Assigns a sentinel object to a variable so it can be used as a distinctive default value, enabling detection of missing arguments via identity comparison.

Provides a unique sentinel value that can be distinguished from legitimate data values such as None, False, or empty containers.

When defining a function or method that requires a default argument distinguishable from None, or when internal state tracking needs a unique marker object.

Function default arguments, class attributes, callbacks, or any code needing a unique marker for identity checks.

{variable} = {module}.{attribute}

variable = module.attribute

{variable}: any valid identifier; {module}: an object or module providing sentinel attributes; {attribute}: the name of a sentinel attribute (e.g., MY_DEPENDENCY).

• sentinel
• MY_DEPENDENCY
• default
• object()
• sentinel object
• default argument

• sentinel_obj = sentinel.MY_DEPENDENCY
• my_sentinel = object()
• _sentinel = object()
• _missing = object()

Using None as a sentinel when None is a valid value; using mutable defaults like [] or {}; reusing the same sentinel across unrelated contexts causing false matches; forgetting to import the sentinel module.

Using object() directly vs. importing a named sentinel; using Ellipsis (...) as sentinel; using a custom class instance as sentinel.

Confusing the sentinel with None, False, 0, or empty containers; reusing the same sentinel in unrelated code leading to ambiguous identity checks.

• sentinel assignment pattern

The sentinel must be unique within the module's namespace to avoid accidental equality; its identity, not its value, is what matters.

Signals to readers that the value is a special marker indicating a missing or default state, not a legitimate data value.

Think of a sentinel as a guard that stands watch for missing data.

Commonly used in libraries to mark missing arguments; typing.Sentinel provides a typed alternative in newer Python versions.

Consider migrating to typing.Sentinel (Python 3.11+) or typing_extensions.Sentinel for better type safety.