@QmlAttached

This decorator declares that the enclosing type attaches the type passed as an attached property to other types. This takes effect if the type is exposed to QML using a QmlElement() or @QmlNamedElement() decorator.

QML_IMPORT_NAME = "com.library.name"
QML_IMPORT_MAJOR_VERSION = 1
QML_IMPORT_MINOR_VERSION = 0 # Optional

@QmlAnonymous
class LayoutAttached(QObject):
    @Property(QMargins)
    def margins(self):
       ...

@QmlElement()
@QmlAttached(LayoutAttached)
class Layout(QObject):
      ...

Afterwards the class may be used in QML:

import com.library.name 1.0

Layout {
   Widget {
       Layout.margins: [2, 2, 2, 2]
   }
}