Working with External Files
Last updated on 2024-03-12 | Edit this page
Estimated time: 12 minutes
Overview
Questions
- How can we load external data?
Objectives
- Be able to load external data into a workflow
- Configure the workflow to rerun if the contents of the external data change
Episode summary: Show how to read and write external files
Treating external files as a dependency
Almost all workflows will start by importing data, which is typically stored as an external file.
As a simple example, let’s create an external data file in RStudio
with the “New File” menu option. Enter a single line of text, “Hello
World” and save it as “hello.txt” text file in
_targets/user/data/.
We will read in the contents of this file and store it as
some_data in the workflow by writing the following plan and
running tar_make():
Save your progress
You can only have one active _targets.R file at a time
in a given project.
We are about to create a new _targets.R file, but you
probably don’t want to lose your progress in the one we have been
working on so far (the penguins bill analysis). You can temporarily
rename that one to something like _targets_old.R so that
you don’t overwrite it with the new example _targets.R file
below. Then, rename them when you are ready to work on it again.
R
library(targets)
library(tarchetypes)
tar_plan(
some_data = readLines("_targets/user/data/hello.txt")
)
OUTPUT
• start target some_data
• built target some_data [0.002 seconds]
• end pipeline [0.083 seconds]If we inspect the contents of some_data with
tar_read(some_data), it will contain the string
"Hello World" as expected.
Now say we edit “hello.txt”, perhaps add some text: “Hello World. How are you?”. Edit this in the RStudio text editor and save it. Now run the pipeline again.
R
library(targets)
library(tarchetypes)
tar_plan(
some_data = readLines("_targets/user/data/hello.txt")
)
OUTPUT
✔ skip target some_data
✔ skip pipeline [0.065 seconds]The target some_data was skipped, even though the
contents of the file changed.
That is because right now, targets is only tracking the
name of the file, not its contents. We need to use a
special function for that, tar_file() from the
tarchetypes package. tar_file() will calculate
the “hash” of a file—a unique digital signature that is determined by
the file’s contents. If the contents change, the hash will change, and
this will be detected by targets.
R
library(targets)
library(tarchetypes)
tar_plan(
tar_file(data_file, "_targets/user/data/hello.txt"),
some_data = readLines(data_file)
)
OUTPUT
• start target data_file
• built target data_file [0.001 seconds]
• start target some_data
• built target some_data [0 seconds]
• end pipeline [0.092 seconds]This time we see that targets does successfully re-build
some_data as expected.
A shortcut (or, About target factories)
However, also notice that this means we need to write two targets
instead of one: one target to track the contents of the file
(data_file), and one target to store what we load from the
file (some_data).
It turns out that this is a common pattern in targets
workflows, so tarchetypes provides a shortcut to express
this more concisely, tar_file_read().
R
library(targets)
library(tarchetypes)
tar_plan(
tar_file_read(
hello,
"_targets/user/data/hello.txt",
readLines(!!.x)
)
)
Let’s inspect this pipeline with tar_manifest():
R
tar_manifest()
OUTPUT
# A tibble: 2 × 2
name command
<chr> <chr>
1 hello_file "\"_targets/user/data/hello.txt\""
2 hello "readLines(hello_file)" Notice that even though we only specified one target in the pipeline
(hello, with tar_file_read()), the pipeline
actually includes two targets, hello_file
and hello.
That is because tar_file_read() is a special function
called a target factory, so-called because it makes
multiple targets at once. One of the main purposes of
the tarchetypes package is to provide target factories to
make writing pipelines easier and less error-prone.
Non-standard evaluation
What is the deal with the !!.x? That may look unfamiliar
even if you are used to using R. It is known as “non-standard
evaluation,” and gets used in some special contexts. We don’t have time
to go into the details now, but just remember that you will need to use
this special notation with tar_file_read(). If you forget
how to write it (this happens frequently!) look at the examples in the
help file by running ?tar_file_read.
Other data loading functions
Although we used readLines() as an example here, you can
use the same pattern for other functions that load data from external
files, such as readr::read_csv(),
xlsx::read_excel(), and others (for example,
read_csv(!!.x), read_excel(!!.x), etc.).
This is generally recommended so that your pipeline stays up to date with your input data.
R
source("R/packages.R")
source("R/functions.R")
tar_plan(
tar_file_read(
penguins_data_raw,
path_to_file("penguins_raw.csv"),
read_csv(!!.x, show_col_types = FALSE)
),
penguins_data = clean_penguin_data(penguins_data_raw)
)
OUTPUT
• start target penguins_data_raw_file
• built target penguins_data_raw_file [0.002 seconds]
• start target penguins_data_raw
• built target penguins_data_raw [0.098 seconds]
• start target penguins_data
• built target penguins_data [0.023 seconds]
• end pipeline [0.217 seconds]Writing out data
Writing to files is similar to loading in files: we will use the
tar_file() function. There is one important caveat: in this
case, the second argument of tar_file() (the command to
build the target) must return the path to the file. Not
all functions that write files do this (some return nothing; these treat
the output file is a side-effect of running the function), so you may
need to define a custom function that writes out the file and then
returns its path.
Let’s do this for writeLines(), the R function that
writes character data to a file. Normally, its output would be
NULL (nothing), as we can see here:
R
x <- writeLines("some text", "test.txt")
x
OUTPUT
NULLHere is our modified function that writes character data to a file
and returns the name of the file (the ... means “pass the
rest of these arguments to writeLines()”):
R
write_lines_file <- function(text, file, ...) {
writeLines(text = text, con = file, ...)
file
}
Let’s try it out:
R
x <- write_lines_file("some text", "test.txt")
x
OUTPUT
[1] "test.txt"We can now use this in a pipeline. For example let’s change the text to upper case then write it out again:
R
library(targets)
library(tarchetypes)
source("R/functions.R")
tar_plan(
tar_file_read(
hello,
"_targets/user/data/hello.txt",
readLines(!!.x)
),
hello_caps = toupper(hello),
tar_file(
hello_caps_out,
write_lines_file(hello_caps, "_targets/user/results/hello_caps.txt")
)
)
OUTPUT
• start target hello_file
• built target hello_file [0.002 seconds]
• start target hello
• built target hello [0 seconds]
• start target hello_caps
• built target hello_caps [0 seconds]
• start target hello_caps_out
• built target hello_caps_out [0.001 seconds]
• end pipeline [0.098 seconds]Take a look at hello_caps.txt in the
results folder and verify it is as you expect.
targets detects that hello_caps_out has
changed (is “invalidated”), and re-runs the code to make it, thus
writing out hello_caps.txt to results
again.
So this way of writing out results makes your pipeline more robust:
we have a guarantee that the contents of the file in
results are generated solely by the code in your plan.
Key Points
-
tarchetypes::tar_file()tracks the contents of a file - Use
tarchetypes::tar_file_read()in combination with data loading functions likeread_csv()to keep the pipeline in sync with your input data - Use
tarchetypes::tar_file()in combination with a function that writes to a file and returns its path to write out data