How to define the number of decimals when adding a new field as double to an attribut

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How to change the expression in order to define the number of decimals?

layer.dataProvider().addAttributes([QgsField("fieldname", QVariant.Double)])As I read in QGIS API Documentation:

Parameters name Field name type Field variant type, currently supported: String / Int / Double typeName Field type (eg. char, varchar, text, int, serial, double). len Field length prec Field precisionSo I try:

layer.dataProvider().addAttributes([QgsField("fieldname", QVariant.Double, 10, 3)])But I get this error:

TypeError: arguments did not match any overloaded call:QgsField(QString name=QString(), Type type=QVariant.Invalid, QString typeName=QString(), int len=0, int prec=0, QString comment=QString()): argument 3 has unexpected type 'int'QgsField(QgsField): argument 1 has unexpected type 'str'

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