In datascienceworkshops/rexpect: Automate Interactions with Textual Interfaces

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rexpect

rexpect is an R package that allows you to automate interactions with programs that expose a text terminal interface. The API is inspired by the original Expect tool by Don Libes.

Installation

You can install the development version from GitHub with:

# install.packages("remotes")
remotes::install_github("jeroenjanssens/rexpect")

Example

Let's say we have a Python script that repeatedly asks for a number. When the user enters sum, the script prints the sum of all the numbers. The script exits when the user presses CTRL-C. Here's the contents of bin/sum.py:

```{verbatim, file = "bin/sum.py", lang = "python"}

We can use `rexpect` to run this script and interact with it.
Let's enter the numbers 7, 13, and 22 and see what the sum is:

```r
library(rexpect)

session <- spawn("bin/sum.py", prompt = "^(.*):$")
send_lines(session, c("7", "13", "22", "sum"), wait = TRUE)

Under the hood, rexpect starts a tmux session using tmuxr:

session

We can capture the output and extract the sum:

print(output <- read_all(session))
as.numeric(gsub("[^0-9.]", "", output[length(output) - 1]))

We've accomplished our goal, so let's exit:

send_lines(session, "exit")
read_all(session)

Oh that's right, we need to press CTRL-C to exit the script:

send_control_c(session)

This was a rather simple example to demonstrate some of the capabilities of rexpect. The session could have taken place inside a Docker container or over SSH. We could also have recorded the session using asciinema. Have a look at the function reference to learn more about what rexpect has to offer.