Question

Shall same datatypes be used to return from functions, or assign default values ? or None ? What is a better coding practice, and why ?

eg. Some pseudo codes in python :

/1

def my_position():   # returns a positive integer if found
    if(object is present):
          position = get_position()
          return position # eg 2,3,4,6
    else: 
          return None     # or return -1 or 0 ??

/2

def get_database_rows():    
    do query to whatever database
    if(rows are found):
       return [list of rows]
    else:
       return None  # or return empty list []  ?

/3

the_dictionary = {'a' : 'john','b':'mike','c': 'robert' }  # values are names i.e. non empty string
my_new_var = the_dictionary.get('z', None)  # or the_dictionary.get('z','')  ?

Answer 1

1. Raise an IndexError if the item is not found. This is what Python’s list does. (Or maybe return the index where the item should have lived when doing a binary search or similar operation.)

2. Think about what your function does, logically: if it returns a list of all items in a DB that satisfy some criterion, and there are no such items, then it makes sense to return an empty list since that allows all the usual list operations (len, in) to function without the need for an explicit check.

   However, if the absence of the required items indicates inconsistency, then raise an exception.

3. My previous remark applies especially to this case: it depends on what you’re going to do with the value you get. An ordinary dict just raises a KeyError when a key is not found. You’re replacing that exception with a value, so you should know which value makes sense in the context of your program. If no value does, then just let the exception fly.

That said, returning None is often a bad idea because it may obscure bugs. None is the default return value in Python, so a function returning it may indicate nothing more than its author’s forgetting a return statement:

def food(what):
    if what == HAM:
        return "HAM!"
    if what == SPAM:
        return " ".join(["SPAM" for i in range(10)])
    # should raise an exception here

lunch = food(EGGS)    # now lunch is None, but what does that mean?