# [Python-ideas] PEP 485: A Function for testing approximate equality

Guido van Rossum guido at python.org
Fri Jan 23 06:54:11 CET 2015

```I'd use an example with round numbers. "For example, to set a tolerance of
5%, pass tol=0.05. The default tolerance is 1e-8."

On Thursday, January 22, 2015, Chris Barker <chris.barker at noaa.gov> wrote:

> Andrew,
>
> I totally agree that it's not going to be that clear to folks -- but I'm
> as stumped as you as to how to make it clear without getting really wordy.
>
> Also, I think the percent error use case is infrequent, more likely would
> be that a relative tolerance of 1e-8 means that the numbers are the same to
> within about 8 significant decimal figures. After all, not many people
> think in terms of 0.0000001%
>
>
> -Chris
>
>
>
> On Thu, Jan 22, 2015 at 7:30 PM, Andrew Barnert <abarnert at yahoo.com
> <javascript:_e(%7B%7D,'cvml','abarnert at yahoo.com');>> wrote:
>
>> Overall I like it, but I'm not sure the help on the tol parameter is
>> clear enough for people who don't already know what they want--in other
>> words, the very people this function should be helping.
>>
>> In my experience, novices understand relative tolerance immediately if
>> you put it in terms of "within X% of expected", but don't always understand
>> it if you put it in terms of "within X * expected" or, worse, "relative to
>> the magnitude of the expected value". Just using % in there somewhere makes
>> people get the concept.
>>
>> Unfortunately, since the API doesn't actually use a percentage--and
>> shouldn't--I'm not sure how to get this across in a one-liner in the help.
>> You can always add something like "(e.g., a relative tolerance of .005
>> means that the actual value must be within 0.5% of the expected value)",
>> but that's way too verbose.
>>
>> (Also, I should note that the people I've explained this to have mostly
>> been people with a US 1960-1990-style basic math education; I can't be sure
>> that people who learned in another country, or in the post-post-new-math
>> era in the US, etc. will respond the same way, although I do have a bit of
>> anecdotal evidence from helping a few people on forums like StackOverflow
>> that seems to imply they do.)
>>
>> Sent from a random iPhone
>>
>> On Jan 22, 2015, at 16:40, Chris Barker <chris.barker at noaa.gov
>> <javascript:_e(%7B%7D,'cvml','chris.barker at noaa.gov');>> wrote:
>>
>> > is the relative tolerance -- it is the amount of error
>> > allowed, relative to the magnitude of the expected value.
>>
>
>
>
> --
>
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
> NOAA/NOS/OR&R            (206) 526-6959   voice
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>
> Chris.Barker at noaa.gov
> <javascript:_e(%7B%7D,'cvml','Chris.Barker at noaa.gov');>
>

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