SymPy: Custom print format for a custom Function expression

问题

What is the correct way to define classes that inherit the Function class in Sympy and create custom print statements for them?

I have followed the examples from https://docs.sympy.org/latest/modules/printing.html that modify the output of Mod and it all works fine.

For example:

With a Custom Printer

from sympy import Symbol, Mod
from sympy.printing.latex import LatexPrinter


class MyLatexPrinter(LatexPrinter):
    def _print_Mod(self, expr, exp=None):
        return 'hey'


def print_my_latex(expr):
    print(MyLatexPrinter().doprint(expr))


x = Symbol('x')
m = Symbol('m')

print_my_latex(Mod(x, m))

# prints: hey

With a Custom Printing Method

from sympy import Symbol, Mod
from sympy.printing.latex import print_latex


class ModOp(Mod):
    def _latex(self, printer=None):
        return "hello"


x = Symbol('x')
m = Symbol('m')

print_latex(ModOp(x, m))

# prints: hello

However when I try to create my own function I can never get either method to alter the output.

With a Custom Printer

from sympy import Symbol, Function
from sympy.printing.latex import LatexPrinter


class MyLatexPrinter(LatexPrinter):
    def _print_Custom(self, expr, exp=None):
        return 'hi'


def print_my_latex(expr):
    print(MyLatexPrinter().doprint(expr))


class Custom(Function):

    @classmethod
    def eval(cls, a, b):
        return a * b


a = Symbol('a')
b = Symbol('b')

print_my_latex(Custom(a, b))

# prints: a b

With a Custom Printing Method

from sympy import Symbol, Function
from sympy.printing.latex import print_latex


class Custom(Function):
    @classmethod
    def eval(cls, a, b):
        return a * b

    def _latex(self, printer=None):
        return "howdy"


a = Symbol('a')
b = Symbol('b')

print_latex(Custom(a, b))

# prints: a b

Can someone please help me to understand what it is that I'm missing?

回答1:

Since Custom has an eval method, Custom(a,b) becomes the Mul, a*b. Since it is not an instance of Custom your printer method never gets called. If you rename your eval method _eval (to disable it) then the print_latex(Custom(a, b)) will produce the output "hi".

来源：https://stackoverflow.com/questions/55070462/sympy-custom-print-format-for-a-custom-function-expression

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sympy