On Thursday, January 21, 2021 at 7:21:52 AM UTC-8 Nikos Apostolakis wrote:
> Dear Vincent,
>
> Thank you very much, that fixes it.
>
> IMHO this is a bug though, or at least an inconsistent behavior. Most of
> the time Sage silently coerces things to the appropriate type so commands
> just work. For example:
>
>
> sage: m = 11
>> sage: l = (m-1)/2 -1
>>
> sage: l
>> 4
>> sage: range(5)[l]
>> 4
>
>
> There's a subtle difference here: The python interface here requires an
index to be something that can be converted into an integer; i.e.,
something for which the __int__ method does the right thing. "Coercion" in
sage has a more restricted notion; see the docs. In sage, there is no
coercion from QQ to ZZ, because there is no natural map from one to the
other. There is a conversion, though, which is allowed to be a partial map.
Generally, coercion maps only exist in one direction. Otherwise, things
like ZZ(1)+QQ(2) whould have an ill-determined type.
Whether "Words" should attempt to convert its second argument to an integer
is a different matter. Generally it is best to start interface design by
doing only a minimal amount of processing to the argument, because being
"smart" for one thing can easily hide errors in another case.
The error message that is currently given is perhaps a little generic, but
it does clearly indicate that the arguments given to "Words" are
problematic. Once you check that the *apparent* arguments work fine if
entered directly, it's clear there's probably a type issue. If you use
IPython's interactive debugger ("%debug") you can dive into the problematic
code and query the arguments and their types directly, within the context
where the error was raised. That can be a big time saver, because it can
help you find the error without having to construct a stand-alone
reproducible test case that you would need if you want to communicate the
error to other people.
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