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" <st...@pearwood.info>
To: <tutor@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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