Null references

In an earlier post, I described my distaste for null references. I thought I should elaborate a little more with an example. So first, in Python, consider the following.

#Example; Null references.

if __name__=="__main__":
ref1=None
ref2=True
ref3=False

print "REF1",ref1==True
print "REF2",ref2==True
print "REF3",ref3==True

Here we have defined three variables; ref1, ref2, and ref3. In this example, ref1 is supposed to represent a null reference. It is defined as None. This means it is pseudo-defined. There is really no way to differentiate between ref1 and ref3 when running this example. They both evaluate to False. From a developer perspective, this doesn't make things very easy. The two variables (ref1 and ref3), are defined differently yet yield the same result. What gives?

Normally this wouldn't make a difference. Certain variables will have different content but will evaluate to True in truth tests. For instance, consider the following.

#Example; Null references.

if __name__=="__main__":
  ref1=""
  ref2="My String"
  ref3=None

  if ref1:
      print "REF1 TRUE"
  else:
      print "REF1 FALSE"      
  if ref2:
      print "REF2 TRUE"
  else:
      print "REF2 FALSE"
  if ref3:
      print "REF3 TRUE"
  else:
      print "REF3 FALSE"

Here, we have altered the definitions of ref1, ref2, and ref3. ref1 is now an empty string. ref2 is now a non-empty string. ref3 is a null reference. Running this example will result in the following:

• ref1 is False because it is empty. But, it is still a string.
• ref2 is True because it is a non-empty string.
• ref3 is False because it is a null reference and hence has no way of knowing if it is True.

Finally, this last example shows how this null reference can really cause havoc.

#Example; Null references.

def print_length(str_obj):
   print len(str_obj)

if __name__=="__main__":
   ref1=""
   ref2="My String"
   ref3=None
  
   print_length(ref1)
   print_length(ref2)
   print_length(ref3)

Here we have the exact some reference variables as the last example. We also have a print_length() function that will print the length of the specified string. The last print_length() invocation will fail because it is a null reference. The first invocation on the empty string works because it is still a string object.

If you want something to evaluate to False until it is non-empty, you can still specify a type that will evaluate to false. If not, build your own Null type.