[docutils:bugs] #430 avoid mutables as default functions arguments

- **status**: open-fixed --> closed-fixed
- **Comment**:

Fixed in Docutils 0.19.
Thank you for the report.



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** [bugs:#430] avoid mutables as default functions arguments**

**Status:** closed-fixed
**Labels:** rst parser 
**Created:** Mon Oct 25, 2021 09:53 AM UTC by Dimitri Papadopoulos
**Last Updated:** Wed Dec 29, 2021 12:19 PM UTC
**Owner:** Günter Milde


Mutables shouldn't be used as default parameters, it is a Python anti-pattern. See for example:
* [Default Parameter Values in Python](https://web.archive.org/web/20200221224620/http://effbot.org/zone/default-values.htm)
* [Python Mutable Defaults Are The Source of All Evil](https://florimond.dev/en/posts/2018/08/python-mutable-defaults-are-the-source-of-all-evil/)
* [The Hitchhiker's Guide to Python - Common Gotchas](https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments)
* [Python Pitfall: Mutable Default Arguments](https://towardsdatascience.com/python-pitfall-mutable-default-arguments-9385e8265422)

Mutable default arguments are used in a few places in docutils. Just grep `=[]` and `={}` and retain function definitions.

Mutable default arguments can also be found in the documentation:
https://docutils.sourceforge.io/docs/howto/rst-roles.html#define-the-role-function
https://docutils.sourceforge.io/docs/howto/rst-roles.html#rfc-reference-role

Of course, that's not an immediate problem if the mutable arguments are actually not mutated by the function, but it remains an anti-pattern that may create problems in the future.


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