ADR0005: Decide on python code style guide

Use Google style guide with refinements, because the Google style
guide is a comprehensive, well-established style guide that is
mostly based on PEP-8 and was accepted by everyone on the TUF team.

There is no need to replicate these recommendations. However, we do
provide a very slim document with additional refinements, in order
to emphasize on items the we consider especially important, want to
be handled differently, or in one specific way, where the Google
guide would allow multiple.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>
Co-authored-by: Joshua Lock <jlock@vmware.com>

Author: 2 people

docs/adr/0005-use-google-python-style-guide.md

@@ -0,0 +1,40 @@
+# Use Google Python style guide with minimal refinements
+
+Technical Story: https://github.com/theupdateframework/tuf/issues/1128
+
+## Context and Problem Statement
+
+The Secure Systems Lab code style guide, which has been used for most of the
+code base, has become outdated. Through the upcoming rewrite, we have the
+chance to ignore consistency considerations with existing code style and can
+choose a more standard and up-to-date style guide.
+
+## Decision Drivers
+
+* Flaws in original Secure Systems Lab style guide
+* Curating a complete custom style guide is time consuming
+* Well-established style rules lower contribution barrier
+* Custom style is not supported by default in common tooling (i.e. editors
+  and linters)
+
+## Considered Options
+
+* Use custom style guide
+* Use Google style guide with refinements
+
+## Decision Outcome
+
+Chosen option: "Use Google style guide with refinements", because the Google
+style guide is a comprehensive, well-established style guide that is mostly
+based on PEP-8 and was accepted by everyone on the TUF team. There is no need
+to replicate these recommendations. However, we do provide a very slim document
+with additional refinements, in order to emphasize items the we consider
+especially important, want to be handled differently, or in one specific way,
+where the Google guide would allow multiple.
+
+
+## Links
+* [New Slim Secure Systems Lab style guide](https://github.com/secure-systems-lab/code-style-guidelines/pull/21)
+* [Google Python style guide](https://google.github.io/styleguide/pyguide.html)
+* [PEP 8](https://www.python.org/dev/peps/pep-0008/)
+* [Issues in original Secure Systems Lab style guide](https://github.com/secure-systems-lab/code-style-guidelines/issues/20)

docs/adr/index.md

@@ -9,6 +9,7 @@ This log lists the architectural decisions for tuf.
 - [ADR-0002](0002-pre-1-0-deprecation-strategy.md) - Deprecation strategy
 - [ADR-0003](0003-where-to-develop-TUF-1-0-0.md) - Develop TUF 1.0.0 in a subdirectory of the current TUF implementation
 - [ADR-0004](0004-extent-of-OOP-in-metadata-model.md) - Add classes for complex metadata attributes
+- [ADR-0005](0005-use-google-python-style-guide.md) - Use Google Python style guide with minimal refinements
 
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