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HACKING.rst: add Type Annotations design section #293

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Mar 30, 2020
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HACKING.rst: add Type Annotations design section #293

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17 changes: 17 additions & 0 deletions HACKING.rst
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Expand Up @@ -182,3 +182,20 @@ The following guidelines should be following:
.. _TestGetPackageMirrorInfo: https://github.com/canonical/cloud-init/blob/42f69f410ab8850c02b1f53dd67c132aa8ef64f5/cloudinit/distros/tests/test_init.py\#L15
.. _TestPrependBaseCommands: https://github.com/canonical/cloud-init/blob/master/cloudinit/tests/test_subp.py#L9
.. _assertion introspection: https://docs.pytest.org/en/latest/assert.html

Type Annotations
----------------

The cloud-init codebase uses Python's annotation support for storing
type annotations in the style specified by `PEP-484`_. Their use in
the codebase is encouraged but with one important caveat: types from
the ``typing`` module cannot be used.

cloud-init still supports Python 3.4, which doesn't have the ``typing``
module in the stdlib. This means that the use of any types from the
``typing`` module in the codebase would require installation of an
additional Python module on platforms using Python 3.4. As such
platforms are generally in maintenance mode, the introduction of a new
dependency may act as a break in compatibility in practical terms.

.. _PEP-484: https://www.python.org/dev/peps/pep-0484/