Tk Source Code

scale, spinbox and place format floating-point values with the C library's locale-sensitive conversions (snprintf("%g"), Tcl_ObjPrintf("%g")), which honour LC_NUMERIC. Under a locale that uses , as the decimal separator they produce strings like "0,3" — but these values go into linked -variables, subcommand results, -command callbacks and place info, where a valid Tcl number is required. "0,3" does not parse:

expected floating-point number but got "0,3"

Tk's number parsing is already locale-independent (Tcl_GetDoubleFromObj; strtod removed in 8.7), so only formatting is affected — a value Tk writes can no longer be read back by Tk.

Affected sites: scale -variable/get/-command (tkScale.c, tkUnixScale.c); spinbox value on step/-format (tkEntry.c); place info -relx/-rely/ -relwidth/-relheight (tkPlace.c).

Reproducing

LC_NUMERIC must be a comma locale while the widget runs. Tcl resets it to "C" at init, so plain tclsh/wish is unaffected; an embedder that calls setlocale(LC_NUMERIC, ...) afterwards triggers it — e.g. a Python program doing locale.setlocale(locale.LC_NUMERIC, "de_DE.UTF-8") before using tkinter.

setlocale(LC_NUMERIC, "de_DE.UTF-8");   /* after Tcl_Init/Tk_Init */
spinbox .s -from 0 -to 1 -increment 0.1 -textvariable v; .s set 0; .s invoke buttonup
scale .sc -from 0 -to 1 -resolution 0.1 -variable w; .sc set 0.3
frame .f; place .f -relx 0.3 -rely 0.6

Observed (9.1b0): v=0,1 w=0,3 [.sc get]=0,3 [place info .f] ... -relx 0,3 ... and expr {$v + 0} raises the error above. Expected: v=0.1 w=0.3 0.3 ... -relx 0.3 ...

Root cause and fix

Tcl numbers always use . (Tcl_PrintDouble, Tcl_GetDoubleFromObj); these widgets bypass that and call the C library, so output follows LC_NUMERIC.

The attached patch adds a helper

int TkFormatDouble(char *buffer, size_t size, const char *format, double value);

that does the snprintf() and, when localeconv()->decimal_point is a single-byte separator other than ., replaces it (a %f/%e/%g conversion emits at most one). The widget value conversions are routed through it. 6 files (tkUtil.c + tkInt.h, tkScale.c, tkEntry.c, tkPlace.c, tkUnixScale.c), +94/-19. unix/tkUnixScale.c is the scale display file compiled on all platforms (win/Makefile.in and the Aqua build both use it), so there is no separate macOS/Windows source.

With the patch the reproducer gives the expected output in any locale, the "C" locale is unchanged, and scale/place/spinbox/entry tests pass (the locally seen entry-3.1 bbox failure is font/display dependent and fails identically without the patch).

Notes

• 8.6 backport: ported and verified the same way; identical sites, only Tcl_NewStringObj(s, -1) instead of ... TCL_INDEX_NONE. Separate patch on request. (8.6 also still has the parsing bug — "bad screen distance" via strtod — fixed in 8.7; out of scope here.)
• Open question: the patch makes these widgets render . in every locale (matching the default "C" behaviour). If display should instead show the localized separator while only machine-readable values stay ., drop the tkUnixScale.c drawing hunks; the rest still fixes the round-trip.

Downstream

• CPython gh-84008 — this bug (scale/spinbox + DoubleVar).
• CPython gh-56767 — the parsing side, already fixed in Tk 8.7+.

ttk::spinbox is not covered by this fix: it formats its value in script with [format $fmt $val], and Tcl's format itself honours LC_NUMERIC. Tracked separately as https://core.tcl-lang.org/tcl/tktview/fcd3dadd79cf.