fix: rename shadowed 'format' builtin in _render_graphviz

[dashitongzhi]

Problem

In burr/core/graph.py, the function _render_graphviz() uses format as a local variable name (lines 98, 100). This shadows Python's built-in format() function, which can lead to confusion and potential bugs if the builtin is needed within the function's scope. Many linters (flake8 A001/A002, pylint W0622) flag this as a code smell.

Fix

Renamed the local variable from format to fmt across all 5 occurrences within the function:

# Before
format = output_file_path.suffix.partition(".")[-1]
format = "png"
graphviz_obj.render(path_without_suffix, format=format, view=view)
pathlib.Path(f"{path_without_suffix}.{format}").write_bytes(...)
graphviz_obj.pipe(format=format)

# After
fmt = output_file_path.suffix.partition(".")[-1]
fmt = "png"
graphviz_obj.render(path_without_suffix, format=fmt, view=view)
pathlib.Path(f"{path_without_suffix}.{fmt}").write_bytes(...)
graphviz_obj.pipe(format=fmt)

The keyword argument format= in calls to graphviz_obj.render() and graphviz_obj.pipe() is preserved as-is — only the local variable assignment and its usages are renamed. All behavior remains identical.

[Copilot]

Pull request overview

This PR removes a local variable name that shadows Python’s built-in format() inside burr/core/graph.py:_render_graphviz(), reducing linter warnings and improving readability without changing behavior.

Changes:

• Renames the local variable format to fmt within _render_graphviz().
• Updates all uses of the renamed variable while preserving the format= keyword argument in Graphviz calls.

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[dashitongzhi]

Friendly ping for review 🙏 This is a small, focused fix. CI is passing. Thank you!

[dashitongzhi]

Hi! 👋 This PR has passed all CI checks and looks ready for review. Could a maintainer please take a look when they have a chance? Thank you!

[skrawcz]

LGTM — clean rename of a shadowed builtin. Variable format → fmt in _render_graphviz(), no behavioral change. All graph tests pass.

[skrawcz]

The pre-commit black check is failing because with the shorter variable name fmt, the line now fits on a single line. Quick fix:

-        pathlib.Path(f"{path_without_suffix}.{fmt}").write_bytes(
-            graphviz_obj.pipe(format=fmt)
-        )
+        pathlib.Path(f"{path_without_suffix}.{fmt}").write_bytes(graphviz_obj.pipe(format=fmt))

Or just run pre-commit run --all-files and commit the result.