[Python-Dev] PEP 565: Show DeprecationWarning in main (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Nov 13 20:05:59 EST 2017

On 14 November 2017 at 02:27, Victor Stinner <victor.stinner at gmail.com> wrote:

The change proposed in this PEP is to update the default warning filter list to be::

default::DeprecationWarning:main ignore::DeprecationWarning ignore::PendingDeprecationWarning ignore::ImportWarning ignore::BytesWarning ignore::ResourceWarning This PEP can break applications parsing Python stderr, application which don't expect to get DeprecationWarning in their output.

Right, but anything affected by this would eventually have broken anyway, when the change being warned about actually happens.

That said, reducing the likelihood of breaking application output parsers is part of the rationale for restricting the change to unpackaged scripts (we know from the initial PEP 538 implementation that there's plenty of code out there that doesn't cope well with unexpected lines appearing on stderr).

Is it possible to disable this PEP using a command line option and/or environment variable to get the Python 3.6 behaviour (always DeprecationWarning)?

The patch for the PEP will also update the documentation for the PYTHONWARNINGS environment variable to explicitly call out the following settings:

PYTHONWARNINGS=error # Convert to exceptions
PYTHONWARNINGS=always # Warn every time
PYTHONWARNINGS=default # Warn once per call location
PYTHONWARNINGS=module # Warn once per calling module
PYTHONWARNINGS=once # Warn once per Python process
PYTHONWARNINGS=ignore # Never warn

And then also cover their respective shorthand command line equivalents (-We, -Wa, -Wd, -Wm,-Wo, -Wi).

While https://docs.python.org/3/using/cmdline.html#cmdoption-w does currently explain this, neither it nor https://docs.python.org/3/using/cmdline.html#envvar-PYTHONWARNINGS show specific examples

They also don't link directly to https://docs.python.org/3/library/warnings.html#the-warnings-filter, and that section doesn't explain the shorthand :-separated notation used in sys.warnoptions.

I guess that it's "PYTHONWARNINGS=ignore::DeprecationWarning:main". Am I right?

PYTHONWARNINGS=ignore::DeprecationWarning would be the shortest way to suppress deprecations everywhere again while still getting other warnings.

Would you mind to mention that in the PEP, please?

Will do - I actually meant to cover this anyway (hence the reference to docs changes in the abstract), but I missed it in the initial draft.

Sorry, I'm not an expert of the warnings module. Is it possible to also configure Python to ignore DeprecationWarning using the warnings module, at the start of the main script? Something like warnings.filterwarnings("ignore", '', DeprecationWarning)? Again, maybe explain that in the PEP?

warnings.simplefilter("ignore", DeprecationWarning) is the simplest runtime option for ensuring that deprecation warnings are never displayed.

The downside of doing this programmatically is that you can end up overriding environmental and command line settings, so the best application level "Only show warnings if my users ask for them" snippet actually looks like:

if not sys.warnoptions:
    warnings.simplefilter("ignore")

(I'll mention this in the PEP and docs patch as well)

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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