ConstantSymbol

[Henry Schreiner]

Is there a way to make a constant symbol in Sympy? I haven't found one yet. Basically, it would be a lot like pi and other constants, but you could make your own. In other words,

>>> c = ConstantSymbol('c', 1.23)

>>> c + 1

c+1

>>> _.evalf()

2.23

That way, formulas could stay in a nice form until evaluation occurs. Also, subs will still allow changing out constants.

Here's example code that does that:

class ConstantSymbol(s.NumberSymbol):
    is_real = True
    is_irrational = False
    is_algebraic = True
    __slots__ = ['name', 'value']

    def __new__(cls, name, value):
        self = super(ConstantSymbol,cls).__new__(cls)
        self.name=name
        self.value=s.Float(value)

        return self

    def __getnewargs__(self):
        return (self.name,self.value)

    def _latex(self, printer):
        return printer.doprint(s.Symbol(self.name))

    def _sympystr(self, printer):
        return printer.doprint(s.Symbol(self.name))

    def _sympyrepr(self, printer):
        return printer.doprint(s.Symbol(self.name))

    def _mathml(self, printer):
        return printer.doprint(s.Symbol(self.name))

    def _as_mpf_val(self, prec):
        return self.value._as_mpf_val(price)

The problem with the above code is that sympy.lamdify uses _sympystr() instead of evaluating it first.

Is there something like this, or would something like this be useful?

[Aaron Meurer]

Sure, it sounds useful.

I think the assumptions should mirror the assumptions of self.value
(currently, Floats set is_rational to None), which I believe is
intentional.

For lambdify, I suppose LambdaPrinter ought to define its own print
method. That might break existing code that expects it to use str, so
we should make sure it falls back.

Aaron Meurer

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[Björn Dahlgren]

Henry, did you get any further with this?
I have often missed this kind of object, I usually keep track of the values of constants in a dictionary and perform xreplace
before calling e.g. lambdify. But using a ConstantSymbol along the lines which you have described would make the
code much cleaner.

Looking a bit further, I think a natural next step would then be to pre-define symbols for the natural constants in sympy.physics.units, e.g.:
Avogadro_constant = ConstantSymbol('N_A', avogadro_number/mol)

using objects like this with lambdify would then require the user to choose a system of units (e.g. SI, imperial units, etc) at invocation
of `lambdify`. The callback returned by lambdify would then assume nondimensionalised input in the chosen unit system
(which means that the user would likely appreciate having a corresponding nondimensionalising-callback for work with input carrying units).

Best,
Björn

[B.L.]

Any updates on this?

Is it now possible to do a Constant Symbol in numpy?

If not, is subclassing NumberSymbol the best option?

I did not find any documentation about how to do it, the sample from Henry is the largest piece of information I found.

Thank you.

[Aaron Meurer]

I don't think so. It would be good to open an issue about this. I
think Henry's example is a good start, but we also need to modify the
lambdify printer as I described.

Aaron Meurer

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[B.L.]

Done: github issue