Simple recursive sum function | what's the cause of the weird behaviour?

Terry Reedy tjreedy at
Sat Jul 6 20:47:27 CEST 2013

On 7/6/2013 8:37 AM, Russel Walker wrote:
> I know this is simple but I've been starring at it for half an hour and trying all sorts of things in the interpreter but I just can't see where it's wrong.
> def supersum(sequence, start=0):
>      result = start
>      for item in sequence:
>          try:
>              result += supersum(item, start)
>          except:

Bare except statements cover up too many sins. I and others *strongly* 
recommend that you only catch what you *know* you actually want to (see 

>              result += item
>      return result

I recommend that you start with at least one test case, and with an edge 
case at that. If you cannot bring yourself to do it before writing a 
draft of the function code, do it immediately after and run. If you do 
not want to use a framework, use assert.

assert supersum([]) == 0
assert supersum([], []) == []

Do the asserts match your intention? The tests amount to a specification 
by example. Any 'kind' of input that is not tested is not guaranteed to 

Back to the except clause: only add try..except xxx when needed to pass 
a test.

Terry Jan Reedy

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