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<H2><A NAME="SECTION0081210000000000000000">
6.12.1 Background, details, hints, tips and caveats</A>
</H2>

<P>
The C standard defines the locale as a program-wide property that may
be relatively expensive to change.  On top of that, some
implementation are broken in such a way that frequent locale changes
may cause core dumps.  This makes the locale somewhat painful to use
correctly.

<P>
Initially, when a program is started, the locale is the "<tt class=samp>C</tt>" locale, no
matter what the user's preferred locale is.  The program must
explicitly say that it wants the user's preferred locale settings by
calling <tt>setlocale(LC_ALL, "")</tt>.

<P>
It is generally a bad idea to call <tt class=function>setlocale()</tt> in some library
routine, since as a side effect it affects the entire program.  Saving
and restoring it is almost as bad: it is expensive and affects other
threads that happen to run before the settings have been restored.

<P>
If, when coding a module for general use, you need a locale
independent version of an operation that is affected by the locale
(e.g. <tt class=function>string.lower()</tt>, or certain formats used with
<tt class=function>time.strftime()</tt>)), you will have to find a way to do it
without using the standard library routine.  Even better is convincing
yourself that using locale settings is okay.  Only as a last resort
should you document that your module is not compatible with
non-"<tt class=samp>C</tt>" locale settings.

<P>
The case conversion functions in the
<tt class=module><a href="module-string.html">string</a></tt><a name="l2h-1246"></a> and
<tt class=module>strop</tt><a name="l2h-1247"></a> modules are affected by the locale
settings.  When a call to the <tt class=function>setlocale()</tt> function changes
the <tt class=constant>LC_CTYPE</tt> settings, the variables
<tt>string.lowercase</tt>, <tt>string.uppercase</tt> and
<tt>string.letters</tt> (and their counterparts in <tt class=module>strop</tt>) are
recalculated.  Note that this code that uses these variable through
`<tt class=keyword>from</tt> ... <tt class=keyword>import</tt> ...', e.g. <tt>from string
import letters</tt>, is not affected by subsequent <tt class=function>setlocale()</tt>
calls.

<P>
The only way to perform numeric operations according to the locale
is to use the special functions defined by this module:
<tt class=function>atof()</tt>, <tt class=function>atoi()</tt>, <tt class=function>format()</tt>,
<tt class=function>str()</tt>.

<P>
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