Is there a way to detect the chunk language when setting knitr chunk options?

Question

Summary - is there any way for the code run when setting knitr chunk options to know the language of the contained chunk?

Detail:

I have a knitr (in fact Quarto) book in which I have both R and Python chunks, where the Python chunks run via Reticulate.

In fact the R chunks are for an R edition of the book, and the Python chunks for a Python edition.

I'd like to tell knitr to eval=FALSE, echo=FALSE for the R chunks, when building the Python edition, and vice versa for the Python edition.

Is there a way for me to detect the chunk language at chunk option evaluation time, in order to do something like this?

# My document

```{r, setup, include=FALSE}
is_py_ed <- TRUE  # Worked out elsewhere.
is_r_ed <- FALSE  # Worked out elsewhere. 
knitr::opts_chunk$set(
  eval=(this_is_python_chunk & is_py_ed) | (this_is_r_chunk & is_r_ed),
  echo=(this_is_python_chunk & is_py_ed) | (this_is_r_chunk & is_r_ed)
)
```

```{python}
# This chunk gets dropped in R edition, kept in Python edition.
```

```{r}
# This chunk gets dropped in Python edition, kept in R edition.
```

Answer 1

You can do this using an option hook and get the current language engine used by knitr to eval and execute the code of the current code chunk using options$engine (where options is a list of chunk options passed to the hook functions).

---
title: "Chunk language"
format: html
---

# My document

```{r, setup, include=FALSE}
is_py_ed <- TRUE  # Worked out elsewhere.
is_r_ed <- FALSE  # Worked out elsewhere.

knitr::opts_chunk$set(echo = "", eval = "")

knitr::opts_hooks$set(
  echo = function(options) {
    if(!is.logical(options$echo)) {
      options$echo <- (options$engine == "python" & is_py_ed) | (options$engine == "R" & is_r_ed) 
    }
    return(options)
  },
  eval = function(options) {
    if(!is.logical(options$eval)) {
      options$eval <- (options$engine == "python" & is_py_ed) | (options$engine == "R" & is_r_ed)
    }
    return(options)
  }
)

```

```{python}
print("Hello from python")
```

```{r}
print("Hello from R")
```

```{r, echo=TRUE, eval=TRUE}
print("Hello from R (evaluated in python edition)")
```

---

Note: As a workaround to enable overriding the chunk option echo and eval locally, I have set them to empty strings globally. Then the option hooks will change the values accordingly if they are not set in the chunk header.

---