5. Localization

QGIS user interface is available in multiple languages.

A global option in the settings allows to override the user interface language which was automatically detected from the operating system.

The representation of numbers, currency and dates varies according to the country and it is also overrideable in the settings.

Another option in the settings allows to define if the numeric thousands separator has to be used.

All these features are implemented through the QLocale QT class which provides a very complete support for numeric and date types representation.

In order to make this system work a few rules need to be carefully followed when displaying numeric values in the QGIS user interface and when converting user input to integral or floating point variables.

5.1. Converting numbers to strings

For strings that are printed on the screen and visible to the users do not use QString::number() because it does not take locale options into consideration and it always uses C locale.

Also do not use string interpolation unless you use the L suffix as explained in QString documentation.

Use QLocale().toString() instead.

Example:

const QString localizedNumber = QLocale().toString( 1234.56 );
// localizedNumber will be:  1,234.56 using english or "C" locale
//                           1.234,56 using italian locale
//                            1234.56 using english or "C" locale without thousands separator
//                            1234,56 using italian locale without thousands separator

Warning

A notable exception to this rule is the generation of strings that will be passed to external applications or scripts that expect C number representation (dot as decimal separator and no thousands separator). QString::number() must be used in this situation.

5.2. Converting user input strings to numbers

When converting strings from user input to numbers do not use QVariant::toDouble(), QString::toDouble() or other ::toDouble() methods available in QT classes because these methods ignore locale settings. The same consideration applies to integral types and ::toInt() ot ::toLongLong() methods.

QLocale().toDouble() or QLocale().toInt() and the others QLocale() conversion methods can be used in this situation.

As a better alternative, QGIS API provides a few classes that should be used for floating point numeric user input:

QgsDoubleSpinBox displays the number according to user locale and settings and it validates the user input using QgsDoubleValidator.

The validator class is tolerant and it will try to interpret the input checking first for the user locale and falling back to C locale if there is no match.

The general recommendation is to use QgsDoubleSpinBox for all floating point types I/O whenever it is possible because it is very well tested and it validates the input correctly. As an alternative it is possible to use the QgsDoubleValidator class independently on a string obtained from another widget (e.g. a simple QLineEdit widget).