[CHORE] Modernize Python syntax with pyupgrade (ruff UP rules)

[oberstet]

Summary

Run ruff check --fix --select UP across the codebase to modernize Python syntax automatically. This is a low-risk, high-value cleanup that improves code consistency and readability.

Suggested by @bblommers in #1838:

"Speaking of: running ruff check --select UP seems to be mostly f-strings violations, which could be fixed automatically using ruff check --fix. I would be happy to add that as part of this PR - but I would recommend running that against the entire codebase in a separate PR for that. It makes reviewing a lot easier!"

What pyupgrade (UP) Does

The UP rule set modernizes Python syntax to use newer, cleaner constructs. Key transformations include:

String Formatting → f-strings

# Before (UP031, UP032)
"Hello, %s" % name
"Hello, {}".format(name)

# After
f"Hello, {name}"

Type Annotations → Modern Syntax

# Before (UP006, UP007)
from typing import Dict, List, Optional, Union
x: Optional[str]
y: Union[int, str]
z: Dict[str, List[int]]

# After
x: str | None
y: int | str
z: dict[str, list[int]]

Other Modernizations

Rule	Before	After
UP005	assertEquals()	assertEqual()
UP008	super(Foo, self)	super()
UP009	# -*- coding: utf-8 -*-	(removed, UTF-8 is default)
UP010	from __future__ import print_function	(removed)
UP015	open("f", "r")	open("f")
UP018	str("literal")	"literal"
UP025	"unicode" encoding	(simplified)
UP028	yield in for loop	yield from
UP034	(x,) extraneous parens	x,
UP035	typing.Dict	dict
UP036	Version checks for old Python	(removed)
UP038	isinstance(x, (A, B))	isinstance(x, A | B)

Implementation

Step 1: Preview Changes

# See what would change (dry run)
ruff check --select UP src/autobahn/

Step 2: Auto-fix

# Apply fixes automatically
ruff check --fix --select UP src/autobahn/

Step 3: Manual Review

Some changes may need manual attention:

• Complex string formatting with many arguments
• Edge cases in type annotations
• Any changes that affect runtime behavior (rare but possible)

Step 4: Run Tests

just test cpy314

Scope

In scope:

• All .py files in src/autobahn/
• Automatic fixes via ruff --fix
• Manual fixes for any remaining violations

Out of scope:

• Test files (can be done in follow-up)
• Changes that would affect public API signatures
• Any behavioral changes (this is purely syntactic)

Why a Separate PR?

As @bblommers noted, keeping this separate from typing work makes review much easier:

1. Single concern — only modernization, no new types
2. Mechanical changes — easy to verify correctness
3. Clean diff — reviewers can focus on unexpected changes
4. Bisectable — if issues arise, easy to identify source

Acceptance Criteria

• ruff check --select UP src/autobahn/ reports zero violations
• All tests pass (just test cpy314)
• CI passes
• No changes to public API behavior

Assignee

@bblommers has hinted at / offered to take this on — thank you, should you decide to invest time & work!

Related

• Autobahn WebSocket Protocol - Improve typing #1838 — Initial typing contribution (merged)
• [FEATURE] Constrain codebase to statically typed and region-based lifetimes Python subset #1839 — Typed subset initiative (parent)
• [CHORE] Add ruff annotation checks to check-typing recipe #1840 — Add ruff ANN checks to check-typing recipe

References

• ruff pyupgrade (UP) rules
• pyupgrade project

Checklist

• I have searched existing issues to avoid duplicates
• I have described the problem clearly
• I have provided use cases
• I have considered alternatives
• I have assessed impact and breaking changes

[oberstet]

fixed via #1843
GH release: https://github.com/crossbario/autobahn-python/releases/tag/master-202601101042