/usr/share/fish/completions/python3.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
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1 2 3 | complete -c python3 -w python
# Override this to use python2 instead of python
complete -c python3 -s m -d 'Run library module as a script (terminates option list)' -xa '(python3 -c "import pkgutil; print(\'\n\'.join([p[1] for p in pkgutil.iter_modules()]))")'
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