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<H2><A NAME="SECTION0082210000000000000000">
6.22.1 Background, details, hints, tips and caveats</A>
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<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 <code>setlocale(LC_ALL, "")</code>.
<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" tppabs="http://www.python.org/doc/current/lib/module-string.html">string</a></tt> and
<tt class="module">strop</tt> 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
<code>string.lowercase</code>, <code>string.uppercase</code> and
<code>string.letters</code> (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. <code>from string
import letters</code>, 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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