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: yaml#439

lib/yaml/resolver.py

@@ -146,8 +146,8 @@ def resolve(self, kind, value, implicit):
                 resolvers = self.yaml_implicit_resolvers.get(u'', [])
             else:
                 resolvers = self.yaml_implicit_resolvers.get(value[0], [])
-            resolvers += self.yaml_implicit_resolvers.get(None, [])
-            for tag, regexp in resolvers:
+            wildcard_resolvers = self.yaml_implicit_resolvers.get(None, [])
+            for tag, regexp in resolvers + wildcard_resolvers:
                 if regexp.match(value):
                     return tag
             implicit = implicit[1]

lib3/yaml/resolver.py

@@ -146,8 +146,8 @@ def resolve(self, kind, value, implicit):
                 resolvers = self.yaml_implicit_resolvers.get('', [])
             else:
                 resolvers = self.yaml_implicit_resolvers.get(value[0], [])
-            resolvers += self.yaml_implicit_resolvers.get(None, [])
-            for tag, regexp in resolvers:
+            wildcard_resolvers = self.yaml_implicit_resolvers.get(None, [])
+            for tag, regexp in resolvers + wildcard_resolvers:
                 if regexp.match(value):
                     return tag
             implicit = implicit[1]