Jason G. recommended I put this discussion about # 4677 on-list:
This works:
{{{
sage: f=x^2
sage: plot(lambda x:f(x),(x,-1,1))
}}}
But this doesn't:
{{{
sage: f=x^2
sage: plot(lambda x:f,(x,-1,1))
verbose 0 (3633: plot.py, _plot) WARNING: When plotting, failed to
evaluate function at 400 points.
verbose 0 (3633: plot.py, _plot) Last error message: 'float()
argument
must be a string or a number'
}}}
The behavior is the same for f(x)=x^2.
This is because in the second example "evaluating" the lambda
function
yields a SymbolicCallableExpression (namely, x |--> x^2 )which needs
to be called again to
actually yield a numerical value. I think this is not hard to fix,
by for instance
{{{
sage: f=x^2
sage: a=lambda x: f
sage: a=a(x)
sage: a(2)
4
}}}
so that in principle upon a TypeError, one could try letting func=func
(x)
and then do float(func(point)). However, it is unclear (to me)
whether this is desired, or whether this is an error which should stay
an error. I'd love it, but there might be other things this could
break.
Just to be clear, I am only suggesting this be done in plot in a
situation which would otherwise not plot (returning the empty plot),
so that it would actually plot.
- kcrisman
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