PEP 635: Explain why mapping patterns ignore extra keys (#1721)

python · Nov 22, 2020 · f01b03e · f01b03e
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diff --git a/foo.py b/foo.py
@@ -0,0 +1 @@
+"""Test program"""
diff --git a/pep-0635.rst b/pep-0635.rst
@@ -1003,8 +1003,13 @@ dictionary.  Particularly common are string keys.
 
 The mapping pattern reflects the common usage of dictionary lookup: it allows
 the user to extract some values from a mapping by means of constant/known
-keys and have the values match given subpatterns.  Moreover, the mapping
-pattern does not check for the presence of additional keys.  Should it be
+keys and have the values match given subpatterns.
+Extra keys in the subject are ignored even if ``**rest`` is not present.
+This is different from sequence patterns, where extra items will cause a
+match to fail.  But mappings are actually different from sequences: they
+have natural structural sub-typing behavior, i.e., passing a dictionary
+with extra keys somewhere will likely just work.
+Should it be
 necessary to impose an upper bound on the mapping and ensure that no
 additional keys are present, then the usual double-star-pattern ``**rest``
 can be used.  The special case ``**_`` with a wildcard, however, is not