Fix: load package (and cache) of external engine only if necessary

R/engine.R

@@ -56,8 +56,8 @@ cache_engines$.get = cache_engines$get
 cache_engines$get = function(name, ...) {
  if (missing(name)) {
  cache_engines$.get(...)
- } else if (!identical(name, 'python') || !isFALSE(options$python.reticulate)) {
- cache_engines$.get(name, ...)
+ } else if (!is.null(cache_importer <- cache_engines$.get(name, ...))) {
+ cache_importer()
  } else {
  NULL
  }
@@ -288,6 +288,17 @@ eng_python = function(options) {
  }
 }
 
+cache_eng_python = function() {
+ # TODO: change this hack to reticulate::cache_eng_python after
+ # https://github.com/rstudio/reticulate/pull/1210 is merged and released
+ if (!isFALSE(options$python.reticulate) &&
+ 'cache_eng_python' %in% ls(asNamespace('reticulate'))) {
+ getFromNamespace('cache_eng_python', 'reticulate')
+ } else {
+ NULL
+ }
+}
+
 ## Java
 # e.g. see http://cran.rstudio.com/package=jvmr
 
@@ -925,11 +936,7 @@ knit_engines$set(
  verbatim = eng_verbatim
 )
 
-# TODO: change this hack to reticulate::cache_eng_python after
-# https://github.com/rstudio/reticulate/pull/1210 is merged and released
-if ('cache_eng_python' %in% ls(asNamespace('reticulate'))) {
- cache_engines$set(python = getFromNamespace('cache_eng_python', 'reticulate'))
-}
+cache_engines$set(python = cache_eng_python)
 
 get_engine = function(name) {
  fun = knit_engines$get(name)