How to define the number of decimals when adding a new field as double to an attribute table with python in Qgis 2.1?

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 precision

So 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'

Answer

Try:

layer.dataProvider().addAttributes([QgsField("fieldname",  QVariant.Double, "double", 10, 3)])

This is the correction that suggests the error message.

I used it in my system for adding two fields (area and perimeter) in the attributes table of my shapefile and it works.

My snipped code

.
.

.
    fields = [ QgsField('area', QVariant.Double, 'double', 20, 2), 
               QgsField('perimeter', QVariant.Double, 'double', 10, 3) ]
.
.
.

The attributes table of my shapefile: