Standard error output

The script

stderr.py

The output

When the output of the script is sent to the “Python 3.7.4 Shell” window of IDLE, the standard output is in blue and the standard error output is in red.

This is standard output.
This is also standard output.
This is standard error output.

Two output channels (or “streams”)

Every Python program is born with two channels through which it can send output to the outside world:

• Use the standard output sys.stdout for normal output and other good news.
• Use the standard error output sys.stderr in a non-interactive script for warnings, error messages, and other bad news.

We have already seen the sep and end keyword parameters of print. The file keyword parameter of print defaults to sys.stdout.

Don’t produce any standard error output yet.

An interactive script is one that prints questions and lets the user type in answers. We have already printed with the print function, and soon we will allow the user to type in input with the input function. So for the time being our scripts are interactive. An interactive script should produce its output via the standard output channel.

The standard error output channel is intended for use by non-interactive scripts that produce voluminous error messages (many pages) that will be stored in a separate output file that people will look at later.