Python developer mode

The Python Development Mode introduces additional runtime checks that are too expensive to be enabled by default. It should not be more verbose than the default if the code is correct; new warnings are only emitted when an issue is detected.

It can be enabled using the -X dev command line option or by setting the PYTHONDEVMODE environment variable to 1.

Some of the more useful things this does for typical Python development includes the following:

• Add default warning filter.

  • The following warnings are shown:
    • DeprecationWarning
    • ImportWarning
    • PendingDeprecationWarning
    • ResourceWarning
  • Normally, the above warnings are filtered by the default warning filters.
  • It behaves as if the -W default command line option is used.
  • Use the -W error command line option or set the PYTHONWARNINGS environment variable to error to treat warnings as errors.

• Enable asyncio debug mode. For example, asyncio checks for coroutines that were not awaited and logs them.

  • It behaves as if the PYTHONASYNCIODEBUG environment variable is set to 1.

• Check the encoding and errors arguments for string encoding and decoding operations. Examples: open(), str.encode() and bytes.decode().

  • By default, for best performance, the errors argument is only checked at the first encoding/decoding error and the encoding argument is sometimes ignored for empty strings.