The log module

Leveled, structured logging: lines with a level, a message, key/value fields and the place of the call, written whole to standard error or to a Writer of your choice, from any thread or task.

log is a module: bring it in with import log (and write log.info("started")).

• error, warning, info, debug and trace write a line at their Level. Each takes optional fields (made with field) and a target, the part of the program the line is about.
• set_level picks the least important level shown, for every target or for one; enabled asks whether a line would be written.
• set_output sends lines to a Writer (a file, a socket) in a Format: readable text or JSON Lines.

By default lines go to standard error as text, in color when it is a terminal (unless NO_COLOR is set), and info and above are shown. The PEPPER_LOG environment variable sets the levels when the program first logs: comma-separated items, each a level for every target or target=level for one, such as PEPPER_LOG=warning,http=debug. The levels are off, error, warning (or warn), info, debug and trace.

A text line reads:

2026-10-17T23:40:01.123Z INFO  [http] request done status=200 path="/" (main.pep:5:5)

Example

import log;

main(): int {
    log.info("started");
    log.warning("disk almost full", [log.field("free_mb", 120)], target = "storage");
    log.debug("not shown unless PEPPER_LOG=debug");
    return 0;
}

Source: runtime/log/module.pep