diff --git a/CHANGES.rst b/CHANGES.rst
index cf7dcf16..4a03eb0b 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -1,3 +1,18 @@
+v6.0.0
+======
+
+* #371: When a key is missing from metadata, raise a ``KeyError``
+ instead of returning ``None``, matching the usual expectation for
+ mapping objects and also the protocol definition.
+
+ Projects should update to expect the ``KeyError`` or wrap the call
+ to replace a ``KeyError`` with a ``None`` return, e.g.::
+
+ try:
+ value = metadata(pkg)['Name']
+ except KeyError:
+ value = None
+
v5.1.0
======
diff --git a/importlib_metadata/_adapters.py b/importlib_metadata/_adapters.py
index aa460d3e..c8576cd9 100644
--- a/importlib_metadata/_adapters.py
+++ b/importlib_metadata/_adapters.py
@@ -39,6 +39,20 @@ def __init__(self, *args, **kwargs):
def __iter__(self):
return super().__iter__()
+ def __getitem__(self, item):
+ """
+ Prefer dict-like behavior for __getitem__ when keys are missing.
+ >>> msg = Message(email.message.Message())
+ >>> msg['thing']
+ Traceback (most recent call last):
+ ...
+ KeyError: 'thing'
+ """
+ res = super().__getitem__(item)
+ if res is None:
+ raise KeyError(item)
+ return res
+
def _repair_headers(self):
def redent(value):
"Correct for RFC822 indentation"
diff --git a/tests/test_api.py b/tests/test_api.py
index f65287a5..d3ae8144 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -141,6 +141,14 @@ def test_importlib_metadata_version(self):
resolved = version('importlib-metadata')
assert re.match(self.version_pattern, resolved)
+ def test_missing_key(self):
+ """
+ Attempting to request missing metadata raises KeyError.
+ """
+ md = metadata('distinfo-pkg')
+ with self.assertRaises(KeyError):
+ md['does-not-exist']
+
@staticmethod
def _test_files(files):
root = files[0].root
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