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Three new matchers that'll rock your world.
Improvements
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* New, powerful matchers that match items in a dictionary:
- ``MatchesDict``, match every key in a dictionary with a key in a
dictionary of matchers. For when the set of expected keys is equal to the
set of observed keys.
- ``ContainsDict``, every key in a dictionary of matchers must be
found in a dictionary, and the values for those keys must match. For when
the set of expected keys is a subset of the set of observed keys.
- ``ContainedByDict``, every key in a dictionary must be found in
a dictionary of matchers. For when the set of expected keys is a superset
of the set of observed keys.
The names are a little confusing, sorry. We're still trying to figure out
how to present the concept in the simplest way possible.