gh-143387: Raise an exception instead of returning None when metadata file is missing. by jaraco · Pull Request #146234 · python/cpython · GitHub
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gh-143387: Raise an exception instead of returning None when metadata file is missing. #146234

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Merged
jaraco merged 2 commits into from
Mar 23, 2026
Merged

gh-143387: Raise an exception instead of returning None when metadata file is missing. #146234

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Raise an exception instead of returning None when metadata file is mi…
jaraco Mar 20, 2026
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jaraco Mar 20, 2026
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35 changes: 27 additions & 8 deletions Lib/importlib/metadata/__init__.py
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118 changes: 118 additions & 0 deletions Lib/importlib/metadata/_context.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,118 @@
from __future__ import annotations

import functools
import operator


# from jaraco.context 6.1
class ExceptionTrap:
"""
A context manager that will catch certain exceptions and provide an
indication they occurred.

>>> with ExceptionTrap() as trap:
... raise Exception()
>>> bool(trap)
True

>>> with ExceptionTrap() as trap:
... pass
>>> bool(trap)
False

>>> with ExceptionTrap(ValueError) as trap:
... raise ValueError("1 + 1 is not 3")
>>> bool(trap)
True
>>> trap.value
ValueError('1 + 1 is not 3')
>>> trap.tb
<traceback object at ...>

>>> with ExceptionTrap(ValueError) as trap:
... raise Exception()
Traceback (most recent call last):
...
Exception

>>> bool(trap)
False
"""

exc_info = None, None, None

def __init__(self, exceptions=(Exception,)):
self.exceptions = exceptions

def __enter__(self):
return self

@property
def type(self):
return self.exc_info[0]

@property
def value(self):
return self.exc_info[1]

@property
def tb(self):
return self.exc_info[2]

def __exit__(self, *exc_info):
type = exc_info[0]
matches = type and issubclass(type, self.exceptions)
if matches:
self.exc_info = exc_info
return matches

def __bool__(self):
return bool(self.type)

def raises(self, func, *, _test=bool):
"""
Wrap func and replace the result with the truth
value of the trap (True if an exception occurred).

First, give the decorator an alias to support Python 3.8
Syntax.

>>> raises = ExceptionTrap(ValueError).raises

Now decorate a function that always fails.

>>> @raises
... def fail():
... raise ValueError('failed')
>>> fail()
True
"""

@functools.wraps(func)
def wrapper(*args, **kwargs):
with ExceptionTrap(self.exceptions) as trap:
func(*args, **kwargs)
return _test(trap)

return wrapper

def passes(self, func):
"""
Wrap func and replace the result with the truth
value of the trap (True if no exception).

First, give the decorator an alias to support Python 3.8
Syntax.

>>> passes = ExceptionTrap(ValueError).passes

Now decorate a function that always fails.

>>> @passes
... def fail():
... raise ValueError('failed')

>>> fail()
False
"""
return self.raises(func, _test=operator.not_)
11 changes: 7 additions & 4 deletions Lib/test/test_importlib/metadata/test_main.py
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
from importlib.metadata import (
Distribution,
EntryPoint,
MetadataNotFound,
PackageNotFoundError,
_unique,
distributions,
Expand Down Expand Up @@ -159,13 +160,15 @@ def test_valid_dists_preferred(self):

def test_missing_metadata(self):
"""
Dists with a missing metadata file should return None.
Dists with a missing metadata file should raise ``MetadataNotFound``.

Ref python/importlib_metadata#493.
Ref python/importlib_metadata#493 and python/cpython#143387.
"""
fixtures.build_files(self.make_pkg('foo-4.3', files={}), self.site_dir)
assert Distribution.from_name('foo').metadata is None
assert metadata('foo') is None
with self.assertRaises(MetadataNotFound):
Distribution.from_name('foo').metadata
with self.assertRaises(MetadataNotFound):
metadata('foo')


class NonASCIITests(fixtures.OnSysPath, fixtures.SiteDir, unittest.TestCase):
Expand Down
7 changes: 7 additions & 0 deletions Misc/NEWS.d/next/Library/2026-03-20-16-17-31.gh-issue-143387.9Waopa.rst
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