Sample code for: Python 3.15 Preview: Sentinel Values

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")  # Made a typo in the variable name
+
+
+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(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(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)

python315-sentinel-values/example_009.py

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+# Both calls intentionally fail. Catch each error so you can see both.
+try:
+    sentinel("UNSET", "<unset>")
+except TypeError as error:
+    print(f"Expected TypeError: {error}")
+
+try:
+    sentinel(name="UNSET")
+except TypeError as error:
+    print(f"Expected TypeError: {error}")

python315-sentinel-values/example_010.py

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+MISSING = sentinel("MISSING")
+print(bool(MISSING))
+print(hash(MISSING))
+config = {MISSING: "unset", "timeout": 30}
+print(config[MISSING])
+print(MISSING in {MISSING, 1, 2})
+print(MISSING == sentinel("MISSING"))

python315-sentinel-values/example_011.py

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+print(sentinel("STOP") is sentinel("STOP"))

python315-sentinel-values/example_012.py

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+A = sentinel("A")
+B = sentinel("B")
+print(type(A) is type(B))
+print(isinstance(A, sentinel))
+print(isinstance(B, sentinel))
+print(A is B)

python315-sentinel-values/example_013.py

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+from copy import copy, deepcopy
+
+MISSING = sentinel("MISSING")
+print(copy(MISSING) is MISSING)
+print(deepcopy(MISSING) is MISSING)

python315-sentinel-values/example_014.py

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+import pickle
+from config import Config, INHERIT, UNSET
+
+print(pickle.loads(pickle.dumps(UNSET)) is UNSET)
+print(pickle.loads(pickle.dumps(Config.AUTO)) is Config.AUTO)
+# These names intentionally don't match their module/class bindings.
+for value in (INHERIT, Config.NO_LIMIT):
+    try:
+        pickle.dumps(value)
+    except pickle.PicklingError as error:
+        print(f"Expected PicklingError: {error}")

python315-sentinel-values/example_015.py

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+import inspect
+
+from settings_legacy import get_setting
+
+help(get_setting)
+print(inspect.signature(get_setting))

python315-sentinel-values/example_016.py

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+import inspect
+
+from settings_sentinel import get_setting
+
+help(get_setting)
+print(inspect.signature(get_setting))

python315-sentinel-values/example_017.py

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+import typing
+from settings_typed_legacy import get_setting
+
+hints = typing.get_type_hints(get_setting)
+print(hints["default"])

python315-sentinel-values/example_018.py

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+import typing
+from settings_typed_sentinel import get_setting
+
+hints = typing.get_type_hints(get_setting)
+print(hints["default"])

python315-sentinel-values/example_019.py

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+from profiles_legacy import apply_patch, parse_patch
+
+profile = {
+    "name": "J. Doe",
+    "bio": "Data scientist",
+    "email": "jane@example.com",
+}
+patch = parse_patch({"name": "jane", "bio": None})
+print(apply_patch(profile, patch))

python315-sentinel-values/example_020.py

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+from copy import deepcopy
+from profiles_legacy import _MISSING, parse_patch
+
+patch = parse_patch({"name": "jane", "bio": None})
+print(deepcopy(patch)["email"] is _MISSING)

python315-sentinel-values/example_021.py

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+import pickle
+from profiles_legacy import _MISSING, parse_patch
+
+patch = parse_patch({"name": "jane", "bio": None})
+print(pickle.loads(pickle.dumps(patch))["email"] is _MISSING)

python315-sentinel-values/example_022.py

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+import pickle
+from copy import deepcopy
+from profiles_sentinel import MISSING, parse_patch
+
+patch = parse_patch({"name": "jane", "bio": None})
+print(deepcopy(patch)["email"] is MISSING)
+print(pickle.loads(pickle.dumps(patch))["email"] is MISSING)

python315-sentinel-values/example_023.py

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+STOP = "done"
+
+
+def producer():
+    words = ["ready", "done", "pending"]  # Simulate produced words
+    for word in words:
+        yield word
+    yield STOP
+
+
+words_stream = producer()
+for word in words_stream:
+    if word == STOP:
+        break
+    print(f"Processing '{word}'...")

python315-sentinel-values/example_024.py

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+STOP = sentinel("STOP")
+
+
+def producer():
+    words = ["ready", "done", "pending"]
+    for word in words:
+        yield word
+    yield STOP
+
+
+words_stream = producer()
+for word in words_stream:
+    if word is STOP:
+        break
+    print(f"Processing '{word}'...")

python315-sentinel-values/missing.py

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+class _MissingType:
+    def __init__(self, name):
+        self._name = name
+
+    def __repr__(self):
+        return self._name
+
+    def __copy__(self):
+        return self
+
+    def __deepcopy__(self, memo):
+        return self
+
+    def __reduce__(self):
+        return self._name
+
+    def __eq__(self, other):
+        return self is other
+
+    def __hash__(self):
+        return id(self)

python315-sentinel-values/profiles_legacy.py

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+# Python 3.14 and older approach
+_MISSING = object()
+
+
+def parse_patch(form):
+    fields = ("name", "bio", "email")
+    return {field: form.get(field, _MISSING) for field in fields}
+
+
+def apply_patch(profile, patch):
+    updated = dict(profile)
+    for field, value in patch.items():
+        if value is not _MISSING:
+            updated[field] = value
+    return updated

python315-sentinel-values/profiles_sentinel.py

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+# Python 3.15 or newer
+MISSING = sentinel("MISSING")
+
+
+def parse_patch(form):
+    fields = ("name", "bio", "email")
+    return {field: form.get(field, MISSING) for field in fields}
+
+
+def apply_patch(profile, patch):
+    updated = dict(profile)
+    for field, value in patch.items():
+        if value is not MISSING:
+            updated[field] = value
+    return updated

python315-sentinel-values/ruff.toml

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+extend = "../pyproject.toml"
+
+# The repository's pinned Ruff predates Python 3.15's sentinel built-in.
+builtins = ["sentinel"]

python315-sentinel-values/sentinel_type.py

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+# Python 3.15 or newer
+print(sentinel)

python315-sentinel-values/settings_legacy.py

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+# Python 3.14 and older approach
+_NO_DEFAULT = object()
+
+
+def get_setting(config, path, default=_NO_DEFAULT):
+    current = config
+    for part in path.split("."):
+        try:
+            current = current[part]
+        except (KeyError, TypeError):
+            if default is _NO_DEFAULT:
+                raise KeyError(path) from None
+            return default
+    return current

python315-sentinel-values/settings_sentinel.py

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+# Python 3.15 or newer
+NO_DEFAULT = sentinel("NO_DEFAULT")
+
+
+def get_setting(config, path, default=NO_DEFAULT):
+    current = config
+    for part in path.split("."):
+        try:
+            current = current[part]
+        except (KeyError, TypeError):
+            if default is NO_DEFAULT:
+                raise KeyError(path) from None
+            return default
+    return current

python315-sentinel-values/settings_typed_legacy.py

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+# Python 3.14 and older approach, with type hints
+_NO_DEFAULT = object()
+
+
+def get_setting(
+    config: dict[str, object],
+    path: str,
+    default: object = _NO_DEFAULT,
+) -> str | int:
+    current = config
+    for part in path.split("."):
+        try:
+            current = current[part]
+        except (KeyError, TypeError):
+            if default is _NO_DEFAULT:
+                raise KeyError(path) from None
+            return default
+    return current

python315-sentinel-values/settings_typed_sentinel.py

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+# Python 3.15 or newer, with type hints
+NO_DEFAULT = sentinel("NO_DEFAULT")
+
+
+def get_setting(
+    config: dict[str, object],
+    path: str,
+    default: str | int | NO_DEFAULT = NO_DEFAULT,
+) -> str | int:
+    current = config
+    for part in path.split("."):
+        try:
+            current = current[part]
+        except (KeyError, TypeError):
+            if default is NO_DEFAULT:
+                raise KeyError(path) from None
+            return default
+    return current