[Python-Dev] Add a developer mode to Python: -X dev command line option (original) (raw)

Victor Stinner victor.stinner at gmail.com
Thu Nov 16 08:04:43 EST 2017

2017-11-16 13:54 GMT+01:00 Antoine Pitrou <antoine at python.org>:

-Wdefault means -Wonce or -Walways? If the former, I don't expect many warnings to be emitted.

It's kind of funny that passing "-W default" changes the "default" behaviour. "-W default" is documented as:

"Explicitly request the default behavior (printing each warning once per source line)."

https://docs.python.org/dev/using/cmdline.html#cmdoption-w

Default warnings filters in release mode:

$ python3 -c 'import pprint, warnings; pprint.pprint(warnings.filters)' [('ignore', None, <class 'DeprecationWarning'>, None, 0), ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0), ('ignore', None, <class 'ImportWarning'>, None, 0), ('ignore', None, <class 'BytesWarning'>, None, 0), ('ignore', None, <class 'ResourceWarning'>, None, 0)]

-Wd adds the a warnings filter with the "default" action matching all warnings (any kind, any message, any line number) if I understand correctly:

$ python3 -Wd -c 'import pprint, warnings; pprint.pprint(warnings.filters)' [('default', re.compile('', re.IGNORECASE), <class 'Warning'>, re.compile(''), 0), ('ignore', None, <class 'DeprecationWarning'>, None, 0), ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0), ('ignore', None, <class 'ImportWarning'>, None, 0), ('ignore', None, <class 'BytesWarning'>, None, 0), ('ignore', None, <class 'ResourceWarning'>, None, 0)]

"default" and "once" actions are different:

default: "print the first occurrence of matching warnings for each location where the warning is issued" once: "print only the first occurrence of matching warnings, regardless of location"

For example, on testos, PYTHONMALLOC=debug increases the peak memory usage from 10.5 MiB to 15.8 MiB: +50%. I see. For my use cases, this would be acceptable :-) But I think this should be documented, for example: """Currently, developer mode adds negligible CPU time overhead, but can increase memory consumption significantly if many small objects are allocated. This is subject to change in the future."""

+50% memory is inacceptable to develop on embedded devices, or more generally with low level. But in that case, you are can enable options enabled by -X dev manually, without PYTHONMALLOC=debug :-)

Ok, I will document that, I like your proposed paragraph.

Victor

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