You want to have your program issue warning messages (e.g., about deprecated features

or usage problems).

To have your program issue a warning message, use the warnings.warn() function. For

example:

def func(x, y, logfile=None, debug=False):

if logfile is not None:

warnings.warn('logfile argument deprecated', DeprecationWarning)

...

The arguments to warn() are a warning message along with a warning class, which is

typically one of the following: UserWarning, DeprecationWarning, SyntaxWarning,

RuntimeWarning, ResourceWarning, or FutureWarning.

The handling of warnings depends on how you have executed the interpreter and other

configuration. For example, if you run Python with the -W all option, you’ll get output

such as the following:

Normally, warnings just produce output messages on standard error. If you want to turn

warnings into exceptions, use the -W error option:

Issuing a warning message is often a useful technique for maintaining software and

assisting users with issues that don’t necessarily rise to the level of being a full-fledged

exception. For example, if you’re going to change the behavior of a library or framework,

you can start issuing warning messages for the parts that you’re going to change while

still providing backward compatibility for a time. You can also warn users about prob‐

lematic usage issues in their code.

As another example of a warning in the built-in library, here is an example of a warning

message generated by destroying a file without closing it:

>>> import warnings

>>> warnings.simplefilter('always')

>>> f = open('/etc/passwd')

>>> del f

>>>

By default, not all warning messages appear. The -W option to Python can control the

output of warning messages. -W all will output all warning messages, -W ignore

ignores all warnings, and -W error turns warnings into exceptions. As an alternative,

you can can use the warnings.simplefilter() function to control output, as just

shown. An argument of always makes all warning messages appear, ignore ignores all

warnings, and error turns warnings into exceptions.

For simple cases, this is all you really need to issue warning messages. The warnings

module provides a variety of more advanced configuration options related to the fil‐

tering and handling of warning messages. See the Python documentation for more

information.