275 lines
12 KiB
TeX
275 lines
12 KiB
TeX
\section{Standard Module \sectcode{locale}}
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\stmodindex{locale}
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\label{module-locale}
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The \code{locale} module opens access to the \POSIX{} locale database
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and functionality. The \POSIX{} locale mechanism allows applications
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to integrate certain cultural aspects into an applications, without
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requiring the programmer to know all the specifics of each country
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where the software is executed.
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The \module{locale} module is implemented on top of the
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\module{_locale}\refbimodindex{_locale} module, which in turn uses an
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ANSI \C{} locale implementation if available.
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The \module{locale} module defines the following exception and
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functions:
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\begin{funcdesc}{setlocale}{category\optional{, value}}
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If \var{value} is specified, modifies the locale setting for the
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\var{category}. The available categories are listed in the data
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description below. The value is the name of a locale. An empty string
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specifies the user's default settings. If the modification of the
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locale fails, the exception \exception{Error} is
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raised. If successful, the new locale setting is returned.
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If no \var{value} is specified, the current setting for the
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\var{category} is returned.
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\function{setlocale()} is not thread safe on most systems. Applications
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typically start with a call of
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\begin{verbatim}
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import locale
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locale.setlocale(locale.LC_ALL,"")
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\end{verbatim}
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This sets the locale for all categories to the user's default setting
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(typically specified in the \code{LANG} environment variable). If the
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locale is not changed thereafter, using multithreading should not
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cause problems.
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\end{funcdesc}
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\begin{excdesc}{Error}
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Exception raised when \function{setlocale()} fails.
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\end{excdesc}
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\begin{funcdesc}{localeconv}{}
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Returns the database of of the local conventions as a dictionary. This
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dictionary has the following strings as keys:
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\begin{itemize}
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\item \code{decimal_point} specifies the decimal point used in
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floating point number representations for the \code{LC_NUMERIC}
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category.
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\item \code{grouping} is a sequence of numbers specifying at which
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relative positions the \code{thousands_sep} is expected. If the
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sequence is terminated with \code{locale.CHAR_MAX}, no further
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grouping is performed. If the sequence terminates with a \code{0}, the last
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group size is repeatedly used.
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\item \code{thousands_sep} is the character used between groups.
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\item \code{int_curr_symbol} specifies the international currency
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symbol from the \code{LC_MONETARY} category.
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\item \code{currency_symbol} is the local currency symbol.
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\item \code{mon_decimal_point} is the decimal point used in monetary
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values.
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\item \code{mon_thousands_sep} is the separator for grouping of
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monetary values.
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\item \code{mon_grouping} has the same format as the \code{grouping}
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key; it is used for monetary values.
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\item \code{positive_sign} and \code{negative_sign} gives the sign
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used for positive and negative monetary quantities.
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\item \code{int_frac_digits} and \code{frac_digits} specify the number
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of fractional digits used in the international and local formatting
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of monetary values.
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\item \code{p_cs_precedes} and \code{n_cs_precedes} specifies whether
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the currency symbol precedes the value for positive or negative
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values.
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\item \code{p_sep_by_space} and \code{n_sep_by_space} specifies
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whether there is a space between the positive or negative value and
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the currency symbol.
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\item \code{p_sign_posn} and \code{n_sign_posn} indicate how the
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sign should be placed for positive and negative monetary values.
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\end{itemize}
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The possible values for \code{p_sign_posn} and \code{n_sign_posn}
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are given below.
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\begin{tableii}{|c|l|}{code}{Value}{Explanation}
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\lineii{0}{Currency and value are surrounded by parentheses.}
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\lineii{1}{The sign should precede the value and currency symbol.}
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\lineii{2}{The sign should follow the value and currency symbol.}
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\lineii{3}{The sign should immediately precede the value.}
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\lineii{4}{The sign should immediately follow the value.}
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\lineii{LC_MAX}{Nothing is specified in this locale.}
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\end{tableii}
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\end{funcdesc}
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\begin{funcdesc}{strcoll}{string1,string2}
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Compares two strings according to the current \constant{LC_COLLATE}
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setting. As any other compare function, returns a negative, or a
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positive value, or \code{0}, depending on whether \var{string1}
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collates before or after \var{string2} or is equal to it.
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\end{funcdesc}
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\begin{funcdesc}{strxfrm}{string}
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Transforms a string to one that can be used for the built-in function
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\function{cmp()}\bifuncindex{cmp}, and still returns locale-aware
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results. This function can be used when the same string is compared
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repeatedly, e.g. when collating a sequence of strings.
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\end{funcdesc}
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\begin{funcdesc}{format}{format, val, \optional{grouping\code{ = 0}}}
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Formats a number \var{val} according to the current
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\constant{LC_NUMERIC} setting. The format follows the conventions of
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the \code{\%} operator. For floating point values, the decimal point
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is modified if appropriate. If \var{grouping} is true, also takes the
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grouping into account.
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\end{funcdesc}
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\begin{funcdesc}{str}{float}
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Formats a floating point number using the same format as the built-in
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function \code{str(\var{float})}, but takes the decimal point into
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account.
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\end{funcdesc}
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\begin{funcdesc}{atof}{string}
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Converts a string to a floating point number, following the
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\constant{LC_NUMERIC} settings.
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\end{funcdesc}
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\begin{funcdesc}{atoi}{string}
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Converts a string to an integer, following the \constant{LC_NUMERIC}
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conventions.
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\end{funcdesc}
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\begin{datadesc}{LC_CTYPE}
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\refstmodindex{string}
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Locale category for the character type functions. Depending on the
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settings of this category, the functions of module \module{string}
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dealing with case change their behaviour.
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\end{datadesc}
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\begin{datadesc}{LC_COLLATE}
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Locale category for sorting strings. The functions
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\function{strcoll()} and \function{strxfrm()} of the \module{locale}
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module are affected.
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\end{datadesc}
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\begin{datadesc}{LC_TIME}
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Locale category for the formatting of time. The function
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\function{time.strftime()} follows these conventions.
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\end{datadesc}
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\begin{datadesc}{LC_MONETARY}
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Locale category for formatting of monetary values. The available
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options are available from the \function{localeconv()} function.
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\end{datadesc}
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\begin{datadesc}{LC_MESSAGES}
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Locale category for message display. Python currently does not support
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application specific locale-aware messages. Messages displayed by the
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operating system, like those returned by \function{os.strerror()}
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might be affected by this category.
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\end{datadesc}
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\begin{datadesc}{LC_NUMERIC}
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Locale category for formatting numbers. The functions
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\function{format()}, \function{atoi()}, \function{atof()} and
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\function{str()} of the \module{locale} module are affected by that
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category. All other numeric formatting operations are not affected.
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\end{datadesc}
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\begin{datadesc}{LC_ALL}
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Combination of all locale settings. If this flag is used when the
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locale is changed, setting the locale for all categories is
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attempted. If that fails for any category, no category is changed at
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all. When the locale is retrieved using this flag, a string indicating
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the setting for all categories is returned. This string can be later
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used to restore the settings.
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\end{datadesc}
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\begin{datadesc}{CHAR_MAX}
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This is a symbolic constant used for different values returned by
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\function{localeconv()}.
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\end{datadesc}
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Example:
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\begin{verbatim}
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>>> import locale
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>>> loc = locale.setlocale(locale.LC_ALL) # get current locale
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>>> locale.setlocale(locale.LC_ALL, "de") # use German locale
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>>> locale.strcoll("f\344n", "foo") # compare a string containing an umlaut
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>>> locale.setlocale(locale.LC_ALL, "") # use user's preferred locale
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>>> locale.setlocale(locale.LC_ALL, "C") # use default (C) locale
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>>> locale.setlocale(locale.LC_ALL, loc) # restore saved locale
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\end{verbatim}
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\subsection{Background, details, hints, tips and caveats}
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The C standard defines the locale as a program-wide property that may
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be relatively expensive to change. On top of that, some
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implementation are broken in such a way that frequent locale changes
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may cause core dumps. This makes the locale somewhat painful to use
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correctly.
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Initially, when a program is started, the locale is the \samp{C} locale, no
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matter what the user's preferred locale is. The program must
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explicitly say that it wants the user's preferred locale settings by
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calling \code{setlocale(LC_ALL, "")}.
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It is generally a bad idea to call \function{setlocale()} in some library
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routine, since as a side effect it affects the entire program. Saving
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and restoring it is almost as bad: it is expensive and affects other
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threads that happen to run before the settings have been restored.
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If, when coding a module for general use, you need a locale
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independent version of an operation that is affected by the locale
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(e.g. \function{string.lower()}, or certain formats used with
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\function{time.strftime()})), you will have to find a way to do it
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without using the standard library routine. Even better is convincing
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yourself that using locale settings is okay. Only as a last resort
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should you document that your module is not compatible with
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non-\samp{C} locale settings.
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The case conversion functions in the
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\module{string}\refstmodindex{string} and
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\module{strop}\refbimodindex{strop} modules are affected by the locale
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settings. When a call to the \function{setlocale()} function changes
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the \constant{LC_CTYPE} settings, the variables
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\code{string.lowercase}, \code{string.uppercase} and
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\code{string.letters} (and their counterparts in \module{strop}) are
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recalculated. Note that this code that uses these variable through
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`\keyword{from} ... \keyword{import} ...', e.g. \code{from string
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import letters}, is not affected by subsequent \function{setlocale()}
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calls.
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The only way to perform numeric operations according to the locale
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is to use the special functions defined by this module:
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\function{atof()}, \function{atoi()}, \function{format()},
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\function{str()}.
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\subsection{For extension writers and programs that embed Python}
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\label{embedding-locale}
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Extension modules should never call \function{setlocale()}, except to
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find out what the current locale is. But since the return value can
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only be used portably to restore it, that is not very useful (except
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perhaps to find out whether or not the locale is \samp{C}).
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When Python is embedded in an application, if the application sets the
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locale to something specific before initializing Python, that is
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generally okay, and Python will use whatever locale is set,
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\emph{except} that the \constant{LC_NUMERIC} locale should always be
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\samp{C}.
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The \function{setlocale()} function in the \module{locale} module contains
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gives the Python progammer the impression that you can manipulate the
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\constant{LC_NUMERIC} locale setting, but this not the case at the \C{}
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level: \C{} code will always find that the \constant{LC_NUMERIC} locale
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setting is \samp{C}. This is because too much would break when the
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decimal point character is set to something else than a period
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(e.g. the Python parser would break). Caveat: threads that run
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without holding Python's global interpreter lock may occasionally find
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that the numeric locale setting differs; this is because the only
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portable way to implement this feature is to set the numeric locale
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settings to what the user requests, extract the relevant
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characteristics, and then restore the \samp{C} numeric locale.
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When Python code uses the \module{locale} module to change the locale,
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this also affect the embedding application. If the embedding
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application doesn't want this to happen, it should remove the
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\module{_locale} extension module (which does all the work) from the
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table of built-in modules in the \file{config.c} file, and make sure
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that the \module{_locale} module is not accessible as a shared library.
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