The with statement (__enter__ & _exit__ methods)
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.. seealso:: https://docs.python.org/3.4/reference/compound_stmts.html#with

Many (modern) classes have an ``__enter__`` and an ``__exit__`` method; this *pair* is used by the
**with**-statement. And are used to create a *context manager*.

Before executing the *block* of statements in the the **with**-statements, the ``__enter__()`` of
the object named in the *with*-statement is called. And, when leaving that *block* the
``__exit__()`` is called, always! This is convenient, as it cleans up the code “within”.

.. tip::

   * It may be convenient to think placeholder to “open” and “close” the object.

   * As the *with*-statement is often used with a newly initiated object, part of the “before” code
     is often located in the ``__init__()``; leaving only ``return self`` in the __init__.

   * The ``__exit__`` has some extra parameters, that describe “how” the block is exited. Without
     a raised exception, they are `None`; else they contain the “context” of that exception.

   * For the design-analyse, they are not that important. Every design can be coded without them!