## 21.2.1 Background, details, hints, tips and caveats

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.

Initially, when a program is started, the locale is the "`C`" 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 `setlocale(LC_ALL, '')`.

It is generally a bad idea to call `setlocale()` 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.

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 `string.lower()`, or certain formats used with `time.strftime()`), 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-"`C`" locale settings.

The case conversion functions in the `string` module are affected by the locale settings. When a call to the `setlocale()` function changes the `LC_CTYPE` settings, the variables `string.lowercase`, `string.uppercase` and `string.letters` are recalculated. Note that code that uses these variable through ``from` ... `import` ...', e.g. `from string import letters`, is not affected by subsequent `setlocale()` calls.

The only way to perform numeric operations according to the locale is to use the special functions defined by this module: `atof()`, `atoi()`, `format()`, `str()`.