Sample code for: Python 3.15 Preview: Sentinel Values (#823)

* Sample code for: Python 3.15 Preview: Sentinel Values

* TR updates, first round

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Apply DR/LE findings from new skills

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* Print the enum's repr() in example_006 to match the tutorial

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Bartosz Zaczyński <bartosz@realpython.com>
Co-authored-by: Real Python Bot <42617967+realpython-bot@users.noreply.github.com>

Author: 4 people

python315-sentinel-values/README.md

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+# Python 3.15 Preview: Sentinel Values
+
+This folder provides the code examples for the Real Python tutorial [Python 3.15 Preview: Sentinel Values](https://realpython.com/python315-sentinel-values/)

python315-sentinel-values/config.py

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+UNSET = sentinel("UNSET")
+INHERIT = sentinel("INHERITED")  # Name doesn't match the variable
+
+
+class Config:
+    AUTO = sentinel("Config.AUTO")
+    # Some code here...
+    NO_LIMIT = sentinel("NO_LIMIT")  # Forgot the qualified name

python315-sentinel-values/example_001.py

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+print("Hello, World!".find("z"))

python315-sentinel-values/example_002.py

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+import configparser
+import inspect
+import typing
+import unittest.mock
+
+print(configparser._UNSET)
+print(inspect.Parameter.empty)
+print(typing.NoDefault)
+print(unittest.mock.DEFAULT)

python315-sentinel-values/example_003.py

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+import pickle
+from copy import deepcopy
+
+_MISSING = object()
+print(_MISSING)
+print(type(_MISSING))
+print(deepcopy(_MISSING) is _MISSING)
+print(pickle.loads(pickle.dumps(_MISSING)) is _MISSING)

python315-sentinel-values/example_004.py

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+grades = {"Jane": 90, "John": None}
+print(grades.get("John") is grades.get("Linda"))
+print("Pythonista!".find("z"))
+print("Pythonista!"["Pythonista!".find("z")])

python315-sentinel-values/example_005.py

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+from copy import deepcopy
+
+
+class _MissingType:
+    def __repr__(self):
+        return "MISSING"
+
+
+MISSING = _MissingType()
+print(MISSING)
+print(deepcopy(MISSING) is MISSING)

python315-sentinel-values/example_006.py

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+import enum
+
+
+class MissingType(enum.Enum):
+    MISSING = "MISSING"
+
+
+print(repr(MissingType.MISSING))

python315-sentinel-values/example_007.py

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+MISSING = sentinel("MISSING")
+print(MISSING)

python315-sentinel-values/example_008.py

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+UNSET = sentinel("UNSET", repr="<unset>")
+print(UNSET)