Avoid repeatedly appending to yaml_implicit_resolvers
Repeated calls to `resolve` can experience performance degredation, if
`add_implicit_resolver` has been called with `first=None` (to add an
implicit resolver with an unspecified first character).
For example, every time `foo` is encountered, the "wildcard implicit
resolvers" (with `first=None`) will be appended to the list of implicit
resolvers for strings starting with `f`, which will normally be the
resolver for booleans. The list `yaml_implicit_resolvers['f']` will keep
getting longer. The same behavior applies for any first-letter matches
with existing implicit resolvers.
This change avoids unintentionally mutating the lists in the class-level
dict `yaml_implicit_resolvers` by looping through a temporary copy.
Fixes: #439
2 files changed
