translate the command codes easily into executable code:

def do_ping(self, arg):
    return 'Pong, {0}!'.format(arg)

def do_ls(self, arg):
    return '\n'.join(os.listdir(arg))

dispatch = {
    'ping': do_ping,
    'ls': do_ls,
}

def process_network_command(command, arg):
    send(dispatch[command](arg))

Note that what function we call now depends entirely on what the value is of command. The key doesn't have to match either; it doesn't even have to be a string, you could use anything that can be used as a key, and fits your specific application.

Using a dispatch method is safer than other techniques, such as eval(), as it limits the commands allowable to what you defined beforehand. No attacker is going to sneak a ls)"; DROP TABLE Students; -- injection past a dispatch table, for example.

You should return boolean when the method/function is useful in making logical decisions.

You should throw an exception when the method/function isn't likely to be used in logical decisions.

You have to make a decision about how important the failure is, and if it should be handled or not. If you could classify the failure as a warning, then return boolean. If the object enters a bad state that make future calls to it unstable, then throw an exception.

Another practice is to return objects instead of a result. If you call open, then it should return a File object or null if unable to open. This ensures the programmers has an object instance that is in a valid state that can be used.

EDIT:

Keep in mind that most languages will discard the result of a function when it's type is boolean or integer. So it's possible to call the function when there is no left hand assignment for the result. When working with boolean results, always assume the programmer is ignoring the returned value and use that to decide if it should rather be an exception.