knitr engine for sympy code

example.Rmd

---
title: "Sympy Engine"
output: html_notebook
editor_options: 
  chunk_output_type: inline
---

Assuming the "sympy" knitr engine has been registered:

```{sympy, results='asis'}
from sympy import *
x = symbols('x')
a = Integral(cos(x) * exp(x), x)
Eq(a, a.doit())
```

knitr-sympy.R

knitr::knit_engines$set(
  sympy = function(options) {
    # intercept the chunk code
    code <- options$code
    output <- paste0("output = ", code[length(code)]) # grab last line
    code <- c(
      code[-length(code)], # sans last line
      output,
      "print('$' + latex(output) + '$')"
    )
    options$code <- code
    options$engine <- "python"
    reticulate::eng_python(options)
  }
)