Tcl Source Code

format converts floating-point values with the C library function snprintf (generic/tclStringObj.c, Tcl_AppendFormatToObj), which honours LC_NUMERIC. Under a comma-decimal locale, format emits a value that Tcl then refuses to parse, because its own number parser is locale-independent:

% format %.1f 0.3
0,3
% expr {[format %.1f 0.3] + 0}
expected floating-point number but got "0,3"

Tcl resets LC_NUMERIC to "C" at init, so this only shows up under an embedder that sets a comma locale afterwards - for example a Python/tkinter program calling locale.setlocale(locale.LC_NUMERIC, "de_DE.UTF-8").

Tcl_GetDoubleFromObj and Tcl_PrintDouble are already locale-independent (Tcl_PrintDouble was fixed in https://core.tcl-lang.org/tcl/tktview/3120139); only format still leaks the locale. Making its floating-point conversions locale-independent (always .) would match that precedent and the new Tk helper TkFormatDouble (https://core.tcl-lang.org/tk/tktview/348b482ff73e).

Concrete fallout: ttk::spinbox formats its value with [format $fmt $val], so under a comma locale it writes "0,3" into its -textvariable and breaks (reported downstream as CPython issue https://github.com/python/cpython/issues/84008; the ttk-side remainder of the Tk fix above).