ADR0005: Decide on python code style guide

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

@@ -0,0 +1,46 @@
+# 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+code base, has become outdated.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.
+
+**Course of Action:**
+* Follow existing style when working on existing code (files)
+* Follow new style in any new code (files)
+* Consider providing linter and formatter configuration (e.g. pylint, flake8,
+  black, yapf) to enforce and facilitate new style
+
+
+## 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
 
 <!-- adrlogstop -->