The ``sentinel`` object provides a convenient way of providing unique

objects for your tests.

Attributes are created on demand when you access them by name. Accessing

the same attribute will always return the same object. The objects

returned have a sensible repr so that test failure messages are readable.

A helper function to create a mock to replace the use of :func:`open`. It works

for :func:`open` called directly or used as a context manager.

The *mock* argument is the mock object to configure. If ``None`` (the

default) then a :class:`MagicMock` will be created for you, with the API limited

to methods or attributes available on standard file handles.

*read_data* is a string for the :meth:`~io.IOBase.read`,

:meth:`~io.IOBase.readline`, and :meth:`~io.IOBase.readlines` methods

of the file handle to return. Calls to those methods will take data from

*read_data* until it is depleted. The mock of these methods is pretty

simplistic: every time the *mock* is called, the *read_data* is rewound to

the start. If you need more control over the data that you are feeding to

the tested code you will need to customize this mock for yourself. When that

is insufficient, one of the in-memory filesystem packages on `PyPI

<https://pypi.org>`_ can offer a realistic filesystem for testing.