different way to specify engines {r engine='python'} -> {python}

[jankatins]

Re: https://twitter.com/xieyihui/status/571162828528185345

Just because I recently wanted to support '''{python} ('''{r engine='python'} -> '''{python}) in a python port of knitr and had to include that in the regex I use:

    chunk_begin = '^\s*```+\s*{[.]?([a-z]+)\s*(.*)}\s*$'
    chunk_end = '^\s*```+\s*$'
    inline_code = '`([a-z]+) +([^`]+)\s*`'

And then in parser.R:36

      params.src = if (group_pattern(chunk.begin)) {
        tmp <- str_trim(gsub(chunk.begin, '\\2', g[1]))
        engine <- str_trim(gsub(chunk.begin, '\\1', g[1]))
        if(engine != "r"){
            tmp <- paste(tmp, ", engine='",engine,"'"
        }
        tmp
      } else ''

(this is probably not valid R code... :-/ )

[yihui]

That sounds like a good idea. I'll think about it. Thanks!

[kevinushey]

@yihui, do you plan on making this work generally for all language modes, so that e.g. all of the following are valid?

```{python}
```

or

```{css}
```

Just wondering because on the RStudio side it might be nice to have snippet insertions for each form.

[yihui]

@kevinushey Yes, it will be a general form like

```{lang}

for all language engines.

[jankatins]

In the interest of adding all arguments, I would like to add this here, which I found out at my side when I implemented chunk options.

It seems that knitr uses eval to build the plotting chunk options, so valid R code is possible to be included in the options. Even if you use engine="python", the plotting options are still evaluated as R code, so a python True will probably error in rmd (-> TRUE).

IMO this is an argument to keep using {r ... engine="python"}, because the options are "R" and not "python". On the other hand, knitpy uses .pymd and not .rmd as a file extensions, so one could argue that this already is enough to distinguish between r and py...

-> all in all, I still think that file extensions are enough and {lang} would be nice.

[yihui]

@JanSchulz Thanks! I'm not trying to throw away {r engine=???}: I just want to add {???} as a shorthand of it. I guess the vast majority of knitr users are R users, so I'm not seriously concerned about the True issue, and I'd encourage them to use your knitpy if they are more comfortable with the Python syntax :)

[yihui]

Done.

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