chore(eval): drop Python 2 coding cookie prepended to evaluated source#6089
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The `# -*- coding: utf-8 -*-` line prepended to every string passed to PyRun_String was a Python 2 workaround. Python 3's PyRun_String already assumes UTF-8 source encoding, so the cookie is dead weight — and harmful: it shifts all line numbers up by one in tracebacks and SyntaxErrors from evaluated code (a `raise` on line 2 would incorrectly appear as line 3). Remove the cookie and replace the stale comment with one that explains the actual reason for the `std::string` conversion (need a C string for PyRun_String). No other location in eval.h uses this pattern (eval_file reads directly from a FILE*). Assisted-by: ClaudeCode:claude-fable-5
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Per https://docs.python.org/3/reference/lexical_analysis.html#encoding-declarations:
If no encoding declaration is found, the default encoding is UTF-8.
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Not only is this a Python 2 holdover, but it makes traceback line numbers be off by one!
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Summary
# -*- coding: utf-8 -*-line that was prepended to every string passed toPyRun_Stringinpy::eval/py::exec.PyRun_Stringalready assumes UTF-8 source — the cookie is a Python 2 artifact that serves no purpose.SyntaxErrors from evaluated code. Code thatraises on line 2 of the evaluated string now correctly appears as line 2 in tracebacks (previously it appeared as line 3).std::stringconversion (obtaining aconst char*forPyRun_String).eval_filereads directly from aFILE*viaPyRun_FileExand was never affected.Verification
Tested on macOS with Python 3.14 by compiling a scratch extension and confirming:
py::eval("1+1")returns 2.héllo) round-trips correctly.py::exec("x = 1\nraise ValueError('line2error')")now correctly reports the traceback at line 2 (previously line 3 due to the prepended cookie).No tests assert on line numbers in evaluated code, so no test updates were needed.
Part of #6084