[Tutor] How to handle exceptions raised inside a function?

[Patty]

This is very interesting to me - the below excerpt is something I was trying 
to do for one of my programs and gave up on it:

> A fifth approach, common in some other languages, is to return some 
> arbitrary value, and set an error flag. The caller then has to write code 
> like this:
>
> result = function(arguments)
> if not last_result_error:
>     # no error occurred
>     print "result is", result
>
>
> If you do this, I will *personally* track you down and beat you to death 
> with a rather large fish.
>
> *wink*

I think I was trying to do something like thius at the end of a function I 
wrote-

return 2  or return my_special_integer_mvar

and then do something or other depending on this value once it passes back 
to calling function or main().  I think I used similar code as you have 
above.  It didn't go well and also there seemed to be a problem related to 
where I was returning this value _to_ (where I actually placed this snippet 
of code like you wrote above) -  a function or module I wrote or main().

So, could you expand on this for me?  I would have to dig around to find the 
actual program I was working on.

Thanks,

Patty



----- Original Message ----- 
From: "Steven D'Aprano" <steve at pearwood.info>
To: <tutor at python.org>
Sent: Tuesday, November 30, 2010 1:23 PM
Subject: Re: [Tutor] How to handle exceptions raised inside a function?


> Richard D. Moores wrote:
>> Please take a look at 2 functions I just wrote to calculate the
>> harmonic and geometric means of lists of positive numbers:
>> <http://tutoree7.pastebin.com/VhUnZcma>.
>>
>> Both Hlist and Glist must contain only positive numbers, so I really
>> need to test for this inside each function. But is there a good way to
>> do this? What should the functions return should a non-positive number
>> be detected? Is there a conventional Pythonic way to do this?
>
> There are two basic approaches to handling errors in Python:
>
> (1) Don't do any error checking at all. If the input is bad, an exception 
> will (hopefully!) be raised. Provided you know that bad input *will* lead 
> to an exception, and not just plausible-looking but incorrect result, this 
> is often the simplest way.
>
> (2) If you don't trust that a sensible exception will be raised, then do 
> your own error checking, and raise an exception.
>
> For numeric work, another approach is to return a floating point NAN ("Not 
> A Number"). Unfortunately Python doesn't give any standard way to specify 
> *which* NAN is returned, but you can return float("nan") to return one of 
> them.
>
> A fourth approach, rare in Python, is to return some sort of magic value 
> to indicate an exceptional case. Just about the only example of this I can 
> think of is string.find(), which returns -1 to indicate "not found".
>
> A fifth approach, common in some other languages, is to return some 
> arbitrary value, and set an error flag. The caller then has to write code 
> like this:
>
> result = function(arguments)
> if not last_result_error:
>     # no error occurred
>     print "result is", result
>
>
> If you do this, I will *personally* track you down and beat you to death 
> with a rather large fish.
>
> *wink*
>
>
> For what it's worth, I have a module of statistics functions (shameless 
> plug: http://pypi.python.org/pypi/stats and 
> http://code.google.com/p/pycalcstats -- feedback and bug reports welcome) 
> that includes the harmonic and geometric mean. My harmonic mean looks like 
> this:
>
> def harmonic_mean(data):
>     try:
>         m = mean(1.0/x for x in data)
>     except ZeroDivisionError:
>         return 0.0
>     if m == 0.0:
>         return math.copysign(float('inf'), m)
>     return 1/m
>
> Notice that if the data includes one or more zeroes, the harmonic mean 
> itself will be zero: limit as x->0 of 1/x -> infinity, and 1/infinity -> 
> 0. If the sum of reciprocals itself cancels to zero, I return the infinity 
> with the appropriate sign. The only exceptions that could occur are:
>
> * mean will raise ValueError if the data is empty;
> * if an argument is non-numeric, TypeError will occur when I take the 
> reciprocal of it.
>
>
>
> -- 
> Steven
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