PEP: 331 Title: Locale-Independent Float/String Conversions Version: $Revision$ Last-Modified: $Date$ Author: Christian R. Reis Status: Final Type: Standards Track Content-Type: text/x-rst Created: 19-Jul-2003 Python-Version: 2.4 Post-History: 21-Jul-2003, 13-Aug-2003, 18-Jun-2004 Abstract ======== Support for the ``LC_NUMERIC`` locale category in Python 2.3 is implemented only in Python-space. This causes inconsistent behavior and thread-safety issues for applications that use extension modules and libraries implemented in C that parse and generate floats from strings. This document proposes a plan for removing this inconsistency by providing and using substitute locale-agnostic functions as necessary. Introduction ============ Python provides generic localization services through the locale module, which among other things allows localizing the display and conversion process of numeric types. Locale categories, such as ``LC_TIME`` and ``LC_COLLATE``, allow configuring precisely what aspects of the application are to be localized. The ``LC_NUMERIC`` category specifies formatting for non-monetary numeric information, such as the decimal separator in float and fixed-precision numbers. Localization of the ``LC_NUMERIC`` category is currently implemented only in Python-space; C libraries invoked from the Python runtime are unaware of Python's ``LC_NUMERIC`` setting. This is done to avoid changing the behavior of certain low-level functions that are used by the Python parser and related code [2]_. However, this presents a problem for extension modules that wrap C libraries. Applications that use these extension modules will inconsistently display and convert floating-point values. James Henstridge, the author of PyGTK [3]_, has additionally pointed out that the ``setlocale()`` function also presents thread-safety issues, since a thread may call the C library ``setlocale()`` outside of the GIL, and cause Python to parse and generate floats incorrectly. Rationale ========= The inconsistency between Python and C library localization for ``LC_NUMERIC`` is a problem for any localized application using C extensions. The exact nature of the problem will vary depending on the application, but it will most likely occur when parsing or formatting a floating-point value. Example Problem =============== The initial problem that motivated this PEP is related to the GtkSpinButton [4]_ widget in the GTK+ UI toolkit, wrapped by the PyGTK module. The widget can be set to numeric mode, and when this occurs, characters typed into it are evaluated as a number. Problems occur when ``LC_NUMERIC`` is set to a locale with a float separator that differs from the C locale's standard (for instance, ',' instead of '.' for the Brazilian locale pt_BR). Because ``LC_NUMERIC`` is not set at the libc level, float values are displayed incorrectly (using '.' as a separator) in the spinbutton's text entry, and it is impossible to enter fractional values using the ',' separator. This small example demonstrates reduced usability for localized applications using this toolkit when coded in Python. Proposal ======== Martin v. Löwis commented on the initial constraints for an acceptable solution to the problem on python-dev: - ``LC_NUMERIC`` can be set at the C library level without breaking the parser. - ``float()`` and ``str()`` stay locale-unaware. - locale-aware ``str()`` and ``atof()`` stay in the locale module. An analysis of the Python source suggests that the following functions currently depend on ``LC_NUMERIC`` being set to the C locale: - ``Python/compile.c:parsenumber()`` - ``Python/marshal.c:r_object()`` - ``Objects/complexobject.c:complex_to_buf()`` - ``Objects/complexobject.c:complex_subtype_from_string()`` - ``Objects/floatobject.c:PyFloat_FromString()`` - ``Objects/floatobject.c:format_float()`` - ``Objects/stringobject.c:formatfloat()`` - ``Modules/stropmodule.c:strop_atof()`` - ``Modules/cPickle.c:load_float()`` The proposed approach is to implement ``LC_NUMERIC``-agnostic functions for converting from (``strtod()``/``atof()``) and to (``snprintf()``) float formats, using these functions where the formatting should not vary according to the user-specified locale. The locale module should also be changed to remove the special-casing for ``LC_NUMERIC``. This change should also solve the aforementioned thread-safety problems. Potential Code Contributions ============================ This problem was initially reported as a problem in the GTK+ libraries [5]_; since then it has been correctly diagnosed as an inconsistency in Python's implementation. However, in a fortunate coincidence, the glib library (developed primarily for GTK+, not to be confused with the GNU C library) implements a number of ``LC_NUMERIC``-agnostic functions (for an example, see [6]_) for reasons similar to those presented in this paper. In the same GTK+ problem report, Havoc Pennington suggested that the glib authors would be willing to contribute this code to the PSF, which would simplify implementation of this PEP considerably. Alex Larsson, the original author of the glib code, submitted a PSF Contributor Agreement [7]_ on 2003-08-20 [8]_ to ensure the code could be safely integrated; this agreement has been received and accepted. Risks ===== There may be cross-platform issues with the provided locale-agnostic functions, though this risk is low given that the code supplied simply reverses any locale-dependent changes made to floating-point numbers. Martin and Guido pointed out potential copyright issues with the contributed code. I believe we will have no problems in this area as members of the GTK+ and glib teams have said they are fine with relicensing the code, and a PSF contributor agreement has been mailed in to ensure this safety. Tim Peters has pointed out [9]_ that there are situations involving threading in which the proposed change is insufficient to solve the problem completely. A complete solution, however, does not currently exist. Implementation ============== An implementation was developed by Gustavo Carneiro , and attached to Sourceforge.net bug 774665 [10]_ The final patch [11]_ was integrated into Python CVS by Martin v. Löwis on 2004-06-08, as stated in the bug report. References ========== .. [2] Python locale documentation for embedding, http://docs.python.org/library/locale.html .. [3] PyGTK homepage, http://www.daa.com.au/~james/pygtk/ .. [4] GtkSpinButton screenshot (demonstrating problem), http://www.async.com.br/~kiko/spin.png .. [5] GNOME bug report, http://bugzilla.gnome.org/show_bug.cgi?id=114132 .. [6] Code submission of g_ascii_strtod and g_ascii_dtostr (later renamed g_ascii_formatd) by Alex Larsson, http://mail.gnome.org/archives/gtk-devel-list/2001-October/msg00114.html .. [7] PSF Contributor Agreement, https://www.python.org/psf/contrib/contrib-form/ .. [8] Alex Larsson's email confirming his agreement was mailed in, https://mail.python.org/pipermail/python-dev/2003-August/037755.html .. [9] Tim Peters' email summarizing LC_NUMERIC trouble with Spambayes, https://mail.python.org/pipermail/python-dev/2003-September/037898.html .. [10] Python bug report, https://bugs.python.org/issue774665 .. [11] Integrated LC_NUMERIC-agnostic patch, https://sourceforge.net/tracker/download.php?group_id=5470&atid=305470&file_id=89685&aid=774665 Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: