Fix watchers support when the Parameterized instance is falsey #769
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Fixes #747
inst or cls
was used to selectcls
wheninst
was None, except that this fails wheninst
is not None andinst
is falsey evaluates, as in the example reported in the issue where__len__
is defined on a Parameterized class and returns0
. This PR fixes a few occurrences of this bug.