Fix mutable default argument in vttests/common.py::sgr_n
#19028
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[sgr_n](cci:1://file:///d:/Github/terminal/src/tools/vttests/common.py:61:0-73:58) previously used a mutable list (`seq=[]`) as its default parameter. In Python, default arguments are instantiated once at function‐definition time and then shared across all calls. If any caller mutated that list (e.g., `seq.append(...)`), later invocations would inherit the mutated state, producing unpredictable or corrupted VT-test sequences. #### What’s fixed * Replaced the default with `seq=None` and create a new list when `None` is passed. * Added a clear docstring detailing the function and the rationale for the change. #### Why it matters * Prevents hidden state leakage between test cases, ensuring reproducible VT escape-sequence output. * Guards against subtle bugs that could invalidate rendering tests or complicate debugging. #### Impact No behavior change for existing callers that pass an explicit sequence. All internal tools and CI VT tests now operate with guaranteed clean state on each call to [sgr_n](cci:1://file:///d:/Github/terminal/src/tools/vttests/common.py:61:0-73:58).
lhecker
approved these changes
Jun 16, 2025
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Overview
sgr_n previously used a mutable list (
seq=[]) as its default parameter.In Python, default arguments are instantiated once at function‐definition time and then shared across all calls.
If any caller mutated that list (e.g.,
seq.append(...)), later invocations would inherit the mutated state, producing unpredictable or corrupted VT-test sequences.What’s fixed
seq=Noneand create a new list whenNoneis passed.Why it matters
Impact
No behavior change for existing callers that pass an explicit sequence.
All internal tools and CI VT tests now operate with guaranteed clean state on each call to sgr_n.