The Samurai Principle

Robert Kern robert.kern at
Sun Sep 12 07:20:46 CEST 2010

On 9/11/10 7:08 PM, Lawrence D'Oliveiro wrote:
> In message<mailman.654.1284158340.29448.python-list at>, Robert
> Kern wrote:
>> On 9/10/10 5:17 PM, Lawrence D'Oliveiro wrote:
>>> In message<mailman.567.1283927599.29448.python-list at>, Ian
>>> Kelly wrote:
>>>> And returning None on failure is dangerous, because if the programmer
>>>> does not take care to handle that case, the program may attempt to
>>>> regard it as actual data.
>>> But None *is* actual data.
>> And that is exactly the reason why the Samurai Principle says to not
>> return None when the function fails to do what it intended to do.
> How can the function “fail” when it returns what it is specified to return?

The Samurai Principle is about what specifications and guarantees a programmer 
should make about his functions. It gives advice for programmers on how to 
specify their functions in the face of failure conditions the function itself 
sees. For example, a function that is supposed to fetch a record from a database 
given a key, a failure condition would be that the key does not exist in the 
database. Or the connection times out, or anything else that prevents the 
function from retrieving a correct record for the key. The Principle says that a 
programmer should specify his function such that it raises an exception to 
signify this failure rather than to return None.

Of course, a programmer may make his functions' contracts as loose as he 
pleases, and thus returning None may be "correct" in the sense that it satisfies 
those contracts. The Principle simply advises that making such contracts is not 
a good idea.

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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