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<H2><A NAME="SECTION0082710000000000000000">
6.27.1 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 <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
(such as <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 id='l2h-2410' xml:id='l2h-2410'></a> module 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> 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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	54
	55	<H2><A NAME="SECTION0082710000000000000000">
	56	6.27.1 Background, details, hints, tips and caveats</A>
	57	</H2>
	58
	59	<P>
	60	The C standard defines the locale as a program-wide property that may
	61	be relatively expensive to change. On top of that, some
	62	implementation are broken in such a way that frequent locale changes
	63	may cause core dumps. This makes the locale somewhat painful to use
	64	correctly.
65
66	<P>
67	Initially, when a program is started, the locale is the "<tt class="samp">C</tt>" locale, no
68	matter what the user's preferred locale is. The program must
69	explicitly say that it wants the user's preferred locale settings by
70	calling <code>setlocale(LC_ALL, '')</code>.
71
72	<P>
73	It is generally a bad idea to call <tt class="function">setlocale()</tt> in some library
74	routine, since as a side effect it affects the entire program. Saving
75	and restoring it is almost as bad: it is expensive and affects other
76	threads that happen to run before the settings have been restored.
77
78	<P>
79	If, when coding a module for general use, you need a locale
80	independent version of an operation that is affected by the locale
81	(such as <tt class="function">string.lower()</tt>, or certain formats used with
82	<tt class="function">time.strftime()</tt>), you will have to find a way to do it
83	without using the standard library routine. Even better is convincing
84	yourself that using locale settings is okay. Only as a last resort
85	should you document that your module is not compatible with
86	non-"<tt class="samp">C</tt>" locale settings.
87
88	<P>
89	The case conversion functions in the
90	<tt class="module"><a href="module-string.html">string</a></tt><a id='l2h-2410' xml:id='l2h-2410'></a> module are affected by the
91	locale settings. When a call to the <tt class="function">setlocale()</tt> function
92	changes the <tt class="constant">LC_CTYPE</tt> settings, the variables
93	<code>string.lowercase</code>, <code>string.uppercase</code> and
94	<code>string.letters</code> are recalculated. Note that this code that uses
95	these variable through `<tt class="keyword">from</tt> ... <tt class="keyword">import</tt> ...',
96	e.g. <code>from string import letters</code>, is not affected by subsequent
97	<tt class="function">setlocale()</tt> calls.
98
99	<P>
100	The only way to perform numeric operations according to the locale
101	is to use the special functions defined by this module:
102	<tt class="function">atof()</tt>, <tt class="function">atoi()</tt>, <tt class="function">format()</tt>,
103	<tt class="function">str()</tt>.
104
105	<P>
106
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132	<b class="navlabel">Previous:</b>
133	<a class="sectref" rel="prev" href="module-locale.html">6.27 locale </A>
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135	<a class="sectref" rel="parent" href="module-locale.html">6.27 locale </A>
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