chronometre: A new package (pair) demo for R and Python

Sun, 08 Feb 2026 12:11:00 CDT

Both R and Python make it reasonably easy to work with compiled extensions. But how to access objects in one environment from the other and share state or (non-trivial) objects remains trickier. Recently (and while r-forge was ‘resting’ so we opened GitHub Discussions) a question was asked concerning R and Python object pointer exchange.

This lead to a pretty decent discussion including arrow interchange demos (pretty ideal if dealing with data.frame-alike objects), but once the focus is on more ‘library-specific’ objects from a given (C or C++, say) library it is less clear what to do, or how involved it may get.

R has external pointers, and these make it feasible to instantiate the same object in Python. To demonstrate, I created a pair of (minimal) packages wrapping a lovely (small) class from the excellent spdlog library by Gabi Melman, and more specifically in an adapted-for-R version (to avoid some R CMD check nags) in my RcppSpdlog package. It is essentially a nicer/fancier C++ version of the tic() and tic() timing scheme. When an object is instantiated, it ‘starts the clock’ and when we accessing it later it prints the time elapsed in microsecond resolution. In Modern C++ this takes little more than keeping an internal chrono object.

Which makes for a nice, small, yet specific object to pass to Python. So the R side of the package pair instantiates such an object, and accesses its address. For different reasons, sending a ‘raw’ pointer across does not work so well, but a string with the address printed works fabulously (and is a paradigm used around other packages so we did not invent this). Over on the Python side of the package pair, we then take this string representation and pass it to a little bit of pybind11 code to instantiate a new object. This can of course also expose functionality such as the ‘show time elapsed’ feature, either formatted or just numerically, of interest here.

And that is all that there is! Now this can be done from R as well thanks to reticulate as the demo() (also shown on the package README.md) shows:

> library(chronometre)
> demo("chronometre", ask=FALSE)


        demo(chronometre)
        ---- ~~~~~~~~~~~

> #!/usr/bin/env r
>
> stopifnot("Demo requires 'reticulate'" = requireNamespace("reticulate", quietly=TRUE))

> stopifnot("Demo requires 'RcppSpdlog'" = requireNamespace("RcppSpdlog", quietly=TRUE))

> stopifnot("Demo requires 'xptr'" = requireNamespace("xptr", quietly=TRUE))

> library(reticulate)

> ## reticulate and Python in general these days really want a venv so we will use one,
> ## the default value is a location used locally; if needed create one
> ## check for existing virtualenv to use, or else set one up
> venvdir <- Sys.getenv("CHRONOMETRE_VENV", "/opt/venv/chronometre")

> if (dir.exists(venvdir)) {
+ >     use_virtualenv(venvdir, required = TRUE)
+ > } else {
+ >     ## create a virtual environment, but make it temporary
+ >     Sys.setenv(RETICULATE_VIRTUALENV_ROOT=tempdir())
+ >     virtualenv_create("r-reticulate-env")
+ >     virtualenv_install("r-reticulate-env", packages = c("chronometre"))
+ >     use_virtualenv("r-reticulate-env", required = TRUE)
+ > }


> sw <- RcppSpdlog::get_stopwatch()                   # we use a C++ struct as example

> Sys.sleep(0.5)                                      # imagine doing some code here

> print(sw)                                           # stopwatch shows elapsed time
0.501220

> xptr::is_xptr(sw)                                   # this is an external pointer in R
[1] TRUE

> xptr::xptr_address(sw)                              # get address, format is "0x...."
[1] "0x58adb5918510"

> sw2 <- xptr::new_xptr(xptr::xptr_address(sw))       # cloned (!!) but unclassed

> attr(sw2, "class") <- c("stopwatch", "externalptr") # class it .. and then use it!

> print(sw2)                                          # `xptr` allows us close and use
0.501597

> sw3 <- ch$Stopwatch(  xptr::xptr_address(sw) )      # new Python object via string ctor

> print(sw3$elapsed())                                # shows output via Python I/O
datetime.timedelta(microseconds=502013)

> cat(sw3$count(), "\n")                              # shows double
0.502657

> print(sw)                                           # object still works in R
0.502721
>

The same object, instantiated in R is used in Python and thereafter again in R. While this object here is minimal in features, the concept of passing a pointer is universal. We could use it for any interesting object that R can access and Python too can instantiate. Obviously, there be dragons as we pass pointers so one may want to ascertain that headers from corresponding compatible versions are used etc but principle is unaffected and should just work.

Both parts of this pair of packages are now at the corresponding repositories: PyPI and CRAN. As I commonly do here on package (change) announcements, I include the (minimal so far) set of high-level changes for the R package.

Changes in version 0.0.2 (2026-02-05)

Removed replaced unconditional virtualenv use in demo given preceding conditional block

Updated README.md with badges and an updated demo

Changes in version 0.0.1 (2026-01-25)

Initial version and CRAN upload

Questions, suggestions, bug reports, … are welcome at either the (now awoken from the R-Forge slumber) Rcpp mailing list or the newer Rcpp Discussions.

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. If you like this or other open-source work I do, you can sponsor me at GitHub.