allow specifying multiple extensions on the command-line with traitlets 5 #12543
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In traitlets 4, only one extension could be loaded with
--ext
, so:would only actually load
autoreload
, the last occurrence, silently ignoring Cython. With traitlets 5, this silent failure to activate two extensions has turned into an error (see ipython/traitlets#614).But this syntax Just Works™ for List traits with traitlets 5, so this PR adds a new
extra_extensions
plural trait that behaves the same as the existing one, while adding support for specifying multiple extensions if the alias is given multiple times.For backwards-compatibility, instead of updating in-place, add a new
extra_extensions
list and point the--ext
alias to the new list on traitlets 5 and leave it on the existingextra_extension
on traitlets 4.closes #12541
closes ipython/traitlets#614