Introduction to logger

If you are not only using R in the interactive console for ad-hoc data analysis, but running eg batch jobs (for ETL, reporting, modeling, forecasting etc) as well, then logging the status(changes) of your script is a must so that later on you can review / debug what have happened.

For most cases, it’s enough to load the package and use the functions with the log prefix to log important and not so important messages, for example:

library(logger)
log_info("Loading data")
#> INFO [2024-11-20 05:52:02] Loading data
data(mtcars)
log_info("The dataset includes {nrow(mtcars)} rows")
#> INFO [2024-11-20 05:52:02] The dataset includes 32 rows
if (max(mtcars$hp) < 1000) {
  log_warn("Oh, no! There are no cars with more than 1K horsepower in the dataset :/")
  log_debug("The most powerful car is {rownames(mtcars)[which.max(mtcars$hp)]} with {max(mtcars$hp)} hp")
}
#> WARN [2024-11-20 05:52:02] Oh, no! There are no cars with more than 1K horsepower in the dataset :/

Interestingly, the most powerful car was not being logged – because by default the logger prints messages with at least the INFO log level:

log_threshold()
#> Log level: INFO

To change that, specify the new log level threshold, eg TRACE to log everything:

log_threshold(TRACE)

The rerunning the above code chunk:

log_info("Loading data")
#> INFO [2024-11-20 05:52:02] Loading data
data(mtcars)
log_info("The dataset includes {nrow(mtcars)} rows")
#> INFO [2024-11-20 05:52:02] The dataset includes 32 rows
if (max(mtcars$hp) < 1000) {
  log_warn("Oh, no! There are no cars with more than 1K horsepower in the dataset :/")
  log_debug("The most powerful car is {rownames(mtcars)[which.max(mtcars$hp)]} with {max(mtcars$hp)} hp")
}
#> WARN [2024-11-20 05:52:02] Oh, no! There are no cars with more than 1K horsepower in the dataset :/
#> DEBUG [2024-11-20 05:52:02] The most powerful car is Maserati Bora with 335 hp

You may also find the ?log_eval function useful to log both an R expression and its result in the same log record:

f <- sqrt
g <- mean
x <- 1:31
log_eval(y <- f(g(x)), level = INFO)
#> INFO [2024-11-20 05:52:02] 'y <- f(g(x))' => '4'
str(y)
#>  num 4

Sometimes, it may be reasonable to log R objects as markdown, e.g. a smallish data.frame or data.table, e.g. mtcars or iris. Calling the formatter using pander instead of glue can help:

log_formatter(formatter_pander)
log_info(head(iris))
#> INFO [2024-11-20 05:52:02] 
#> INFO [2024-11-20 05:52:02] 
#> INFO [2024-11-20 05:52:02]  Sepal.Length   Sepal.Width   Petal.Length   Petal.Width   Species 
#> INFO [2024-11-20 05:52:02] -------------- ------------- -------------- ------------- ---------
#> INFO [2024-11-20 05:52:02]      5.1            3.5           1.4            0.2       setosa  
#> INFO [2024-11-20 05:52:02]      4.9             3            1.4            0.2       setosa  
#> INFO [2024-11-20 05:52:02]      4.7            3.2           1.3            0.2       setosa  
#> INFO [2024-11-20 05:52:02]      4.6            3.1           1.5            0.2       setosa  
#> INFO [2024-11-20 05:52:02]       5             3.6           1.4            0.2       setosa  
#> INFO [2024-11-20 05:52:02]      5.4            3.9           1.7            0.4       setosa  
#> INFO [2024-11-20 05:52:02]

For more details, check the function reference in the manual, or start with the The Anatomy of a Log Request and Customizing the Format and the Destination of a Log Record vignettes.